Glossary of cellular and molecular biology (0–L)

This glossary of cellular and molecular biology is a list of definitions of terms and concepts commonly used in the study of cell biology, molecular biology, and related disciplines, including genetics, biochemistry, and microbiology. It is split across two articles:
 * This page, Glossary of cellular and molecular biology (0–L), lists terms beginning with numbers and with the letters A through L.
 * Glossary of cellular and molecular biology (M–Z) lists terms beginning with the letters M through Z.

This glossary is intended as introductory material for novices (for more specific and technical detail, see the article corresponding to each term). It has been designed as a companion to Glossary of genetics and evolutionary biology, which contains many overlapping and related terms; other related glossaries include Glossary of virology and Glossary of chemistry.

0–9
3' untranslated region (3'-UTR):

3'-end:

One of two ends of a single linear strand of or, specifically the end at which the chain of terminates at the third carbon atom in the furanose ring of  or  (i.e. the terminus at which the 3' carbon is not attached to another nucleotide via a ; ', the 3' carbon is often still bonded to a hydroxyl group). By convention, sequences and structures positioned nearer to the 3'-end relative to others are referred to as. Contrast '.

5' cap:

A specially altered attached to the of some  as part of the set of  which convert raw transcripts into mature RNA products. The precise structure of the 5' cap varies widely by organism; in eukaryotes, the most basic cap consists of a   bonded to the triphosphate group that terminates the 5'-end of an RNA sequence. Among other functions, capping helps to regulate the export of mature RNAs from the, prevent their degradation by, and promote in the cytoplasm. Mature can also be decapped.

5' untranslated region (5'-UTR):

5-bromodeoxyuridine: See.

5'-end:

One of two ends of a single linear strand of or, specifically the end at which the chain of terminates at the fifth carbon atom in the furanose ring of  or  (i.e. the terminus at which the 5' carbon is not attached to another nucleotide via a ; ', the 5' carbon is often still bonded to a phosphate group). By convention, sequences and structures positioned nearer to the 5'-end relative to others are referred to as. Contrast '.

5-methyluracil: See.

A
acentric: (of a linear or chromosome fragment) Having no.

acetyl coenzyme A (acetyl-CoA): A biochemical compound consisting of a molecule to which an acetyl group is attached via a high-energy thioester bond. of coenzyme A occurs as part of the of, , and  , after which it participates as an energy carrier in several important , notably the , in which hydrolysis of the acetyl group releases energy which is ultimately captured in 11  and one.

acetylation: The covalent attachment of an acetyl group to a chemical compound, protein, or other biomolecule via an esterification reaction with acetic acid, either spontaneously or by catalysis. Acetylation plays important roles in several and in. Contrast.

acetyltransferase: Any of a class of which catalyze the covalent bonding of an acetyl group  to another compound, protein, or biomolecule, a process known as.

acrocentric: (of a linear or chromosome fragment) Having a positioned very close to one end of the chromosome, as opposed to  or.

action potential: The local change in voltage that occurs when the of a specific location along the of a  rapidly depolarizes, such as when a nerve impulse is transmitted between.

activation: See.

activator: A type of that increases the of a  or set of genes. Most activators work by binding to a specific located within or near an  or  and facilitating the binding of  and other transcription machinery in the same region. See also '; contrast '.

active site:

The region of an to which one or more bind, causing the substrate or another molecule to undergo a chemical reaction. This region usually consists of one or more residues (commonly three or four) which, when the enzyme is  properly, are able to form temporary chemical bonds with the atoms of the substrate molecule; it may also include one or more additional residues which, by interacting with the substrate, are able to catalyze a specific reaction involving the substrate. Though the active site constitutes only a small fraction of all the residues comprising the enzyme, its specificity for particular substrates and reactions is responsible for the enzyme's biological function.

active transport: Transport of a substance (such as a or ) across a against a concentration gradient. Unlike, active transport requires an expenditure of energy.

adenine: A used as one of the four standard nucleobases in both  and  molecules. Adenine forms a with  in DNA and with  in RNA.

adenosine: One of the four standard used in molecules, consisting of an   with its N9 nitrogen  to the C1 carbon of a  sugar. Adenine bonded to is known as, which is the version used in.

adenosine diphosphate (ADP):

adenosine monophosphate (AMP):

adenosine triphosphate (ATP): An organic compound derived from that functions as the major source of energy for chemical reactions inside living. It is found in all forms of life and is often referred to as the "molecular currency" of intracellular energy transfer.

A-DNA: One of three main biologically active of the, along with and. The A-form helix has a right-handed twist with 11 per full turn, only slightly more compact than B-DNA, but its bases are sharply tilted with respect to the helical axis. It is often favored in dehydrated conditions and within sequences of consecutive nucleotides (e.g. GAAGGGGA); it is also the primary conformation adopted by  and.

affected relative pair: Any pair of organisms which are related genetically and both affected by the same. For example, two cousins who both have blue eyes are an affected relative pair since they are both affected by the that codes for blue eyes.

alkaline lysis:

allele: One of multiple alternative versions of an individual, each of which is a viable sequence occupying a given position, or, on a. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the same gene produces brown eyes.

allosome:

Any that differs from an ordinary in size, form, or behavior and which is responsible for determining the sex of an organism. In humans, the and the  are sex chromosomes.

alpha helix (α-helix): A common structural in the secondary structures of consisting of a right-handed helix conformation resulting from hydrogen bonding between  residues which are not immediately adjacent to each other.

alternative splicing:

A regulated phenomenon of eukaryotic in which specific or parts of exons from the same  are variably included within or removed from the final, mature  transcript. A class of, alternative splicing allows a single to code for multiple protein isoforms and greatly increases the diversity of proteins that can be produced by an individual. See also.

amber: One of three used in the ; in, it is specified by the nucleotide triplet UAG. The other two stop codons are named and.

amino acid: Any of a class of organic compounds whose basic structural formula includes a central carbon atom bonded to amine and carboxyl functional groups and to a variable side chain. Out of nearly 500 known amino acids, a set of 20 are coded for by the and incorporated into long polymeric chains as the building blocks of and hence of  and. The specific sequences of amino acids in the polypeptide chains that form a protein are ultimately responsible for determining the protein's structure and function.

amino terminus: See.

aminoacyl-tRNA synthetase:

Any of a set of enzymes which catalyze the transesterification reaction that results in the attachment of a specific (or a precursor) to one of its cognate molecules, forming an. Each of the 20 different amino acids used in the is recognized and attached by its own specific synthetase enzyme, and most synthetases are cognate to several different tRNAs according to their specific.

aminoacyl-tRNA (aa-tRNA):

A to which a cognate is chemically bonded; i.e. the product of a transesterification reaction catalyzed by an. Aminoacyl-tRNAs bind to the of the  during.

amplicon: Any DNA or RNA sequence or fragment that is the source and/or product of an reaction. The term is most frequently used to describe the numerous copied fragments that are the products of the or, though it may also refer to sequences that are amplified naturally within a genome, e.g. by.

amplification: The of a biomolecule, in particular the production of one or more copies of a, known as an, either naturally (e.g. by spontaneous ) or artificially (e.g. by ), and especially implying many repeated replication events resulting in thousands, millions, or billions of copies of the target sequence, which is then said to be amplified.

anaphase: The stage of and that occurs after  and before, when the replicated chromosomes are segregated and each of the  are moved to opposite sides of the.

anaphase lag: The failure of one or more pairs of or to properly migrate to opposite sides of the cell during  of  or  due to a defective. Consequently, both daughter cells are : one is missing one or more chromosomes (creating a ) while the other has one or more extra copies of the same chromosomes (creating a ).

aneucentric: (of a linear or chromosome fragment) Having an abnormal number of, i.e. more than one.

aneuploidy: The condition of a cell or organism having an abnormal number of one or more particular (but excluding abnormal numbers of complete sets of chromosomes, which instead is known as ).

annealing: The of two  molecules containing  sequences, creating a  molecule with. The term is used in particular to describe steps in laboratory techniques such as, where double-stranded DNA molecules are repeatedly into single strands by heating and then exposed to cooler temperatures, causing the strands to reassociate with each other or with complementary. The exact is strongly influenced by the length and specific sequence of the individual strands.

antibody:

anticodon: A consecutive within a  which  the three nucleotides of a  within an  transcript. During, each tRNA recruited to the contains a single anticodon triplet that pairs with its complementary codon from the mRNA sequence, allowing each codon to specify a particular  to be added to the growing peptide chain. Anticodons containing in the first position are capable of pairing with more than one codon due to a phenomenon known as.

antimetabolite: Any molecule that functions as an antagonist to a process, limiting or inhibiting normal cellular metabolism; i.e. a metabolic poison.

antimitotic: Any compound that suppresses normal in a cell or population of cells.

antioncogene: A which helps to regulate cell growth and suppress tumors when functioning correctly, such that its absence or malfunction can result in uncontrolled cell growth and possibly cancer. Compare.

antiparallel: The orientation of two strands of a double-stranded (and more generally any pair of biopolymers) which are parallel to each other but with opposite. For example, the two strands of a  molecule run side-by-side but in opposite directions, with one strand oriented -to- and the other 3'-to-5'.

antiporter: A which works by exchanging two different ions or small molecules across a in opposite directions, either at the same time or consecutively.

antisense: See.

antisense RNA (asRNA):

A molecule containing an  sequence that is  to a sense strand, such as a, with which it readily, thereby inhibiting the sense strand's further activity (e.g.  into protein). Many different classes of naturally occurring RNA such as function by this principle, making them potent gene  in various  mechanisms. Synthetic antisense RNA has also found widespread use in gene studies, and in practical applications such as antisense therapy.

anucleate:

(of a cell or organism) Lacking a, i.e. a discrete, membrane-bound organelle enclosing the cell's, used especially of cells which normally have a nucleus but from which the nucleus (e.g. in artificial ), and also of specialized cell types that develop without nuclei despite that the cells of other tissues comprising the same organism ordinarily do have nuclei (e.g. mammalian erythrocytes).

apical constriction:

apoptosis: A highly regulated form of that occurs in organisms.

aptamer:

artificial gene synthesis: A set of laboratory methods used in the of a (or any other ) from free, i.e. without relying on an existing.

assimilatory process: Any process by which chemical compounds containing various essential elements (C, H, O, N, P, S, Se, Fe, Co, Ni, Cu, Zn, Mo, or other oligo-elements) are uptaken by microorganisms and incorporated into complex in order to synthesize various cellular components. In contrast, a uses the energy released by decomposing exogenous molecules to power the cell's  and then   out of the cell, instead of reusing them to build new molecules.

asynapsis: The failure of to properly pair with each other during. Contrast ' and '.

attached X:

A single containing two or more physically attached copies of the normal  as a result of either a natural internal  or any of a variety of  methods. The resulting compound chromosome effectively carries two or more doses of all genes and sequences included on the X, yet functions in all other respects as a single chromosome, meaning that haploid 'XX' (rather than the ordinary 'X' gametes) will be produced by  and inherited by progeny. In mechanisms such as in which the sex of an organism is determined by the total dosage of X-linked genes, an abnormal 'XXY', fertilized by one XX gamete and one Y gamete, will develop into a female.

autolysis: The or digestion of a by its own ; or of a particular enzyme by another instance of the same enzyme. See also.

autosome: Any that is not an and hence is not involved in the determination of the sex of an organism. Unlike the sex chromosomes, the autosomes in a cell exist in pairs, with the members of each pair having the same structure, morphology, and genetic.

autozygote: A cell or organism that is for a at which the two homologous  are identical by descent, both having been derived from a single gene in a common ancestor. Contrast.

auxesis: The growth of a multicellular organism due to an increase in the size of its cells rather than an increase in the number of cells.

axenic: Describing a in which only a single species, variety, or strain is present, and which is therefore entirely free of contaminating organisms including symbiotes and parasites.

B
B chromosome: Any supernumerary molecule which is not a duplicate of nor to any of the standard complement of normal "A" chromosomes comprising a genome. Typically very small and devoid of structural genes, B chromosomes are by definition not necessary for life. Though they occur naturally in many eukaryotic species, they are and thus  even between closely related individuals.

back mutation: A that reverses the effect of a previous mutation which had inactivated a gene, thus restoring function. See also.

backcrossing:

The breeding of a organism with one of its parents or an individual genetically similar to one of its parents, often intentionally as a type of, with the aim of producing offspring with a genetic identity which is closer to that of the parent. The reproductive event and the resulting progeny are both referred to as a backcross, often abbreviated in genetics shorthand with the symbol BC.

bacterial artificial chromosome (BAC):

balancer chromosome:

base: An abbreviation of and.

base pair (bp): A pair of two on complementary or  strands which are bonded to each other by hydrogen bonds. The ability of consecutive base pairs to stack one upon another contributes to the long-chain structures observed in both  and  molecules.

baseline: A measure of the level of a or genes prior to a perturbation in an experiment, as in a negative control. Baseline expression may also refer to the expected or historical measure of expression for a gene.

basic local alignment search tool (BLAST): A computer algorithm widely used in for and comparing primary biological sequence information such as the  of DNA or RNA or the  of proteins. BLAST programs enable scientists to quickly check for homology between two or more sequences by directly comparing the nucleotides or amino acids present at each position within each sequence; a common use is to search for matches between a specific query sequence and a digital sequence database such as a, with the program returning a list of sequences from the database which resemble the query sequence above a specified threshold of similarity. Such comparisons can permit the identification of an organism from an unknown sample or the inference of evolutionary relationships between genes, proteins, or species.

