Glossary of cellular and molecular biology (M–Z)

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 molecular genetics, biochemistry, and microbiology. It is split across two articles:


 * Glossary of cellular and molecular biology (0–L) lists terms beginning with numbers and those beginning with the letters A through L.
 * Glossary of cellular and molecular biology (M–Z) (this page) 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.

M
M phase: See.

macromolecule: Any very large molecule composed of dozens, hundreds, or thousands of covalently bonded atoms, especially one with biological significance. Many important biomolecules, such as and, are consisting of a repeated series of smaller ; others such as  and  may not be polymeric but are nevertheless large and complex molecules.

macronucleus:

The larger of the two types of which occur in pairs in the cells of some ciliated protozoa. Macronuclei are highly and responsible for directing vegetative reproduction, in contrast to the, which have important functions during.

macrophage: Any of a class of relatively long-lived of the mammalian immune system which are activated in response to the presence of foreign materials in certain tissues and subsequently play important roles in antigen presentation, stimulating other types of immune cells, and killing or parasitic microorganisms, diseased cells, or tumor cells.

major groove:

map-based cloning: See.

massively parallel sequencing:

medical genetics: The branch of medicine and medical science that involves the study, diagnosis, and management of, and more broadly the application of knowledge about human to medical care.

megabase (Mb): A unit of length equal to one million (1) in  molecules or one million  in duplex molecules such as.

meiosis: A specialized type of that occurs exclusively in sexually reproducing eukaryotes, during which is followed by two consecutive rounds of division to ultimately produce four genetically unique  daughter cells, each with half the number of  as the original  parent cell. Meiosis only occurs in cells of the sex organs, and serves the purpose of generating haploid such as sperm, eggs, or spores, which are later fused during fertilization. The two meiotic divisions, known as Meiosis I and Meiosis II, may also include various events between.

meiotic spindle: See.

melting: The of a  into two, especially in the context of the.

membrane: A supramolecular aggregate of amphipathic molecules which when suspended in a polar solvent tend to arrange themselves into structures which minimize the exposure of their tails by sheltering them within a ball created by their own  heads (i.e. a micelle). Certain types of lipids, specifically and other, commonly occur as  sheets of molecules when immersed in an aqueous environment, which can themselves assume approximately spherical shapes, acting as semipermeable barriers surrounding a water-filled interior space. This is the basic layout used for the biological membranes enclosing all, , and.

membrane protein: Any that is closely associated either or  with the   surrounding a,, or.

membrane-bound organelle: An or enclosed by its own dedicated lipid, separating its interior from the rest of the.

messenger RNA (mRNA): Any of a class of molecules which function as molecular messengers, carrying sequence information encoded in the  genome to the  where protein synthesis occurs. The primary products of, mRNAs are synthesized by, which builds a chain of that complement the  of a ; in this way, the DNA sequence of a protein-coding  is effectively preserved in the , which is subsequently processed into a mature mRNA by a series of.

metabolism: The complete set of chemical reactions which sustain and account for the basic processes of life in all living cells, especially those involving: 1) the conversion of energy from food into energy available for cellular activities; 2) the breakdown of food into simpler compounds which can then be used as to build complex such as,, and ; and 3) the degradation and excretion of toxins, byproducts, and other unusable compounds known as . In a broader sense the term may include all chemical reactions occurring in living organisms, even those which are not strictly necessary for life but instead serve accessory functions. Many specific cellular activities are accomplished by  in which one chemical is ultimately transformed through a stepwise series of reactions into another chemical, with each reaction  by a specific . Most metabolic reactions can be subclassified as  or.

metabolite: An intermediate or end product of, especially degradative metabolism (catabolism); or any substance produced by or taking part in a metabolic reaction. Metabolites include a huge variety of small molecules generated by cells from various and having various functions, including as inputs to other pathways and reactions, as molecules, and as stimulators, inhibitors, and  of. Metabolites may result from the degradation and elimination of naturally occurring compounds as well as of pharmaceutical compounds.

metabolome:

metacentric: (of a linear or chromosome fragment) Having a positioned in the middle of the chromosome, resulting in  arms of approximately equal length.

metaphase: The stage of and that occurs after  and before, during which the  of the replicated chromosomes align along the equator of the cell, with each  attached to the.

methylation: The covalent attachment of a methyl group to a chemical compound, protein, or other biomolecule, either spontaneously or by catalysis. Methylation is one of the most widespread natural mechanisms by which and  are. The in a DNA molecule inhibits recognition of the methylated sequence by, which can effectively  the expression of genes. Specific in  are also commonly methylated, which can change  positioning and similarly activate or repress nearby loci. Contrast.

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

MicroArray and Gene Expression (MAGE): A group that "aims to provide a standard for the representation of data that would facilitate the exchange of microarray information between different data systems".

microbody: Any of a diverse class of small or found in the cells of many eukaryotes, especially plants and animals, usually having some specific metabolic function and occurring in great numbers in certain specialized cell types.,, , and  are often considered microbodies.

microchromosome: A type of very small, generally less than 20,000 in size, present in the of some organisms.

microdeletion: A that is too short to cause any apparent change in morphology under a light microscope, though it may still be detectable with other methods such as.

microfilament: A long, thin, flexible, rod-like structure composed of polymeric strands of proteins, usually actins, that occurs in abundance in the of eukaryotic cells, forming part of the. Microfilaments comprise the cell's structural framework. They are modified by and interact with numerous other cytoplasmic proteins, playing important roles in cell stability, motility, contractility, and facilitating changes in cell shape, as well as in.

micronucleus: The smaller of the two types of that occur in pairs in the cells of some ciliated protozoa. Whereas the larger is, the micronucleus is  and generally transcriptionally inactive except for the purpose of sexual reproduction, where it has important functions during.

microRNA (miRNA): A type of small,, molecule that functions in  of, particularly RNA , by base-pairing with complementary sequences in , which typically results in the cleavage or destabilization of the transcript or inhibits its  by ribosomes.

microsatellite:

A type of consisting of a relatively short of, in which certain  (ranging in length from one to six or more ) are repeated, typically 5–50 times. Microsatellites are widespread throughout most organisms' genomes and tend to have higher mutation rates than other regions. They are classified as (VNTR) DNA, along with longer.

microspike: See.

microtome:

microtrabecula:

A fine protein filament of the cytoskeleton. Multiple filaments form the microtrabecular network.

microtubule:

microtubule-organizing center (MTOC):

microvesicle:

microvillus:

mid body: The centrally constricted region that forms across the central axis of a cell during, constricted by the closing of the until the are finally separated.

middle lamella: In plant cells, the outermost layer of the ; a continuous, unified layer of extracellular pectins which is the first layer deposited by the cell during and which serves to cement together the primary cell walls of adjacent cells.

Minimal information about a high-throughput sequencing experiment (MINSEQE): A commercial standard developed by for the storage and sharing of  data.

Minimum information about a microarray experiment (MIAME): A commercial standard developed by and based on in order to facilitate the storage and sharing of  data.

minisatellite: A region of, in which certain DNA  (typically 10–60 bases in length) are  (typically 5–50 times). In the human genome, minisatellites occur at more than 1,000 loci, especially in and, and exhibit high mutation rates and high variability between individuals. Like the shorter, they are classified as (VNTRs) and are a type of.

minor groove:

minus-strand: See.

miRNA: See.

mismatch:

An incorrect of on   of  or ; i.e. the presence in one strand of a duplex molecule of a base that is not complementary (by Watson–Crick pairing rules) to the base occupying the corresponding position in the other strand, which prevents normal hydrogen bonding between the bases. For example, a paired with a  would be a mismatch, as guanine normally pairs with.

mismatch repair (MMR):

missense mutation: A type of which results in a that codes for a different  than in the unmutated sequence. Compare.

mistranslation: The insertion of an incorrect in a growing chain during, i.e. the inclusion of any amino acid that is not the one specified by a particular  in an  transcript. Mistranslation may originate from a  or from a malfunctioning.

mitochondrial DNA (mtDNA):

mitochondrial fusion:

mitochondrion:

mitogen: Any substance or stimulus that promotes or induces, or more generally which causes cells to re-enter the.

mitophagy: The selective degradation of by means of ; i.e. the mitochondrion initiates its own degradation. Mitophagy is a regular process in healthy populations of cells by which defective or damaged mitochondria are recycled, preventing their accumulation. It may also occur in response to the changing needs of the cell, e.g. during certain developmental stages.

mitosis:

In eukaryotic cells, the part of the during which the of the  takes place and replicated  are separated into two distinct nuclei. Mitosis is generally preceded by the of, when the cell's, and either occurs simultaneously with or is followed by , when the  and  are divided into two new. Colloquially, the term "mitosis" is often used to refer to the entire process of cell division, not just the division of the nucleus.

mitotic index (MI): The proportion of cells within a sample which are undergoing at the time of observation, typically expressed as a percentage or as a value between 0 and 1. The number of cells dividing by mitosis at any given time can vary widely depending on organism,, developmental stage, and media, among other factors.

mitotic recombination:

The abnormal between during  (as opposed to, where it occurs normally). during mitosis is relatively uncommon; in the laboratory, it can be induced by exposing dividing cells to high-energy electromagnetic radiation such as X rays. As in meiosis, it can separate alleles and thereby propagate potentially significant changes in zygosity to, though unless it occurs very early in development this often has little or no phenotypic effect, since any phenotypic variance shown by mutant lineages arising in terminally differentiated cells is generally masked or compensated for by neighboring  cells.