B-DNA: The "standard" or classical of the  ', thought to represent an average of the various distinct conformations assumed by very long DNA molecules under physiological conditions. The B-form double helix has a right-handed twist with a diameter of 23.7 ångströms and a of 35.7 ångströms or about 10.5  per full turn, such that each nucleotide pair is rotated 36° around the helical axis with respect to its neighboring pairs. See also ' and.

beta oxidation:

bidirectional replication: A common mechanism of in which two move in opposite directions away from the same ; this results in a  where the  molecule is locally separated into two.

binary fission:

binding site:

bioassay:

biochemical pathway:

bioenergetics:

biofilm: A community of symbiotic microorganisms, especially bacteria, where cells produce and embed themselves within a slimy, sticky composed of various high-molecular weight biopolymers, to each other and sometimes also to a, which may be a biotic or abiotic surface. Many bacteria can exist either as independent single cells or switch to a physiologically distinct biofilm phenotype; those that create biofilms often do so in order to shelter themselves from harmful environments. Cells residing within biofilms can easily share nutrients and, and subpopulations of cells may to perform specialized functions supporting the whole biofilm.

biomarker: A measurable indicator of some biological state, especially a compound or whose presence or absence in a biological system is a reliable sign of a normal or abnormal process, condition, or disease. Things that may serve as biomarkers include direct measurements of the concentration of a particular compound or molecule in a tissue or fluid sample, or any other characteristic physiological, histological, or radiographic signal (e.g. a change in heart rate, or a distinct under a microscope). They are regularly used as predictive or diagnostic tools in clinical medicine and laboratory research.

biomolecular gradient:

bivalent:

blot: Any of a variety of molecular biology methods by which or chromatographically separated,, or samples are transferred from a support medium such as a polyacrylamide or agarose gel onto an immobilizing carrier such as a nitrocellulose or PVDF membrane. Some methods involve the transfer of molecules by capillary action (e.g. and ), while others rely on the transport of charged molecules by electrophoresis (e.g. ). The transferred molecules are then visualized by colorant staining, by autoradiography, or by  for specific sequences or  with  or  bound to chemiluminescent reporters.

blunt end: A term used to describe the end of a molecule where the terminal nucleobases on each are  with each other, such that neither strand has a single-stranded "overhang" of unpaired bases, in contrast to a so-called "", where an overhang is created by one strand being one or more bases longer than the other. Blunt ends and sticky ends are relevant when multiple DNA molecules, e.g. in, because sticky-ended molecules will not readily anneal to each other unless they have matching overhangs; blunt-ended molecules do not anneal in this way, so special procedures must be used to ensure that fragments with blunt ends are joined in the correct places.

bromodeoxyuridine (BUDR, BrdU):

C
cadastral gene: A that restricts the  of other genes to specific tissues or body parts in an organism, typically by producing  which variably inhibit or permit  of the other genes in different cell types. The term is used most commonly in plant genetics.

cadherin: Any of a class of which are dependent on calcium ions (Ca2+) and whose extracellular function as mediators of cell–cell adhesion at  in eukaryotic tissues.

callus: An unorganized mass of parenchymal cells that forms naturally at the site of wounds in plant tissues, and which is commonly artificially induced to form in plant as a means of initiating somatic embryogenesis.

Calvin cycle:

candidate gene: A whose location on a chromosome is with a particular  (often a disease-related phenotype), and which is therefore suspected of causing or contributing to the phenotype. Candidate genes are often selected for study based on a priori knowledge or speculation about their functional relevance to the trait or disease being researched.

canonical sequence: See.

carbohydrate: Any of a class of organic compounds having the generic chemical formula, and one of several major classes of found universally in biological systems. Carbohydrates include individual as well as the larger  they form (,, etc.) when multiple monosaccharide units are joined by. Abundant and ubiquitous, these compounds are involved in numerous essential biochemical processes and ; they are widely used as an energy source for cellular, as a form of energy storage, as molecules, and as  to  the activity of other molecules. Carbohydrates are often colloquially described as "sugars"; the prefix glyco- is used to indicate a compound or process containing or related to carbohydrates, and the suffix -ose usually signifies that a compound is a carbohydrate or a derivative.

carboxyl terminus: See.

carrier protein: A that functions as a, binding to a solute and facilitating its movement across the membrane by undergoing a series of. A protein to which a specific or has been conjugated and which thereby carries an  capable of eliciting an  response. A protein which is included in an at high concentrations in order to prevent non-specific interactions of the assay's reagents with vessel surfaces, sample components, or other reagents. For example, in many techniques, albumin is intentionally allowed to bind non-specifically to the blotted membrane prior to, so as to "block" potential off-target binding of the  to the membrane, which might otherwise cause background fluorescence that obscures genuine signal from the target.

caspase:

cassette: A pre-existing nucleic acid sequence or construct, especially a DNA with an annotated sequence and precisely positioned, into which one or more can be readily  or recombined by various  methods. Recombinant vectors containing reliable, , and antibiotic resistance genes are commonly manufactured and sold as cassettes to allow scientists to easily replace one  with another by swapping it into and out of an active "slot" or locus within the plasmid. See also.

CCAAT box:

A highly DNA sequence located approximately 75 base pairs  (i.e. -75) of the  for many eukaryotic genes.

cDNA: See.

cell:

cell biology:

The branch of biology that studies the structures, functions, processes, and properties of biological, the self-contained units of life common to all living organisms.

cell compartmentalization: The subdivision of the interior of a into distinct, usually compartments, including the  and  (,,, , etc.), a defining feature of the Eukarya.

cell cortex: A specialized layer of proteins lining the inner face of the in most eukaryotic cells, composed primarily of   and   and usually 100–1000 nanometres thick, which functions as a modulator of membrane behavior and cell surface properties.

cell counting: The process of determining the number of within a biological sample or by any of a variety of methods. Counting cells is an important aspect of used widely in research and clinical medicine. It is generally achieved by using a manual or digital to count the number of cells present in small fractions of a sample, and then extrapolating to estimate the number present in the entire sample. The resulting quantification is typically expressed as a concentration, i.e. the number of cells per unit area or volume.

cell culture: The process by which living cells are grown and maintained, or "cultured", under carefully controlled conditions, generally outside of their natural environment. Optimal growth conditions vary widely for different cell types but usually consist of a suitable vessel (e.g. a or ) containing a specifically formulated or  that supplies all of the nutrients essential for life (,, vitamins, minerals, etc.) plus any desirable growth factors and, permits gas exchange (if necessary), and regulates the environment by maintaining consistent physico-chemical properties (e.g. , osmotic pressure, and temperature). Some cell types require a solid surface to which they can adhere in order to reproduce, while others can be grown freely floating in a liquid or gelatinous. Most cells have a genetically determined reproduction limit, but cells will divide indefinitely if provided with optimal conditions.

cell cycle:

cell division: The separation of an individual into two by any process. Cell division generally occurs by a complex, carefully structured sequence of events involving the reorganization of the parent cell's internal contents, the physical cleaving of the and, and the even distribution of contents between the two resulting cells, so that each ultimately contains approximately half of the original cell's starting material. It usually implies reproduction via the of the parent cell's genetic material prior to division, though cells may also divide without replicating their DNA. In prokaryotic cells, is the primary form of cell division. In eukaryotic cells, asexual division occurs by and, while specific lineages of cells reserved for sexual reproduction can additionally divide by.

cell fusion: The merging or coalescence of two or more cells into a single cell, as occurs in the fusion of to form a. Generally this occurs by the destabilization of each cell's and the formation of  bridges between them which then expand until the two cytoplasms are completely mixed; intercellular structures or  such as  may or may not fuse as well. Cells can also be artificially induced to fuse with each other by treating them with a fusogen such as polyethylene glycol or by passing an electric current through them.

cell membrane:

The selectively permeable surrounding all prokaryotic and eukaryotic cells, defining the outermost boundary of the cell and physically separating the from the  environment. Like all, the cell membrane is a flexible, fluid, sheet-like with, , and numerous other molecules embedded within or interacting with it from both the interior and exterior sides. Embedded molecules often have freedom to move laterally alongside the membrane's lipids. Though the cell membrane can be freely crossed by many ions, small organic molecules, and water, most other substances require through special pores or  or by  or  in order to enter or exit the cell, usually very large or electrically charged molecules such as proteins and nucleic acids. Besides regulating the transport of substances into and out of the cell, the cell membrane creates an organized interior space in which to perform life-sustaining activities and plays fundamental roles in all of the cell's interactions with the external environment, making it important in, , defense, and , among numerous other processes.

cell physiology:

cell polarity:

cell signaling:

The diverse set of processes by which cells transmit information to and receive information from themselves, other cells, or their environment. occurs in all cell types, prokaryotic and eukaryotic, and is critically important to the cell's ability to navigate and survive its physical surroundings. Countless mechanisms of signaling have evolved in different organisms, and these are often categorized according to the proximity between sender and recipient (,,, , or ).

cell surface receptor: Any of a class of proteins embedded within or attached to the external surface of the, with one or more facing the  environment and one or more  that couple the binding of a particular  to an  event or process. Cell surface receptors are a primary means by which environmental signals are received by the cell and across the membrane into the cell interior. Some may also bind exogenous ligands and transport them into the cell in a process known as.

cell wall:

cell-free DNA (cfDNA): Any molecule that exists outside of a or, freely floating in an  such as blood plasma.

cellular: Of, relating to, consisting of, produced by, or resembling a or cells.

cellular differentiation:

cellular immunity:

cellular noise:

cellular reprogramming: The conversion of a fully from one -specific cell type to another. This involves to a  state; an example is the conversion of mouse  to an undifferentiated embryonic state, which relies on the  Oct4, Sox2, Myc, and Klf4.

cellular senescence:

centimorgan (cM):

A unit for measuring defined as the distance between chromosomal for which the expected average number of intervening  in a single generation is 0.01. Though not an actual measure of physical distance, it is used to infer the actual distance between two loci based on the apparent likelihood of a crossover occurring between them.

central dogma of molecular biology: A generalized framework for understanding the flow of genetic information between macromolecules within biological systems. The central dogma outlines the fundamental principle that the sequence information encoded in the three major classes of biopolymer—,, and —can only be transferred between these three classes in certain ways, and not in others: specifically, information transfer between the and from nucleic acid to protein is possible, but transfer from protein to protein, or from protein back to either type of nucleic acid, is impossible and does not occur naturally.

centriole: A cylindrical composed of, present only in certain eukaryotes. A pair of centrioles migrate to and define the two opposite poles of a where, as part of a, they initiate the growth of the.

centromere: A specialized DNA sequence within a that links a pair of. The primary function of the centromere is to act as the site of assembly for, protein complexes which direct the attachment of to the centromere and facilitate  of the chromatids during  or.

centromeric index: The proportion of the total length of a encompassed by its, typically expressed as a percentage; e.g. a chromosome with a centromeric index of 15 is, with a short arm comprising only 15% of its overall length.

centrosome:

cfDNA: See.

channel protein: A type of whose shape forms an aqueous pore in a, permitting the passage of specific solutes, often small ions, across the membrane in either or both directions.

Chargaff's rules: A set of axioms which state that, in the of any chromosome, species, or organism, the total number of (A)  will be approximately equal to the total number of  (T) residues, and the number of  (G) residues will be equal to the number of  (C) residues; accordingly, the total number of  (A + G) will equal the total number of  (T + C). These observations illustrate the highly specific nature of the  that occurs in all  DNA molecules: even though non-standard pairings are technically possible, they are exceptionally rare because the standard ones are strongly favored in most conditions. Still, the 1:1 equivalence is seldom exact, since at any given time nucleobase ratios are inevitably distorted to some small degree by, missing bases, and non-canonical bases. The presence of polymers also alters the proportions, as the sequence of an individual  may contain any number of any of the bases.

charged tRNA: A to which an has been attached; i.e. an. lack amino acids.

chDNA: See.

chemiosmosis:

chemokinesis: A non-directional, random change in the movement of a molecule, cell, or organism in response to a chemical stimulus, e.g. a change in speed resulting from exposure to a particular chemical compound.

chemotaxis: A directed, non-random change in the movement of a molecule, cell, or organism in response to a chemical stimulus, e.g. towards or away from an area with a high concentration of a particular chemical compound.

chiasma:

A cross-shaped junction that forms the physical point of contact between two non-sister belonging to during. As well as ensuring proper of the chromosomes, these junctions are also the  at which  may occur during  or, which results in the reciprocal exchange of DNA between the synapsed chromatids.

chimerism: The presence of two or more populations of cells with distinct in an individual organism, known as a chimera, which has developed from the fusion of cells originating from separate ; each population of cells retains its own genome, such that the organism as a whole is a mixture of genetically non-identical tissues. Genetic chimerism may be inherited (e.g. by the fusion of multiple embryos during pregnancy) or acquired after birth (e.g. by allogeneic transplantation of cells, tissues, or organs from a genetically non-identical donor); in plants, it can result from grafting or errors in cell division. It is similar to but distinct from.

chloroplast: A type of small, lens-shaped found in the cells of green algae and plants which contains light-sensitive photosynthetic pigments and in which the series of biochemical reactions that comprise photosynthesis takes place. Like, chloroplasts are bound by a double membrane, contain their own from which they direct transcription of a unique set of genes, and replicate independently of the nuclear genome.

chloroplast DNA (cpDNA, chDNA, ctDNA): The set of molecules contained within, a type of photosynthetic  located within the cells of some eukaryotes such as plants and algae, representing a semi-autonomous  separate from that within the cell's nucleus. Like other types of plastid DNA, cpDNA usually exists in the form of small circular.

chondriome: The complete set of or of within a cell, tissue, organism, or species.

chromatid: One copy of a newly copied, which is joined to the original chromosome by a. Paired copies of the same individual chromosome are known as.

chromatin: A complex of,, and found in eukaryotic cells that is the primary substance comprising. Chromatin functions as a means of very long DNA molecules into highly organized and densely compacted shapes, which prevents the strands from becoming tangled, reinforces the DNA during, helps to prevent DNA damage, and plays an important role in regulating  and.

chromatin immunoprecipitation (ChIP):

chromocenter: A central amorphous mass of found in the nuclei of cells of the salivary glands in Drosophila larvae and resulting from the fusion of regions surrounding the  of the somatically paired chromosomes, with the distal  arms radiating outward.

chromomere:

A region of a that has been locally compacted or into, conspicuous under a microscope as a "bead", node, or dark-staining band, especially when contrasted with nearby uncompacted strings of DNA.

chromosomal crossover:

chromosomal DNA: contained in, as opposed to. The term is generally used synonymously with.

chromosomal duplication: The of an entire, as opposed to a segment of a chromosome or an.

chromosomal instability:

chromosome: A molecule containing part or all of the genetic material of an organism. Chromosomes may be considered a sort of molecular "package" for carrying DNA within the of cells and, in most eukaryotes, are composed of long strands of DNA coiled with  which bind to and  the strands to prevent them from becoming an unmanageable tangle. Chromosomes are most easily distinguished and studied in their completely condensed forms, which only occur during. Some simple organisms have only one chromosome made of circular DNA, while most eukaryotes have multiple chromosomes made of linear DNA.

chromosome condensation: The process by which eukaryotic chromosomes become shorter, thicker, denser, and more conspicuous under a microscope during due to systemic coiling and of  strands of DNA in preparation for cell division.

chromosome segregation:

The process by which or  separate from each other and migrate to opposite sides of the dividing cell during  or.

chromosome walking: See.

cilium:

A slender, thread-like, projection extending from the surface of a eukaryotic cell, longer than a but shorter than a. Most eukaryotic cells have at least one primary cilium serving sensory or signaling functions; some cells employ thousands of motile cilia covering their entire surface in order to achieve locomotion or to move extracellular material past the cell.

circular DNA: Any molecule, or, which forms a continuous closed loop without ends; e.g. bacterial chromosomes,  and, as well as many other varieties of , including  and some viral DNA. Contrast.

circulating tumor DNA (ctDNA): Any fragments derived from tumor cells which are circulating freely in the bloodstream.

cis: On the same side; adjacent to; from the same molecule. Contrast.

cis-acting: Affecting a or sequence on the same nucleic acid molecule. A or sequence within a particular DNA molecule such as a  is said to be cis-acting if it influences or acts upon other sequences located within short distances (i.e. physically nearby, usually but not necessarily ) on the same molecule or chromosome; or, in the broadest sense, if it influences or acts upon other sequences located anywhere (not necessarily within a short distance) on the same chromosome of a. Cis-acting factors are often involved in the of  by acting to inhibit or to facilitate. Contrast.

cis-dominant mutation: A occurring within a (such as an ) which alters the functioning of a nearby  or genes on the same. Cis-dominant mutations affect the of genes because they occur at sites that control transcription rather than within the genes themselves.

cisgenesis:

cis-regulatory element (CRE):

Any sequence or region of which the  of nearby  (e.g. a,, , or ), typically by serving as a binding site for one or more. Contrast.

cisterna:

Any of a class of flattened, membrane-bound or of the  and   and the. By traveling through one or more cisternae, each of which contains a distinct set of enzymes, newly created proteins and polysaccharides undergo chemical modifications such as and, which are used as packaging signals to direct their transport to specific destinations within the cell.

cistron:

citric acid cycle:

classical genetics: The branch of based solely on observation of the visible results of reproductive acts, as opposed to that made possible by the modern techniques and methodologies of molecular biology. Contrast.

cleavage:

cleavage furrow: A trough-like indentation in the surface of the, often conspicuous when viewed through a microscope, that initiates the of the cytoplasm as the  begins to narrow during.

clonal selection:

cloning: The process of producing, either naturally or artificially, individual organisms or cells which are genetically identical to each other. Clones are the result of all forms of asexual reproduction, and cells that undergo produce daughter cells that are clones of the parent cell and of each other. Cloning may also refer to biotechnology methods which artificially create copies of organisms or cells, or, in, copies of DNA fragments or other molecules.

closed chromatin: See.

coactivator: A type of that increases the of one or more genes by binding to an.

coding strand:

The strand of a double-stranded DNA molecule whose nucleotide sequence corresponds directly to that of the RNA transcript produced during (except that bases are substituted with  bases in the RNA molecule). Though it is not itself transcribed, the coding strand is by convention the strand used when displaying a DNA sequence because of the direct analogy between its sequence and the of the RNA product. Contrast '; see also '.

codon: A series of three consecutive in a coding region of a sequence. Each of these triplets codes for a particular or  during. and molecules are each written in a language using four "letters" (four different ), but the language used to construct proteins includes 20 "letters" (20 different amino acids). Codons provide the key that allows these two languages to be into each other. In general, each codon corresponds to a single amino acid (or stop signal). The full set of codons is called the.

codon usage bias: The preferential use of a particular to code for a particular rather than alternative codons that are synonymous for the same amino acid, as evidenced by differences between organisms in the frequencies of the synonymous codons occurring in their coding DNA. Because the is, most amino acids can be specified by multiple codons. Nevertheless, certain codons tend to be overrepresented (and others underrepresented) in different species.

coenocyte: A mass of bounded by a  and resulting from continuous cytoplasmic growth and repeated nuclear division without, found in some species of algae and fungi, e.g. Vaucheria and Physarum.

coenzyme: A relatively small, independent molecule which associates with a specific and participates in the reaction that the enzyme catalyzes, often by forming a covalent bond with the. Examples include biotin,, and.

coenzyme A (CoA):

cofactor: Any non-protein organic compound that is bound to an. Cofactors are required for the initiation of catalysis.

comparative genomic hybridization (CGH):

competence:

complementarity: A property of biopolymers whereby two polymeric chains or "" aligned to each other will tend to form  consisting of hydrogen bonds between the individual  comprising each chain, with each type of nucleobase pairing almost exclusively with one other type of nucleobase; e.g. in   molecules,  pairs only with  and  pairs only with. Strands that are paired in such a way, and the bases themselves, are said to be complementary. The degree of complementarity between two strands strongly influences the stability of the molecule; certain sequences may also be internally complementary, which can result in a single strand. Complementarity is fundamental to the mechanisms governing,, and.

complementary DNA (cDNA):

that is synthesized from a single-stranded template (typically or ) in a reaction catalyzed by the enzyme. cDNA is produced both naturally by retroviruses and artificially in certain laboratory techniques, particularly. In, the term may also be used to refer to the sequence of an mRNA transcript expressed as its DNA counterpart (i.e. with  replacing ).

complementation:

compound X: See.

conditional expression: The controlled, inducible of a, either ' or '.

confluence:

In, a measure of the proportion of the surface area of a that is covered by adherent cells, commonly expressed as a percentage. A culture in which the entire surface is completely covered by a continuous, such that all cells are immediately adjacent to and in direct physical contact with other cells, with no gaps or voids, is said to be 100-percent confluent. Different cell lines exhibit differences in growth rate or depending on the degree of confluence. Because of, most show a significant reduction in the rate of as they approach complete confluence, though some  may continue to divide, expanding vertically rather than horizontally by stacking themselves on top of the , until all available nutrients are depleted.

conformation: The three-dimensional spatial configuration of the atoms comprising a molecule or structure. The conformation of a is the physical shape into which its  chains arrange themselves during, which is not necessarily rigid and may  with the protein's particular chemical environment.

conformational change: A change in the spatial or physical shape of a molecule or macromolecule such as a protein or nucleic acid, rarely spontaneously but more commonly as a result of some alteration in the molecule's chemical environment (e.g. temperature, pH, salt concentration, etc.) or an interaction with another molecule. Changes in the of proteins can affect whether or how strongly they bind  or ; inducing these changes is a common means (both naturally and artificially) of activating, inactivating, or otherwise controlling the function of many enzymes and receptor proteins.

congression: The movement of chromosomes to the during the and  stages of.

consensus sequence:

A calculated order of the most frequent (of either or ) found at each position in a common  and obtained by comparing multiple closely related sequence alignments.

conservative replication: A hypothetical mode of in which the two parental of the original  molecule ultimately remain complemented to each other at the end of the replication process, and the two daughter strands end up forming their own separate molecule; hence one molecule is composed of both of the starting strands while the other is composed entirely of newly synthesized strands. This is in contrast to, in which each molecule is a hybrid of one old and one new strand.See also.

conserved sequence: A or sequence that is highly similar or identical across many species or within a, indicating that it has remained relatively unchanged through a long period of evolutionary time.

conspecific: Belonging to the same species.

constitutive expression: The continuous of a, as opposed to, in which a gene is only transcribed as needed. A gene that is transcribed continuously is called a constitutive gene. A gene whose expression depends only on the efficiency of its in binding, and not on any or other  which might  or  its transcription.

contact inhibition:

In, the phenomenon by which most normal eukaryotic cells cease to grow and upon reaching a critical cell density, usually as they approach full  or come into physical contact with other cells. As a result, many types of cells cultured on plates or in will continue to proliferate until they cover the whole surface of the culture vessel, at which point the rate of cell division abruptly decreases or is arrested entirely, thus forming a confluent  with minimal overlap between neighboring cells, even if the nutrient medium remains plentiful, rather than stacking themselves on top of each other. or cells tend not to respond to cell density in the same way and may continue to proliferate at high densities. This type of density-dependent inhibition of growth is similar to and may occur simultaneously with, but is nonetheless distinct from, the related phenomenon of contact inhibition of movement, whereby moving cells respond to physical contact by temporarily stopping and then reversing their direction of locomotion away from the point of contact.

contig: A continuous sequence of generated by assembling cloned fragments by means of their overlapping sequences.

cooperativity:

copy DNA (cDNA): See.

copy error: A resulting from a mistake made during.

copy-number variation (CNV): A phenomenon in which sections of a are repeated and the number of varies between individuals in the population, usually as a result of  or  events that affect entire genes or sections of chromosomes. Copy-number variations play an important role in generating within a population.

coregulator: A protein that works together with one or more to regulate.

corepressor: A type of that reduces (represses) the of one or more genes by binding to and activating a.

cosmid:

cpDNA: See.

CpG island:

A region of a in which occur  or with high frequency.

CpG site:

A sequence of DNA in which a nucleotide is immediately followed by a nucleotide on the same  in the 5'-to-3' ; the "p" in CpG refers simply to the intervening  group linking the two consecutive nucleotides.

CRISPR gene editing:

crista:

Any of numerous folds or in the inner membrane, which give this membrane its characteristic wrinkled shape and increase the surface area across which  and supporting  reactions can occur. Cristae are studded with proteins such as ATP synthase and various cytochromes.

crossing over: See.

crosslink: An abnormal chemical bond between two or more on opposite of a  molecule (interstrand), or between bases on the same strand (intrastrand), specifically via the formation of covalent bonds that are stronger than the hydrogen bonds of. Crosslinks can be generated by a variety of exogenous and endogenous agents, and tend to interfere with normal cellular processes such as and. They are common targets of pathways.

cryptic unstable transcript (CUT):

ctDNA: An abbreviation of. An abbreviation of.