mitotic rounding:

mitotic segregation:

mitotic spindle: See.

mixoploidy: The presence of more than one different level, i.e. more than one number of sets of, in different cells of the same cellular population.

mobile genetic element (MGE): Any genetic material that can move between different parts of a or be transferred from one species or to another within a single. The many types of MGEs include, bacterial, bacteriophage elements which integrate into host genomes by viral , and.

mobilome: The entire set of within a particular, cell, species, or other taxon, including all, , , and other self-splicing nucleic acid molecules.

molecular biology: The branch of biology that studies biological activity at the molecular level, in particular the various mechanisms underlying the biological processes that occur in and between, including the structures, properties, synthesis, and modification of such as and, their interactions with the chemical environment and with other biomolecules, and how these interactions explain the observations of classical biology (which in contrast studies biological systems at much larger scales). Molecular biology relies largely on laboratory techniques of physics and chemistry to manipulate and measure microscopic phenomena. It is closely related to and overlaps with the fields of, , and.

molecular cloning: Any of various methods designed to a particular molecule, usually a, many times inside the cells of a natural host. Commonly, a fragment containing a  is  into a , which  bacterial cells are then induced to uptake in a process known as. The bacteria, carrying the recombinant plasmid, are then allowed to proliferate naturally in, so that each time the bacterial cells divide, the plasmids are replicated along with the rest of the bacterial genome. Any functioning gene of interest will be by the bacterial cells, and thereby its  will also be cloned. The plasmids or gene products, which now exist in many copies, may then be extracted from the bacteria and purified. Molecular cloning is a fundamental tool of which is employed for a wide variety of purposes, often to study, to amplify a specific gene product, or to generate a.

molecular genetics: A branch of that employs methods and techniques of to study the structure and function of  and  at the molecular level. Contrast.

monad: A set of as it exists inside the  of an immature  such as an ootid or spermatid, i.e. a cell which has completed  but is not yet a mature gamete.

monocentric: (of a linear or chromosome fragment) Having only one. Contrast ' and '.

monoclonal: Describing cells, proteins, or molecules descended or derived from a single (i.e. from the same genome or genetic lineage) or made in response to a single unique compound. Monoclonal are raised against only one  or can only recognize one unique  on the same antigen. Similarly, the cells of some and  may be described as monoclonal if they are all the asexual progeny of one original. Contrast.

monokaryotic: (of a cell) Having a single, as opposed to or.

monomer: A molecule or compound which can exist individually or serve as a building block or of a larger aggregate known as a. Polymers form when multiple monomers of the same or similar molecular species are connected to each other by chemical bonds, either in a linear chain or a non-linear conglomeration. Examples include the individual which form  polymers; the individual  which form ; and the individual proteins which form.

monoploid:

monosaccharide: Any of a class of organic compounds which are the simplest forms of and the most basic structural subunits or from which larger carbohydrate  such as,, and  are built. With few exceptions, all monosaccharides are variations on the empirical formula, where typically ranges from 3 (trioses) to 7 (heptoses). Common examples include, , and.

monosomy: The abnormal and frequently pathological presence of only one of a normal pair. It is a type of.

Morpholino:

A synthetic connecting a short sequence of into an artificial, used in  to   by  with  sequences in naturally occurring RNA or DNA molecules, especially , thereby inhibiting interactions with other biomolecules such as proteins and. Morpholino oligomers are not themselves, and neither they nor their hybrid duplexes with RNA are attacked by ; also, unlike the negatively charged of normal nucleic acids, the synthetic backbones of Morpholinos are electrically neutral, making them less likely to interact non-selectively with a host cell's charged proteins. These properties make them useful and reliable tools for artificially generating phenotypes in living cells.

mosaicism: The presence of two or more populations of cells with different in an individual organism which has developed from a single fertilized egg. A mosaic organism can result from many kinds of genetic phenomena, including of chromosomes,, or mutations in individual stem cell lineages during the early development of the embryo. Mosaicism is similar to but distinct from.

motif: Any distinctive or recurring of in a  or of  in a  that is or is conjectured to be biologically significant, especially one that is reliably  by other biomolecules or which has a  that permits unique or characteristic chemical interactions such as. In nucleic acids, motifs are often short (three to ten nucleotides in length), highly which act as recognition sites for  or RNAs involved in the regulation of.

motor protein: Any which converts chemical energy derived from the hydrolysis of such as  and  into mechanical work in order to effect its own locomotion, by propelling itself along a filament or through the.

mRNA: See.

mtDNA: See.

multicellular: Composed of more than one cell. The term is especially used to describe organisms or tissues consisting of many cells descendant from the same original parent cell which work together in an organized way, but may also describe colonies of nominally single-celled organisms such as protists