C-terminus:

The end of a linear chain of (i.e. a ) that is terminated by the free carboxyl group of the last amino acid to be added to the chain during. This amino acid is said to be C-terminal. By convention, sequences, domains, active sites, or any other structure positioned nearer to the C-terminus of the or the folded  it forms relative to others are described as. Contrast.

cut:

C-value: The total amount of contained within a  (e.g. a ) of a particular organism or species, expressed in number of  or in units of mass (typically picograms); or, equivalently, one-half the amount in a. For simple diploid eukaryotes the term is often used interchangeably with, but in certain cases, e.g. in hybrid descended from parents of different species, the C-value may actually represent two or more distinct  contained within the same nucleus. C-values apply only to, and notably exclude.

C-value enigma:

A term used to describe a diverse variety of questions regarding the immense variation in nuclear or among eukaryotic species, in particular the observation that genome size does not correlate with the perceived complexity of organisms, nor necessarily with the number of  they possess; for example, many single-celled protists have genomes containing thousands of times more DNA than the human genome. This was considered paradoxical until the discovery that eukaryotic genomes consist mostly of, which lacks genes entirely. The focus of the enigma has since shifted to understanding why and how genomes came to be filled with so much non-coding DNA, and why some genomes have a higher gene content than others.

cyclic adenosine monophosphate (cAMP):

cyclosis: See.

cytidine: One of the four standard used in molecules, consisting of a   with its N9 nitrogen  to the C1 carbon of a  sugar. Cytosine bonded to is known as, which is the version used in.

cytochemistry: The branch of cell biology involving the detection and identification of the various structures and components within cells by means of biochemical analysis and visualization, in particular the localization of cellular constituents by using techniques such as chemical and, spectrophotometry and spectroscopy, radioautography, and electron microscopy.

cytogenetics: The branch of that studies how influence and relate to cell behavior and function, particularly during  and.

cytokine:

cytokinesis: The final stage of in both and, usually immediately following the division of the, during which the  of the parent cell is  and divided approximately evenly between two. In animal cells, this process occurs by the closing of a microfilament in the equatorial region of the dividing cell. Contrast.

cytology: The study of the morphology, processes, and life history of living, particularly by means of light and electron microscopy. The term is also sometimes used as a synonym for the broader field of.

cytolysis: See.

cytometer:

cytomics: The interdisciplinary field that studies,, and at the level of an individual cell by making use of single-cell molecular techniques and advanced microscopy to visualize the interactions of cellular components.

cytoplasm:

cytoplasmic streaming:

The flow of the inside a cell, driven by forces exerted upon cytoplasmic fluids by the. This flow functions partly to speed up the transport of molecules and suspended in the cytoplasm to different parts of the cell, which would otherwise have to rely on passive diffusion for movement. It is most commonly observed in very large eukaryotic cells, for which there is a greater need for transport efficiency.

cytoplast: An eukaryotic cell; or all other cellular components besides the nucleus (i.e. the cell membrane, cytoplasm, organelles, etc.) considered collectively. The term is most often used in the context of experiments, during which the cytoplast can sometimes remain viable in the absence of a nucleus for up to 48 hours.

cytosine: A used as one of the four standard nucleobases in both  and  molecules. Cytosine forms a with.

cytosol:

The soluble aqueous phase of the, in which small particles such as, , , and many other molecules are suspended or dissolved, excluding larger structures and such as , , , and the.

D
daughter cell: A resulting from the of an initial progenitor, known as the. Generally two daughter cells are produced per division.

de novo mutation: A spontaneous in the genome of an individual organism that is new to that organism's lineage, having first appeared in a of one of the organism's parents or in the fertilized egg that develops into the organism; i.e. a mutation that was not present in either parent's genome.

de novo synthesis: The assembly of a synthetic from free without relying on an existing, i.e. de novo, by any of a variety of laboratory methods. De novo synthesis makes it theoretically possible to construct completely with no naturally occurring equivalent, and no restrictions on size or sequence. It is performed routinely in the commercial production of customized, made-to-order sequences such as.

deacetylation:

decellularization:

degeneracy: The redundancy of the, exhibited as the multiplicity of different that specify the same. For example, in the, the amino acid serine is specified by six unique codons (UCA, UCG, UCC, UCU, AGU, and AGC). Codon degeneracy accounts for the existence of.

degranulation:

deletion:

A type of in which one or more are removed from a.

demethylation:

denaturation: The process by which or lose their,, and/or , either reversibly or irreversibly, through the application of some external chemical or mechanical stress, e.g. by heating, agitation, or exposure to a strong acid or base, all of which can disrupt intermolecular forces such as hydrogen bonding and thereby change or destroy chemical activity. Denatured proteins may be both a cause and a consequence of cell death. Denaturation may also be a normal process; the denaturation of molecules, for example, which breaks the hydrogen bonds between  and causes the separation of the duplex molecule into two, is a necessary step in  and  and hence is routinely performed by enzymes such as. The same mechanism is also fundamental to laboratory methods such as.

deoxyadenosine:

One of the four standard used in molecules, consisting of an   with its N9 nitrogen  to the C1 carbon of a  sugar. Adenine bonded to forms an alternate compound known simply as, which is used in.

deoxycytidine:

One of the four standard used in molecules, consisting of a   with its N9 nitrogen  to the C1 carbon of a  sugar. Cytosine bonded to forms an alternate compound known simply as, which is used in.

deoxyguanosine:

One of the four standard used in molecules, consisting of a   with its N9 nitrogen  to the C1 carbon of a  sugar. Guanine bonded to forms an alternate compound known simply as, which is used in.

deoxyribonuclease (DNase): Any of a class of enzymes which catalyze the hydrolytic cleavage of in  molecules, thereby severing  of  and causing the degradation of DNA polymers into smaller components. Compare.

deoxyribonucleic acid (DNA): A molecule composed of a series of covalently linked, each of which incorporates one of four :  (A),  (G),  (C), and  (T). DNA is most often found in form, which consists of two,  nucleotide chains in which each of the nucleobases on each individual  is  via hydrogen bonding with one on the opposite strand; this structure commonly assumes the shape of a. DNA can also exist in form. By storing and encoding genetic information in the of these nucleobases, DNA serves as the universal molecular basis of biological inheritance and the fundamental template from which all proteins, cells, and living organisms are constructed.

deoxyribonucleotide: A containing as its  sugar component, and the  or subunit used to build  (DNA) molecules. Deoxyribonucleotides canonically incorporate any of four : (A),  (G),  (C), and  (T). Compare.

deoxyribose:

A sugar derived from  by the replacement of the hydroxyl group attached to the C2 carbon with a single hydrogen atom. D-deoxyribose, in its cyclic ring form, is one of three main functional groups of and hence of  (DNA) molecules.

deoxythymidine: See.

dephosphorylation:

derivatization: The artificial modification of a molecule or protein with the intent of altering its solubility or other chemical properties so as to enable analysis (e.g. by mass spectroscopy or chromatography), or of it by attaching a detectable chemical moiety (e.g. a fluorescent tag) to make it easier to identify and track. Molecules modified in this way are described as derivatives of their naturally occurring counterparts and are said to have been derivatized.

desmosome: A specialized cell junction between neighboring epithelial cells in which the cells are held together by a network of keratin filaments and structural proteins bridging the gap between the plasma membranes.

destination vector:

desynapsis: The failure of that have normally during  to remain paired during. Desynapsis is usually caused by the improper formation of. Contrast.

developmental biology:

diakinesis: In, the fifth and final substage of, following and preceding. During diakinesis, the chromosomes are further condensed, the two reach opposite poles of the cell, and the  begins to extend from the poles to the equator.

dicentric: (of a linear or chromosome fragment) Having two instead of the normal one.

differentiation:

dimer: A molecular aggregate consisting of two. The term is often used to describe a composed of two proteins, either the same protein (a ) or different proteins (a ); or to an individual protein composed of two. Compare ', ', and.

dinucleotide: A molecular consisting of exactly two covalently linked ; or any two nucleotides which are immediately adjacent to each other on the same of a longer  polymer.

diploid:

(of a cell or organism) Having two copies of each. Contrast ' and '.

diplonema:

In, the fourth of the five substages of, following and preceding. During diplonema, the disassembles and the paired  begin to separate from one another, though they remain tightly bound at the  where  has occurred.

direct repeat: Any two or more of a specific occurring in the same orientation (i.e. in precisely the same order and not ) and on the same, either separated by intervening nucleotides or not. An example is the sequence TACCGnnnnnnTACCG, in which TACCG occurs twice, though separated by six nucleotides that are not part of the repeated sequence. A direct repeat in which the repeats are immediately adjacent to each other is known as a.

directionality: The end-to-end chemical orientation of a single linear or of a  polymer or a. The nomenclature used to indicate nucleic acid directionality is based on the chemical convention of identifying individual carbon atoms in the or  sugars of nucleotides, specifically the  and  of the  ring. The sequence of nucleotides in a polymeric chain may be read or interpreted in the 5'-to-3' direction – i.e. starting from the terminal nucleotide in which the 5' carbon is not connected to another nucleotide, and proceeding to the other terminal nucleotide, in which the 3' carbon is not connected to another nucleotide – or in the opposite 3'-to-5' direction. Most types of nucleic acid synthesis, including and, work exclusively in the 5'-to-3' direction, because the  involved can only catalyze the addition of free nucleotides to the open 3'-end of the previous nucleotide in the chain. Because of this, the convention when writing any nucleic acid sequence is to present it in the 5'-to-3' direction from left to right. In nucleic acids, the two paired strands must be  in order to  with each other. Polypeptide directionality is similarly based on labeling the functional groups comprising, specifically the amino group, which forms the , and the carboxyl group, which forms the ; amino acid sequences are assembled in the N-to-C direction during , and by convention are written in the same direction.

disaccharide: A composed of two (either the same or different) joined by a covalent. See also ' and '.

dispersive replication:

dissimilatory process: Any exergonic process of microbial by which redox-active chemical species participate in oxidation-reduction reactions (exchange of electrons) to provide the cell with energy needed for sustaining activities. External substances are absorbed by the cell from its environment and then decomposed to release energy, with the subsequently  out of the cell. This is in contrast to an, in which the atoms of the external substances are reused in the synthesis of biomolecules or the fabrication of cellular components.

distance measure: Any quantity used to measure the dissimilarity between the levels of different.

DNA: See.

DNA barcoding: A method of taxonomic identification in which short DNA sequences from one or more specific genes are isolated from unidentified samples and then with sequences from a in order to uniquely identify the species or other taxon from which the samples originated. The sequences used in the comparison are chosen carefully from genes that are both widely and that show greater  between species than within species, e.g. the cytochrome c oxidase gene for eukaryotes or certain  genes for prokaryotes. These genes are present in nearly all living organisms but tend to evolve different mutations in different species, such that a unique sequence variant can be linked to one particular species, effectively creating a unique identifier akin to a retail barcode. DNA barcoding allows unknown specimens to be identified from otherwise indistinct tissues or body parts, where identification by morphology would be difficult or impossible, and the library of organismal barcodes is now comprehensive enough that even organisms previously unknown to science can often be classified with confidence. The simultaneous identification of multiple different species from a mixed sample is known as metabarcoding.

DNA condensation: The process of compacting very long molecules into densely packed, orderly configurations such as, either ' or '.

DNA fingerprinting:

DNA methylation:

DNA microarray: A technology used to measure levels of  transcripts or to detect certain changes in. It consists of an array of thousands of microscopic spots of, called features, each containing picomoles of a specific DNA sequence. This can be a short section of a or any other DNA element, and is used as a  to hybridize a, cRNA or  sample (called a target) under  conditions. Probe-target is usually detected and quantified by fluorescence-based detection of  targets.

DNA polymerase: Any of a class of which synthesize molecules from individual. DNA polymerases are essential for and usually work in pairs to create identical copies of the two  of an original double-stranded molecule. They build long chains of DNA by adding nucleotides one at a time to the of a DNA strand, usually relying on the  provided by the  strand to copy the nucleotide sequence faithfully.

DNA repair: The set of processes by which a cell identifies and corrects structural damage or in the molecules that encode its. The ability of a cell to repair its DNA is vital to the integrity of the genome and the normal functionality of the organism.

DNA replication: The process by which a molecule copies itself, producing two identical copies of one original DNA molecule.

DNA sequencing: The process of determining, by any of a variety of different methods and technologies, the order of the in the long chain of nucleotides that constitutes a of.

DNA turnover: Any mechanism by which are exchanged non-reciprocally (e.g. via,, or ) that causes continual fluctuations in the  of DNA  during an organism's lifetime. Such mechanisms are often major drivers of speciation between populations.

DNA-binding domain (DBD): A protein containing at least one structural motif capable of recognizing and interacting with the of a  or   molecule. DNA-binding domains may bind to specific sequences or have a non-specific affinity for DNA. They are the primary functional components of, including many and regulatory proteins.

DNA-binding protein (DBP): Any or containing one or more  capable of interacting chemically with one or more parts of a  molecule, and consequently having a specific or general affinity for  and/or. DNA-binding activity often depends on the presence and physical accessibility of a specific nucleobase sequence, and mostly occurs at the, since it exposes more of the functional groups which uniquely identify the bases. Binding is also influenced by the spatial conformation of the DNA chain and the occupancy of other proteins near the binding site; many proteins cannot bind to DNA without first undergoing induced by interactions with other molecules.

DNase: See.

domain: A discrete, usually continuous region of a, or the corresponding sequence of a, which serves a particular function or is defined by particular physico-chemical properties (e.g. , polar, non-polar, , etc.), and especially one which assumes a unique, recognizable spatial as part of the protein's  and which contributes to or defines its biological activity. Large proteins are generally composed of several domains linked together by short, intervening polypeptide sequences. Domains are commonly grouped into classes with similar properties or functions, e.g. . More broadly, the term may also be used to refer to a discrete structural entity within any biomolecule, including functionally or compositionally distinct subregions of and.

donor vector:

dosage compensation: Any mechanism by which organisms neutralize the large difference in caused by the presence of differing numbers of in the different sexes, thereby equalizing the  of sex-linked genes so that the members of each sex receive the same or similar amounts of the  of such genes. An example is in female mammals.

double helix: The shape most commonly assumed by molecules, resembling a ladder that has been twisted upon its long axis, with the rungs of the ladder consisting of. This is the most energetically stable conformation of the double-stranded forms of both  and  under most naturally occurring conditions, arising as a consequence of the  of the  and the stacking of the  bonded to it. In, the most common DNA variant found in nature, the double helix has a right-handed twist with about 10 base pairs per full turn, and the molecular geometry results in an alternating pattern of "grooves" of differing widths (a and a ) between the parallel backbones.

double-strand break (DSB): The loss of continuity of the in both strands of a molecule, in particular when the two breaks occur at sites that are directly across from or very close to each other on the complementary strands. Contrast.

double-stranded: Composed of two, molecules or  (either, , or a ) which are held together by hydrogen bonds between the complementary  of each strand, known as. Compare.

double-stranded DNA (dsDNA): Any molecule that is composed of two,  polymers, known as, which are bonded together by hydrogen bonds between the complementary. Though it is possible for DNA to exist as a, it is generally more stable and more common in double-stranded form. In most cases, the complementary causes the twin strands to coil around each other in the shape of a.

double-stranded RNA (dsRNA): Any molecule that is composed of two,  polymers, known as, which are bonded together by hydrogen bonds between the complementary. Though RNA usually occurs in, it is also capable of forming duplexes in the ; an example is an transcript pairing with an  of the same transcript, which effectively  the gene from which the mRNA was transcribed by preventing translation. As in dsDNA, the in dsRNA usually causes the twin strands to coil around each other in the shape of a.

downregulation:

Any process, natural or artificial, which decreases the level of of a certain. A gene which is observed to be expressed at relatively low levels (such as by detecting lower levels of its transcripts) in one sample compared to another sample is said to be downregulated. Contrast.

downstream: Towards or closer to the of a chain of nucleotides, or to the of a  chain. Contrast.

dsDNA: See.

dsRNA: See.

duplex: See.

duplication: The production of a second copy of part or all of a or, either naturally or artificially, and the retention of both copies; especially when both the copy and the original sequence are retained ' within the same molecule, often but not necessarily to each other. See also ', ', and '.

dyad: See.

dysplasia: The abnormal growth or development of a or organ; a change in the growth, behavior, or organization of cells within a tissue, or the presence of cells of an abnormal type, such that the tissue becomes disordered, an event which often precedes the development of cancer.

E
eat-me signal: A molecule exposed on the surface of a cell which effectively tags the cell for, inducing to engulf or "eat" it. The presence of oxidized or phosphatidylserine, or the absence of sialic acid from cell surface  or, are all commonly used as eat-me signals in certain cell types. See also.

electron transport chain:

electroporation:

A molecular biology technique in which a strong electric field is applied to living cells in order to temporarily increase the permeability of their cell membranes, allowing exogenous nucleic acids, proteins, or chemical compounds to easily pass through the membrane and thereby enter the cells. It is a common method of achieving and.

elongation:

The linear growth of a polymer by the sequential addition of individual monomers to a, e.g. during  or , especially when it occurs by  with a. The term is often used to describe steps in certain laboratory techniques such as the.

embryo:

emergenesis: The quality of genetic that results from a specific configuration of interacting, rather than simply their combination.

endocytosis: Any process by which a substance is uptaken by or brought inside of a, crossing the from an  into an, which includes the subclasses of , , and  processes. All of these involve surrounding an extracellular molecule, protein, or even another cell or organism with an extension or of the cell membrane, which then "buds off" or separates from the rest of the membrane on the cytoplasmic side, forming a membrane-enclosed  containing the ingested materials. By this mechanism the material can cross the without being exposed to the hydrophobic space in between, instead remaining suspended in the fluid of the extracellular space. Many large, polar macromolecules which cannot simply diffuse across the membrane, such as and, are transported into the cell by endocytosis. It is distinguished from alternative routes such as passing through or being chaperoned by. The reverse process is called.

endomembrane: Any surrounding an  or, e.g. that of the, , , ,  (the ), etc.

endonuclease: Any whose activity is to cleave within a chain of, including those that cleave relatively nonspecifically (without regard to ) and those that cleave only at very specific sequences (so-called ). When recognition of a specific sequence is required, endonucleases make their cuts in the middle of the sequence. Contrast.

endoplasmic reticulum (ER):

endosome:

enhancer: A region of DNA near a that can be bound by an to increase  or by a  to decrease expression.

enhancer RNA (eRNA): A subclass of transcribed from regions of DNA containing sequences. The expression of a given eRNA generally correlates with the activity of the corresponding enhancer in enhancing transcription of its target genes, suggesting that eRNAs play an active role in gene regulation or.

enucleate: To artificially remove the from a, e.g. by micromanipulation in the laboratory or by destroying it through irradiation with ultraviolet light, rendering the cell.

envagination:

enzyme: A which acts as a for a biological process by accelerating a specific chemical reaction, typically by binding one or more  molecules and decreasing the activation energy necessary for the initiation of a particular reaction involving the substrate(s). Enzymatic catalysis often results in the chemical conversion of the substrate(s) into one or more products, which then inhibit or permit subsequent reactions. All consist of a series of individual reactions which each depend upon one or more specific enzymes to drive them forward at rates fast enough to sustain life.

epigenetics:

epigenome:

episome: Another name for a, especially one that is capable of integrating into a. In eukaryotes, any non-integrated circular molecule that is stably maintained and replicated in the  simultaneously with the rest of the host cell. Such molecules may include viral genomes, bacterial plasmids, and aberrant chromosomal fragments.

epistasis: The collective action of multiple genes interacting during. A form of gene action, epistasis can be either additive or multiplicative in its effects on specific.

epitope:

The specific site or region within an such as a  or  which is recognized by B or T cells of the immune system, against which a specific  is produced, and with which the antibody's  specifically interacts or binds. In proteins, epitopes are typically of 4–5 amino acid residues, sequential or discontiguous, which by virtue of the distinct spatial  they adopt upon  are able to uniquely interact with a particular paratope. In this sense they may be considered, though they do not necessarily overlap with ligand binding sites and need not be in any way relevant to the protein's normal function. Very large molecules may have multiple epitopes, each of which is recognized by a different antibody.

ergosome: See.

euchromatin:

A relatively open, lightly compacted form of in which is only sporadically bound in  and thus broadly accessible to binding and manipulation by  and other molecules. Euchromatic regions of a genome are often enriched in and actively undergoing, in contrast to, which is relatively gene-poor, nucleosome-rich, and less accessible to transcription machinery.

euploidy: The condition of a cell or organism having an abnormal number of complete sets of, possibly excluding the. Euploidy differs from, in which a cell or organism has an abnormal number of one or more specific individual chromosomes.

evolution: The change in the characteristics of biological populations over successive generations. In the most traditional sense, it occurs by changes in the frequencies of in a population's.

ex vivo: Occurring outside of a cell or organism, as with observations made or experiments performed in or on cells or which have been isolated or removed from their natural context to an external environment (usually a carefully controlled environment with minimal alteration of natural conditions, such as a being grown in a laboratory). This is in contrast to observations, which are made in an entirely natural context.

excision: The enzymatic removal of a polynucleotide sequence from one or more strands of a, or of a polypeptide sequence from a, typically implying both the breaking of the polymeric molecule in two locations and the subsequent rejoining of the two breakpoints after the sequence between them has been removed. The term may be used to describe a wide variety of processes performed by distinct enzymes, including most and pathways.

exocytosis: Any process by which a substance is secreted from or transported out of a, crossing the from the  into the, especially that which occurs by the fusion of the membrane surrounding a secretory  with the larger cell membrane. This fusion causes the intra-vesicular space to merge with the extracellular fluid, releasing the vesicle's contents on the exterior side of the cell without exposing them to the hydrophobic space between the. More narrowly the term may refer in particular to the bulk transport of a large amount of molecules out of the cell all at once, often or  which are too large and polar to passively diffuse across the membrane themselves. The reverse process, whereby materials are invaginated into the cell, is known as.

exome: The entire set of within a particular, including of mature mRNAs as well as.

exon: Any part of a that encodes a part of the final mature produced by that gene after  have been removed by. The term refers to both the sequence as it exists within a DNA molecule and to the corresponding sequence in RNA transcripts.

exon skipping:

exonuclease: Any whose activity is to cleave within a chain of, including those that cleave only upon recognition of a specific sequence (so-called ). Exonucleases make their cuts at either the or  of the sequence (rather than in the middle, as with ).

exosome: (protein complex) An intracellular multi-protein complex which serves the function of degrading various types of molecules. (vesicle) A type of membrane-bound produced in many eukaryotic cells by the inward budding of an and the subsequent fusion of the endosome with the, causing the release of the vesicle into various extracellular spaces, including biological fluids such as blood and saliva, where they may serve any of a wide variety of physiological functions, from waste management to intercellular signaling.

exosome complex: An intracellular multi-protein complex which serves the function of degrading various types of molecules.

expression vector:

A type of, usually a or viral vector, designed specifically for the of a   in a target cell, rather than for some other purpose such as.

extein: Any part of an which is retained within a precursor, i.e. not excised by, and is therefore present in the mature , analogous to the  of RNA transcripts. Contrast.

extension: See.

extracellular: Outside the of a or cells; i.e. located or occurring externally to a cell. Contrast '; see also '.

extracellular matrix (ECM):

The network of interacting and minerals secreted by and existing outside of and between cells in structures such as  and, forming a hydrated, mesh-like, semi-solid suspension which not only holds the cells together in an organized fashion but also provides structural and biochemical support, acting as an elastic, compressible buffer against external stresses as well as both regulating and influencing numerous aspects of cell behavior, among them, , , , and. The composition and properties of the ECM vary enormously between organisms and tissue types, but generally it takes the form of a gel in which various fibrous proteins (especially collagen and elastin), enzymes, and  are embedded. Cells themselves both produce the matrix components and respond constantly to local matrix composition, a source of environmental feedback which is critical for, tissue organization, and development.

extrachromosomal DNA:

Any that is not found in or in the  of a cell and hence is not. This may include the DNA contained in or  such as  or, or, in the broadest sense, DNA introduced by viral infection. Extrachromosomal DNA usually shows significant structural differences from nuclear DNA in the same organism.

F
facultative expression: The of a only as needed, as opposed to, in which a gene is transcribed continuously. A gene that is transcribed as needed is called a facultative gene.

fatty acid:

fermentation:

filopodium:

find-me signal: A molecule exposed on the surface of a cell destined for which is used to attract to engulf and eliminate the cell by. See also.

five-prime cap: See.

five-prime end: See.

five-prime untranslated region: See.

fixation: (histology) The preservation of biological material by treating it with a chemical that prevents or delays the natural postmortem processes of decay (e.g. and putrefaction) which would otherwise eventually cause cells, tissues, and biomolecules to lose their characteristic structures and properties. Biological specimens are usually fixed with the broad objective of arresting or slowing biochemical reactions for long enough to study them in detail, essentially 'freezing' cellular processes in their natural state at a specific point in time, while minimizing disruption to existing structures and arrangements, all of which can improve subsequent and microscopy of the fixed samples. Though fixation tends to irreversibly terminate any ongoing reactions, thus killing the fixed cells, it makes it possible to obtain information about molecular details that occur too rapidly or transiently to study in living samples. Common fixatives such as formaldehyde work by disabling enzymes, coagulating and  macromolecules, creating  between them, and protecting specimens from decomposition by bacteria and fungi. (population genetics) The process by which a single for a particular with multiple different alleles increases in  in a given population such that it becomes permanently established as the only allele at that  within the population's.

fixative:

flagellate: (of a cell) Having one or more.

flagellum:

fluorescence in situ hybridization (FISH):

forward genetics: An experimental approach in in which a researcher starts with a specific known and attempts to determine the genetic basis of that phenotype by any of a variety of laboratory techniques, commonly by  random  in the organism's genome and then  for changes in the phenotype of interest. Observed phenotypic changes are assumed to have resulted from the mutation(s) present in the screened sample, which can then be to specific genomic  and ultimately to one or more specific. This methodology contrasts with, in which a specific gene or its gene product is individually manipulated in order to identify the gene's function.

forward mutation:

frameshift mutation: A type of in a caused by the  or  of a number of  that is not divisible by three. Because of the triplet nature by which nucleotides code for amino acids, a mutation of this sort causes a shift in the of the nucleotide sequence, resulting in the sequence of  downstream of the mutation site being completely different from the original.

freeze-drying: See.

Functional Genomics Data (FGED) Society:

An organization that works with others "to develop standards for biological research data quality, annotation and exchange" as well as software tools that facilitate their use.

G
G banding:

A technique used in to produce a visible by staining the condensed chromosomes with Giemsa stain. The staining produces consistent and identifiable patterns of dark and light "bands" in regions of, which allows specific chromosomes to be easily distinguished.

G1:

G2:

gamete: A cell that is the product of a progenitor  and the final product of the  in sexually reproducing multicellular organisms. Gametes are the means by which an organism passes its genetic information to its offspring; during fertilization, two gametes (one from each parent) are fused into a single.

gametogenesis:

GC content: See.

gDNA: See.

gene: Any segment or set of segments of a molecule that contains the information necessary to produce a functional transcript in a controlled manner. In living organisms, genes are often considered the fundamental units of and are typically encoded in. A particular gene can have multiple different versions, or, and a single gene can result in a that influences many different.

gene cassette: See.

gene dosage: The number of copies of a particular present in a. Gene dosage directly influences the amount of a cell is able to express, though a variety of controls have evolved which tightly. Changes in gene dosage caused by mutations include.

gene duplication:

A type of defined as any of a region of  that contains a. Compare.

gene expression: The set of processes by which the information encoded in a is used in the synthesis of a, such as a protein or, or otherwise made available to influence one or more. Canonically, the first step is, which produces a molecule complementary to the  molecule in which the gene is encoded; for protein-coding genes, the second step is , in which the messenger RNA is read by the  to produce a. The information contained within a DNA sequence need not necessarily be transcribed and translated to exert an influence on molecular events, however; broader definitions encompass a huge variety of other ways in which genetic information can be expressed.

Gene Expression Omnibus (GEO): A database of functional genomics and data derived from experimental chips and  and managed by the National Center for Biotechnology Information.

gene fusion: The union, either by natural mutation or by laboratory techniques, of two or more previously independent genes that code for different gene products such that they become subject to control by the same systems. The resulting hybrid sequence is known as a.

gene mapping: Any of a variety of methods used to precisely identify the of a particular within a DNA molecule (such as a chromosome) and/or the physical or  distances between it and other genes.

gene of interest (GOI): A being studied in a scientific experiment, especially one that is the focus of a technique such as.

gene product: Any of the biochemical material resulting from the of a, most commonly interpreted as the functional produced by  of the gene or the fully constructed  produced by  of the transcript, though  molecules such as  may also be considered gene products. A measurement of the quantity of a given gene product that is detectable in a cell or tissue is sometimes used to infer how active the corresponding gene is.

gene regulation: The broad range of mechanisms used by cells to control the activity of their genes, especially to allow, prohibit, increase, or decrease the production or of specific, such as or. Gene regulation increases an organism's versatility and adaptability by allowing its cells to express different gene products when required by changes in its environment. In multicellular organisms, the regulation of gene expression also drives cellular differentiation and morphogenesis in the embryo, enabling the creation of a diverse array of cell types from the same.

gene silencing: Any mechanism of which drastically reduces or completely prevents the of a particular gene. Gene silencing may occur naturally during either or. Laboratory techniques often exploit natural silencing mechanisms to achieve.

gene therapy: The insertion of a functional or gene or part of a gene into an organism (especially a patient) with the intention of correcting a, either by direct substitution of the defective gene or by supplementation with a second, functional version.

gene trapping: A technology used to simultaneously inactivate, identify, and report the of a target gene in a mammalian genome by introducing an insertional  consisting of a   gene and/or a selectable  flanked by an upstream  site and a downstream  termination sequence.

generation: In any given organism, a single reproductive cycle, or the phase between two consecutive reproductive events, i.e. between an individual organism's reproduction and that of the progeny of that reproduction; or the actual or average length of time required to complete a single reproductive cycle, either for a particular or for a population or species as a whole. In a given population, those individuals (often but not necessarily living contemporaneously) who are equally removed from a given by virtue of the same number of reproductive events having occurred between them and the ancestor.

genetic background:

genetic code: A set of rules by which information encoded within is into  by living cells. These rules define how sequences of triplets called  specify which  will be added next during. The vast majority of living organisms use the same genetic code (sometimes referred to as the ) but variant codes do exist.

genetic disorder: Any illness, disease, or other health problem directly caused by one or more abnormalities in an organism's which are congenital (present at birth) and not acquired later in life. Causes may include a to one or more, or a  such as an  of a particular chromosome. The mutation responsible during embryonic development or may be  from one or both parents, in which case the genetic disorder is also classified as a. Though the abnormality itself is present before birth, the actual disease it causes may not develop until much later in life; some genetic disorders do not necessarily guarantee eventual disease but simply of developing it.

genetic distance: A measure of the genetic divergence between species, populations within a species, or individuals, used especially in to express either the time elapsed since the existence of a or the degree of differentiation in the  comprising the  of each population or individual.

genetic engineering:

The direct, deliberate manipulation of an organism's genetic material using any of a variety of biotechnology methods, including the insertion or of, the transfer of genes within and between species, the of existing sequences, and the construction of novel sequences using. Genetic engineering encompasses a broad set of technologies by which the genetic composition of individual cells, tissues, or entire organisms may be altered for various purposes, commonly in order to study the functions and of individual genes, to produce hormones, vaccines, and other drugs, and to create  for use in research and agriculture.

genetic marker: A specific, easily identifiable, and usually highly or other   with a known location on a  that can be used to identify the individual or species possessing it.

genetic recombination: Any reassortment or exchange of genetic material within an individual organism or between individuals of the same or different species, especially that which creates. In the broadest sense, the term encompasses a diverse class of naturally occurring mechanisms by which are copied or physically transferred into different genetic environments, including during  or  or as a normal part of ;  events such as,, or ; or errors in  or cell division. Artificial recombination is central to many techniques which produce.

genetic redundancy: The redundant encoding of two or more distinct that ultimately perform the same biochemical function. in one of these genes may have a smaller effect on fitness than might be expected, since the redundant genes often compensate for any and obviate any.

genetic regulatory network (GRN): A graph that represents the regulatory complexity of. The vertices (nodes) are represented by various regulatory elements and while the edges (links) are represented by their interactions. These network structures also represent functional relationships by approximating the rate at which genes are.

genetic testing:

A broad class of various procedures used to identify features of an individual's particular chromosomes, genes, or proteins in order to determine parentage or ancestry, diagnose vulnerabilities to heritable diseases, or detect alleles associated with increased risks of developing. Genetic testing is widely used in human medicine, agriculture, and biological research.

genetically modified organism (GMO): Any organism whose genetic material has been altered using techniques, particularly in a way that does not occur naturally by mating or by natural.

genetics: The field of biology that studies,, and in living organisms.

genome: The entire complement of genetic material contained within the of an organism, organelle, or virus. The term is also used to refer to the collective set of genetic shared by every member of a population or species, regardless of the different  that may be present at these loci in different individuals.

genome instability:

genome size: The total amount of contained within one copy of a, typically measured by mass (in picograms or daltons) or by the total number of (in  or ). For organisms, genome size is often used interchangeably with.

genome-wide association study (GWAS):

genomic DNA (gDNA):

The contained in, as opposed to the contained in separate structures such as  or organelles such as  or.

genomic imprinting: An phenomenon that causes to be  in a manner dependent upon the particular parent from which the gene was inherited. It occurs when epigenetic marks such as or  are established or "imprinted" in the  of a parent organism and subsequently maintained through cell divisions in the  of the organism's progeny; as a result, a gene in the progeny that was inherited from the father may be expressed differently than another copy of the same gene that was inherited from the mother.

genomic island (GI): A region of a that shows evidence of from another organism. The term is used especially in describing microbial genomes such as those of bacteria, where genomic islands having the same or similar sequences commonly occur in species or strains that are otherwise only distantly related, implying that they were not passed on through vertical descent from a common ancestor but through some form of lateral transfer such as conjugation. These islands often contain functional genes which confer adaptive traits such as antibiotic resistance.

genomics: An interdisciplinary field that studies the structure, function, evolution, mapping, and editing of entire, as opposed to individual.

genotoxicity: The ability of certain chemical agents to cause damage to genetic material within a living cell (e.g. through single- or double-stranded breaks,, or ), which may or may not result in a permanent. Though all are genotoxic, not all genotoxic compounds are mutagenic.

genotype: The entire complement of present in a particular individual's, which gives rise to the individual's.

genotyping: The process of determining differences in the of an individual by examining the in the individual's  using  and comparing them to another individual's sequences or a reference sequence.

germ cell: Any that gives rise to the of a sexually reproducing organism. Germ cells are the vessels for the genetic material which will ultimately be passed on to the organism's descendants and are usually distinguished from, which are entirely separate from the.

germ line: In multicellular organisms, the subpopulation of cells which are capable of passing on their genetic material to the organism's progeny and are therefore (at least theoretically) distinct from, which cannot pass on their genetic material except to their own immediate daughter cells. Cells of the germ line are called. The of germ cells, spanning many generations, that contains the genetic material which has been passed on to an individual from its ancestors.

gigabase (Gb): A unit of length equal to one billion (1) in  molecules or one billion  in duplex molecules such as.

glucogenic amino acid: Any that can be converted into via, as opposed to the, which can be converted into ketone bodies. In humans, 18 of the 20 amino acids are glucogenic; only leucine and lysine are not. Five amino acids (phenylalanine, isoleucine, threonine, tryptophan, and tyrosine) are both glucogenic and ketogenic.

gluconeogenesis (GNG): The chain of reactions that results in the generation of from some non-carbohydrate carbon substrates, including the. It is one of two primary used by most animals to maintain blood sugar levels (the other being ), especially during periods of fasting, starvation, and intense exercise.

glucose:

glycogen:

glycogenolysis:

glycolipid: Any of a subclass of consisting of a central polar molecule (most commonly glycerol or sphingosine) which is covalently attached to one or more or  via, as well as to one or more long, non-polar  chains. Glycolipids are one of three major types of comprising all biological membranes, along with  and.

glycolysis: The in which sugars such as  are broken down into simpler molecules, releasing chemical energy which can then be used for various cellular functions. In a series of ten enzyme-catalyzed reactions, each molecule of glucose is converted into two molecules of, with the free energy liberated in this process simultaneously being used to form high-energy bonds in two molecules of reduced (NADH) and two molecules of  (ATP). In conditions pyruvate and NADH are further oxidized in the ; in  conditions NADH itself subsequently reduces pyruvate to lactate.

glycoprotein: A with one or more molecules, typically short  chains, covalently attached to one or more of its amino acid side chains. Proteins which are exposed on the outer surface of the or are secreted into the extracellular space are commonly  in this way.

glycoside: Any chemical compound in which a molecule is covalently bonded to another molecule containing a hydroxyl group (including other carbohydrates) via one or more. When both molecules are carbohydrates, the glycoside is a or.

glycosidic bond:

glycosylation: The attachment of an (e.g. ) to an asparagine residue within a or  by covalent bonding, a process which takes place in or near the.

Goldberg-Hogness box: See.

Golgi apparatus:

gRNA: See.

guanine: A used as one of the four standard nucleobases in both  and  molecules. Guanine forms a with.

guanine-cytosine content:

The proportion of in a that are either  (G) or  (C), typically expressed as a percentage. DNA and RNA molecules with higher GC-content are generally more thermostable than those with lower GC-content due to molecular interactions that occur during base stacking.

guanosine: One of the four standard used in molecules, consisting of a   with its N9 nitrogen  to the C1 carbon of a  sugar. Guanine bonded to is known as, which is the version used in.

guide RNA (gRNA):

H
hairpin: See.

haploid:

(of a cell or organism) Having one copy of each, with each copy not being part of a pair. Contrast ' and '.

Hayflick limit:

helicase: Any of a class of -dependent that move directionally along the  and catalyze the separation of the two complementary  of  molecules, permitting a wide variety of vital processes to take place, e.g.,, and.

hemizygous: In a organism, having just one at a given  (where there would ordinarily be two). Hemizygosity may be observed when only one copy of a is present in a normally diploid cell or organism, or when a segment of a chromosome containing one copy of an allele is, or when a gene is located on a  in the heterogametic sex (in which the sex chromosomes do not exist in matching pairs); for example, in human males with normal chromosomes, almost all  genes are said to be hemizygous because there is only one  and few of the same genes exist on the.

heredity:

The storage, transfer, and expression of molecular information in biological organisms, as manifested by the passing on of from parents to their, either through sexual or asexual reproduction. Offspring cells or organisms are said to inherit the genetic information of their parents.

heterochromatin:

heterochromosome: See.

heterodimer: A or protein composed of two different  which are  in the quaternary structure of a multimeric complex. Contrast.

heterogeneous nuclear RNA (hnRNA):

heterokaryon: A cell containing with different genotypes, resulting from the  of two or more genetically distinct cells, either naturally (e.g. in certain types of sexual reproduction) or artificially (e.g. in genetic engineering).

heterologous expression: The of a foreign or any other foreign DNA sequence within a host organism which does not naturally contain the same gene. Insertion of foreign into heterologous hosts using   is a common biotechnology method for studying gene structure and function.

heterozygous: In a organism, having two different at a given. In genetics shorthand, heterozygous are represented by a pair of non-matching letters or symbols, often an uppercase letter (indicating a  allele) and a lowercase letter (indicating a  allele), such as "Aa" or "Bb". Contrast.

high-throughput:

histology: The study or analysis of the microscopic anatomy of biological or of within tissues, particularly by making use of specialized techniques to distinguish structures and functions based on visual morphology and differential staining. In practice the term is sometimes used more broadly to include.

histone: Any of a class of highly alkaline responsible for  DNA into structural units called  in eukaryotic cells. Histones are the chief protein components of, where they associate into which act as "spools" around which the linear DNA molecule winds. They play a major role in and.

histone core:

The complex of eight proteins around which double-stranded DNA wraps within a. The canonical histone octamer consists of two each of histones H2A, H2B, H3, and H4, which pair with each other symmetrically to form a ball-shaped cluster around which DNA winds through interactions with the histones' surface, though may replace their analogues in certain contexts.

histone modification:

histone variant:

hnRNA: See.

holocentric: (of a linear or chromosome fragment) Having no single but rather multiple  assembly sites dispersed along the entire length of the chromosome. During cell division, the of holocentric chromosomes move apart in parallel and do not form the classical V-shaped structures typical of  chromosomes.

homeobox: Any of a class of DNA approximately 180 base pairs in length occurring near the of certain eukaryotic genes and encoding a  which is capable of  via a characteristic helix-turn-helix motif in proteins containing it. Homeobox-containing genes are translated into homeodomain-containing proteins, which commonly transcription or translation by binding to other genes or messenger RNAs containing specific. The products of many homeotic genes, exemplified by the, are of critical importance in developmental pathways.

homeobox responsive element (HRE):

Any DNA or RNA that is specifically and bound by the  of a homeodomain-containing protein.

homeodomain: A type of, typically 60 in length, found in certain eukaryotic, characterized by a highly helix-turn-helix  that binds with strong affinity to the backbone of specific  in DNA or RNA molecules. A protein may have one or more homeodomains, generally situated towards the of the polypeptide containing it. Many homeodomain-containing proteins function as by binding to sequences within  and blocking or recruiting other proteins, such as, to the. Homeodomains are the versions of, though the terms are often used interchangeably.

homodimer: A or protein composed of two identical  which are  in the quaternary structure of a multimeric complex. Contrast.

homologous chromosomes:

A set of two matching, one maternal and one paternal, which pair up with each other inside the nucleus during. They have the same at the same, but may have different.

homologous recombination: A type of in which nucleotide sequences are exchanged between two similar or identical ("homologous") molecules of, especially that which occurs between. The term may refer to the recombination that occurs as a part of any of a number of distinct cellular processes, most commonly or  during  in eukaryotes and  in prokaryotes. Contrast.

homozygous: In a organism, having two identical at a given. In genetics shorthand, homozygous are represented by a pair of matching letters or symbols, such as "AA" or "aa". Contrast.

horizontal gene transfer (HGT):

Any process by which genetic material is transferred between unicellular and/or multicellular organisms other than by vertical transmission from parent to offspring, e.g. bacterial conjugation.

housekeeping gene: Any that is at a relatively constant level across many or all known conditions and cell types. The of housekeeping genes typically play critical roles in the maintenance of cellular integrity and basic metabolic function. It is generally assumed that their expression is unaffected by experimental or pathological conditions.

Hox genes: A subset of highly -containing which are essential for the proper organization of the body plan in developing animal embryos, ensuring that the correct structures are formed in the correct places. Hox genes are usually arranged on a chromosome in arrays and are  sequentially during development, with the sequence of gene activation corresponding to their physical arrangement within the genome and/or the physical layout of the tissues in which they are expressed along the organism's anterior–posterior axis.

Human Genome Project (HGP): A collaborative international scientific research project with the goal of all of the and identifying and  all of the  within human cells, and ultimately of assembling a complete  for the human species. The project was launched in 1990 by a consortium of federal agencies, universities, and research institutions and was declared complete in 2003. Because each individual human being has a unique genome, the finished reference genome is a of sequences obtained by sampling DNA from thousands of individuals across the world and does not represent any one individual.

hyaloplasm: See.

hybrid: The that results from combining the qualities of two organisms of different genera, species, breeds, or varieties through sexual reproduction. Hybrids may occur naturally or artificially, as during of domesticated animals and plants. Reproductive barriers typically prevent hybridization between distantly related organisms, or at least ensure that hybrid offspring are sterile, but fertile hybrids may result in speciation.

hybridization: The process by which a organism is produced from two organisms of different genera, species, breeds, or varieties. The process by which two or more molecules with  nucleotide sequences  with each other in solution, creating  or  molecules via the formation of hydrogen bonds between the complementary nucleobases of each strand. In certain laboratory contexts, especially ones in which long strands hybridize with short, hybridization is often referred to as. A step in some experimental assays in which a single-stranded DNA or RNA preparation is added to an array surface and anneals to a.

hybridization probe:

hydrophilic: Soluble in or having an affinity for water or other polar compounds; describing a polar molecule, or a moiety or functional group within a molecule, which participates in intermolecular interactions such as hydrogen bonding with other polar molecules and therefore readily dissolves in polar solvents such as water or aqueous solutions. Unlike compounds, hydrophilic compounds can form energetically favorable contacts with the aqueous phase of biological fluids and so can often be suspended directly in the or exposed to extracellular spaces. Together, the contrasting properties of hydrophilicity and hydrophobicity play major roles in determining the structural and functions of most.

hydrophobic:

Having a low solubility in or affinity for water or other polar solvents; describing a non-polar molecule, or a moiety or functional group within a molecule, which cannot form energetically favorable interactions with polar compounds and which therefore tends to "avoid" or be repulsed by such compounds, instead clustering together with other hydrophobic molecules or arranging itself in a way that minimizes its exposure to its polar surroundings. This phenomenon is not so much due to the affinity of the hydrophobic molecules for each other as it is a consequence of the strong intermolecular forces that allow polar compounds such as water molecules to bond with each other; hydrophobic species are unable to form alternative bonds of equivalent strength with the polar compounds, hence they tend to be excluded from aqueous solutions by the tendency of the polar solvent to maximize interactions with itself. Hydrophobicity is a major determinant of countless chemical interactions in biological systems, including the spatial assumed by such as  and, the binding of  and  to proteins, and the structure and properties of lipid. Contrast.

hypertonic:

hypomorph: A mutant that permits a subnormal expression of the gene's normal phenotype, e.g. by encoding an unstable enzyme which degrades too quickly to fully serve its function but which nevertheless is functional in some limited capacity, being generated in quantities sufficient for its reaction to proceed slowly or at low levels.

hypotonic:

hypoxanthine: A naturally occurring non-canonical that is used in some  molecules and pairs with standard nucleobases in a phenomenon known as. Its form is known as, which is the reason it is commonly abbreviated with the letter I in sequence reads.

I
idiochromosome: See.

idiogram:

A diagrammatic or schematic of the entire set of within a cell or genome, in which annotated illustrations depict each chromosome in its most idealized form (e.g. with straight lines and obvious ) so as to facilitate the easy identification of sequences, structural features, and physical distances, which may be less apparent in photomicrographs of the actual chromosomes.

idiomere: See.

immortalization:

immunogenic: Capable of provoking or inducing an immune response, as with an or a vaccine.

immunohistochemistry (IHC):

immunostaining:

in silico: (of a scientific experiment or research) Conducted, produced, or analyzed by means of computer modeling or simulation, as opposed to a real-world trial.

in situ: (of a scientific experiment or biological process) Occurring or made to occur in a natural, uncontrolled setting, or in the natural or original position or place, as opposed to in a foreign cell or tissue type or.

in situ hybridization:

in vitro: (of a scientific experiment or biological process) Occurring or made to occur in a laboratory vessel or other controlled artificial environment, e.g. in a test tube or a petri dish, as opposed to or.

in vivo: (of a scientific experiment or biological process) Occurring or made to occur inside the cells or tissues of a living organism; or, in the broadest sense, in any natural, unmanipulated setting. Contrast ' and '.

indel: A term referring to either an or a of one or more  in a nucleic acid sequence.

inducer: A protein that binds to a (to disable it) or to an (to enable it).

inducible gene: A whose is either responsive to environmental change or dependent on its host cell's position within the cell cycle.

in-frame: (of a gene or sequence) Read or transcribed in the same as another gene or sequence; not requiring a shift in reading frame to be intelligible or to result in a functional. (of a mutation) Not causing a.

inheritance: See.

initiation codon: See.

inosine:

insertion: A type of in which one or more are added to a. Contrast.

insertion sequence (IS):

Any nucleotide sequence that is naturally or artificially into another sequence. The term is used in particular to refer to the part of a that codes for those proteins directly involved in the transposition process, e.g. the  enzyme. The coding region in a transposable insertion sequence is usually flanked by short, and the structure of larger transposable elements may include a pair of flanking insertion sequences which are themselves inverted.

insertional mutagenesis: The alteration of a DNA sequence by the of one or more nucleotides into the sequence, either naturally or artificially. Depending on the precise location of the insertion within the target sequence, insertions may partially or totally inactivate or even upregulate a or biochemical pathway, or they may be, leading to no substantive changes at all. Many techniques rely on the insertion of exogenous genetic material into host cells in order to study gene function and expression.

insulator: A specific DNA sequence that prevents a gene from being influenced by the or of nearby genes.

integral membrane protein (IMP):

Any of a class of which are permanently embedded within or attached to the (as opposed to those which are ). Integral membrane proteins can be subclassified into, which span the entirety of the membrane, and, which adhere only to one side.

integral monotopic protein: Any of a class of which are permanently attached to one side of the by any means but which do not completely span the membrane. Contrast.

integral polytopic protein:

Any of a class of which span the entirety of the, extending from the interior or side of the membrane to the exterior or  side. Transmembrane proteins typically have hydrophilic exposed to each side as well as one or more hydrophobic domains crossing the nonpolar space inside the, by which they are further classified as single-pass or multipass membrane proteins. As such many transmembrane proteins function as or  to permit or prohibit the movement of specific molecules or ions across the membrane, often undergoing conformational changes in the process, or as  in  pathways. Contrast.

integration:

integron: A consisting of a containing the gene for a, -specific recognition sites, and a  that governs the expression of one or more genes conferring adaptive traits on the host cell. Integrons usually exist in the form of circular DNA fragments, through which they facilitate the rapid adaptation of bacteria by enabling  of antibiotic resistance genes between different bacterial species.

intein: Any sequence of one or more within a precursor that is excised by  during  and is therefore absent from the mature protein, analogous to the  spliced out of RNA transcripts. Contrast.

intercalating agent: Any chemical compound (e.g. ethidium bromide) that disrupts the alignment and in the complementary strands of a molecule by.

intercalation: The insertion, naturally or artificially, of chemical compounds between the planar of a molecule, which generally disrupts the hydrogen bonding necessary for.

intercellular: Between two or more. Contrast '; see also '.

intercistronic region: Any DNA sequence that is located between the of one and the  of the following gene in a  transcription unit. See also.

intercross: A in which both the male and female parents are at a particular.

intergenic region (IGR): Any sequence of that is located between functional.

intergenic spacer (IGS): See.

interkinesis:

The abbreviated pause in activities related to cell division that occurs during in some species, between the first and second meiotic divisions (i.e. meiosis I and meiosis II). No occurs during interkinesis, unlike during the normal  that precedes meiosis I and.

internal ribosome entry site (IRES): A sequence present in some that permits recognition by the and thus the initiation of  even in the absence of a, which in eukaryotes is otherwise required for assembly of the initiation complex. IRES elements are often located in the, but may also be found in other positions.

interphase: All stages of the excluding. A typical cell spends most of its life in interphase, during which it conducts everyday as well as the complete  of its genome in preparation for  or.

intracellular: Within a or cells; i.e. inside the. Contrast ' and '.

intragenic region: See.

intragenic suppression:

intragenomic conflict:

intrinsic membrane protein: See.

intrinsically disordered protein (IDP):

intron:

Any within a functional that is removed by  during  of the   and is therefore absent from the final mature mRNA. The term refers to both the sequence as it exists within a DNA molecule and to the corresponding sequence in RNA transcripts. Contrast.

intron intrusion:

intron-mediated recombination: See.

invagination: The infolding of a toward the interior of a cell or organelle, or of a sheet of cells toward the interior of a developing,, or organ, forming a distinct membrane-lined pocket. In the case of individual cells, the invaginated pocket may proceed to separate from the source membrane entirely, creating a membrane-bound within the cell, as in.

inverted repeat: A followed on the same  by its own. The initial sequence and the reverse complement may be separated by any number of nucleotides, or may be immediately adjacent to each other; in the latter case, the composite sequence is also called a. Inverted repeats are by definition, a property which involves them in many biological functions and dysfunctions. Contrast.

ion channel: A type of complex which forms an spanning the  of a membrane, through which specific inorganic, electrically charged ions can diffuse down their electrochemical gradients.

ionophore: Any chemical compound or macromolecule that facilitates the movement of ions across biological membranes, or more specifically, any chemical species that reversibly binds electrically charged atoms or molecules. Many ionophores are lipid-soluble that catalyze the transport of monovalent and divalent cations across the hydrophobic surrounding cells and vesicles.

isochore: A large region of with a relatively homogeneous composition of, distinguished from other regions by the proportion of pairs that are - or -. The genomes of most plants and vertebrates are composed of different classes of GC-rich and AT-rich isochores.

isochromosome: A type of abnormal in which the arms of the chromosome are mirror images of each other. Isochromosome formation is equivalent to simultaneous and  events such that two copies of either the  or the  comprise the resulting chromosome.

isomerase: Any of a class of which catalyze the conversion of a molecule from one isomer to another, such that the product of the reaction has the same molecular formula as the original substrate but differs in the connectivity or spatial arrangement of its atoms.

isomeric genes: Two or more that are equivalent and redundant in the sense that, despite coding for distinct, they each result in the same when set within the same. If several isomeric genes are present in a single they may be either cumulative or non-cumulative in their contributions to the phenotype.

isotonic:

J
jumping gene: See.

junctional diversity:

junk DNA: Any DNA sequence that appears to have no known biological function, or which acts in a way that has no positive or a net negative effect on the fitness of the in which it is located. The term was once more broadly used to refer to all, though much of this was later discovered to have a function; in modern usage it typically refers to broken or vestigial sequences and, including , , , and fragments of and retroviruses, which together constitute a large proportion of the genomes of most eukaryotes. Despite not contributing productively to the host organism, these sequences are able to persist indefinitely inside genomes because the disadvantages of continuing to copy them are too small to be acted upon by.

junk RNA: Any RNA-encoded sequence, especially a, that appears to have no known biological function, or whose function has no positive or a net negative effect on the fitness of the genome from which it is transcribed. Despite remaining, many still serve important functions, whereas junk RNAs are truly useless: often they are the product of accidental transcription of a sequence, or they may result from  of , as with. Junk RNA is usually quickly degraded by and other cytoplasmic enzymes.

K
karyogram: A which depicts the entire set of in a cell or organism by using photomicrographs of the actual chromosomes as they appear  (usually during, in their most condensed forms), as opposed to the idealized illustrations of chromosomes used in. The photomicrographs are often still arranged in pairs and by size for easier identification of particular chromosomes, whereas in the actual nucleus there is seldom any apparent organization.

karyokinesis: See.

karyolymph: See.

karyon: See.

karyoplasm: See.

karyopyknosis: See.

karyorrhexis: The fragmentation and degeneration of the of a dying cell, during which the is destroyed and the contents of the nucleus, including, are dispersed throughout the  and degraded by enzymes. Karyorrhexis is usually preceded by and may occur as a result of, , or.

karyosome:

karyotype: The number and appearance of within the of a eukaryotic cell, especially as depicted in an organized  or  (in pairs and arranged by size and by position of the ). The term is also used to refer to the complete set of chromosomes in a species or individual organism or to any test that detects this complement or measures the chromosome number.

ketogenesis:

ketogenic amino acid: Any that can be converted directly into, which can then be oxidized for energy or used as a precursor for many. This is in contrast to the, which can be converted into. In humans, seven of the 20 amino acids are ketogenic, though only leucine and lysine are exclusively ketogenic; the other five (phenylalanine, isoleucine, threonine, tryptophan, and tyrosine) are both ketogenic and glucogenic.

ketolysis:

kilobase (kb): A unit of length equal to one thousand (1) in  molecules or one thousand  in duplex molecules such as.

kinase: Any of a class of which catalyze the transfer of groups from high-energy, phosphate-donating molecules such as  to one or more specific, a process known as. The opposite process, known as, is catalyzed by enzymes.

kinesis: Any movement or change in activity by a cell or a population of cells in response to a stimulus, such that the rate of the movement or activity is dependent on the intensity of the stimulus but not on the direction from which the stimulus occurs. Kinesis is often defined as any non-specific, non-directional response, in contrast to and.

kinetochore: A disc-shaped protein complex which assembles around the of a during  of  and, where it functions as the attachment point for microtubules of the.

knob: In, an enlarged, heavily staining that can be used as a visual marker, allowing specific chromosomes to be easily identified in the nucleus.

knockdown: A technique by which the normal rate of of one or more of an organism's  is reduced, either through direct modification of a DNA sequence or through treatment with a reagent such as a short DNA or RNA  with a sequence  to either an  transcript or a gene.

knockin:

knockout: A technique in which an organism is modified to carry that have been made inoperative ("knocked out"), such that their  is disrupted at some point in the pathway that produces their  and the organism is deprived of their normal effects. Contrast.

Kozak consensus sequence:

A highly  which functions as the recognition site for the initiation of  in most eukaryotic, generally a sequence of 10 bases immediately surrounding and inclusive of the : GCCCCAUGG. As the scans the transcript, recognition of this sequence (or a close variant) causes the complex to commit to full  assembly and the start of translation. The Kozak sequence is distinct from other recognition sequences relevant to translation such as and.

Krebs cycle: See.

L
labelling:

The chemical attachment of a highly selective substance, known as a label, tag, or ', to a particular cell, protein, amino acid, or other molecule of interest, either naturally or artificially, ' or. Natural labelling is a primary mechanism by which biomolecules specifically identify and interact with other biomolecules; important examples include,, and. Labelling is also a common laboratory technique, where the label is typically a reactive derivative of a naturally fluorescent compound (e.g. green fluorescent protein), dye, enzyme, antibody, radioactive molecule, or any other substance that makes its target distinguishable in some way. The labelled targets are thereby rendered distinct from their unlabelled surroundings, allowing them to be easily detected, identified, quantified, or isolated for further study.

lagging strand: In, the nascent for which 's direction of synthesis is away from the, which necessitates a complex and discontinuous process in contrast to the streamlined, continuous synthesis of the other nascent strand, known as the , which occurs simultaneously. Because DNA polymerase works only in the to  direction, but the lagging strand's overall direction of chain elongation must ultimately be the opposite (i.e. 3' to 5', toward the replication fork), elongation must occur by an indirect mechanism in which a  enzyme synthesizes short   complementary to the template DNA, and DNA polymerase then extends the primed segments into short chains of nucleotides known as. The RNA primers are then removed and replaced with DNA, and the Okazaki fragments are joined by.

lampbrush chromosome:

lateral gene transfer (LGT): See.

leader sequence: See.

leading strand: In, the nascent for which both the direction of synthesis by and the direction of overall chain elongation are toward the ; i.e. both occur in the  to  direction, resulting in a single, continuous elongation process with few or no interruptions. By contrast, the other nascent strand, known as the, is assembled in a discontinuous process involving the ligation of short synthesized in the opposite direction, away from the replication fork.

left splicing junction:

The boundary between the left end (by convention, the ) of an and the right end of an adjacent  in a.

leptonema:

In, the first of five substages of, following and preceding. During leptonema, the replicated chromosomes condense from diffuse into long, thin strands that are much more visible within the.

lethal mutation: Any that results in the premature death of the organism carrying it. lethal mutations are fatal only to, whereas lethals are fatal even in.

leucine zipper (ZIP): A common structural in  and some other types of proteins, approximately 35 amino acids in length, characterized chiefly by the recurrence of the amino acid leucine every seven residues. When modeled in an idealized conformation, the leucine residues are positioned in such a way that they can interdigitate with the same or similar motifs in an alpha helix belonging to another similar polypeptide, facilitating  and the formation of a complex resembling a zipper.

ligand: In biochemistry, any molecule that binds to or interacts with a on a or other, usually reversibly via intermolecular forces; or any substance that forms a complex with a biomolecule as part of a biological process. The binding of specific ligands to DNA or proteins is important in many ; for example, protein–ligand binding may result in the protein undergoing a which alters its function or affinity for other molecules.

ligase: A class of enzymes which catalyze the joining of large molecules such as by forming one or more chemical bonds between them, typically,, , or bonds via condensation reactions. An example is, which catalyzes the formation of between adjacent  on one or both strands of a DNA molecule, a reaction known as.

ligation:

linkage: The tendency of DNA sequences which are physically near to each other on the same chromosome to be together during. Because the physical distance between them is relatively small, the chance that any two nearby parts of a DNA sequence (often or ) will be separated on to different  during  is statistically very low; such loci are then said to be more linked than loci that are farther apart. Loci that exist on entirely different chromosomes are said to be perfectly unlinked. The standard unit for measuring genetic linkage is the (cM).

linkage disequilibrium:

linker DNA: A short, synthetic DNA duplex containing the for a particular. In, linkers are often deliberately included in molecules in order to make them easily modifiable by permitting cleavage and  of foreign sequences at precise locations. A segment of an engineered containing many such restriction sites is sometimes called a. A section of chromosomal DNA connecting adjacent by binding to  H1.

linking number: The number of times that the two strands of a circular molecule cross each other, equivalent to the  (which measures the torsion of the double helix) plus the  (which measures the degree of supercoiling). The linking number of a closed molecule cannot be changed without breaking and rejoining the strands. DNA molecules which are identical except for their linking numbers are known as topological isomers.

lipid: Any of a heterogeneous class of organic compounds, including (fats), waxes, sterols, and some vitamins, united only by their amphipathic or nature and consequently their very low solubility in water. Some lipids such as tend to form lamellar structures or micelles in aqueous environments, where they serve as the primary constituents of biological. Others such as can be  for energy, have important functions in energy storage, or serve as  molecules. Colloquially, the term "lipids" is sometimes used as a synonym for fats, though fats are more correctly considered a subclass of lipids.

lipid bilayer:

A lamellar structure composed of numerous amphipathic molecules packed together in two back-to-back sheets or layers, with their  "tails" directed inward and their  "heads" exposed on the outer surface. This is the basic structural motif for all biological, including the surrounding all cells as well as the membranes surrounding  and. Though bilayers are sometimes colloquially described as phospholipid bilayers, are just one of several classes of  which form bilayers; most membranes are actually a  of phospholipids,, and , interspersed and studded with various other molecules such as.

lipophilic: See.

lncRNA: See.

locus:

A specific, fixed position on a where a particular or  resides.

LOD score:

long arm:

In condensed where the positioning of the creates two segments or "arms" of unequal length, the longer of the two arms of a. Contrast.

long interspersed nuclear element (LINE): Any of a large family of non- which together comprises one of the most widespread in eukaryotic genomes. Each LINE is on average about 7,000 base pairs in length.

long non-coding RNA (lncRNA): A class of consisting of all of more than 200  in length that are not. This limit distinguishes lncRNA from the numerous smaller non-coding RNAs such as. See also.

lyonization: See.

lysis: The disruption and decomposition of the surrounding a cell, or more generally of any membrane-bound or, especially by, , or other chemical or mechanical processes which compromise the membrane's integrity and thereby cause the unobstructed interchange of the contents of  and  spaces. Lysis generally implies the complete and irreversible loss of intracellular organization as a result of the release of the cell's internal components and the dilution of the, and therefore the death of the cell. Such a cell is said to be lysed, and a fluid containing the contents of lysed cells (usually including, , and many other organic molecules) is called a lysate. Lysis may occur both naturally and artificially, and is a normal part of the cellular life cycle.

lysosome: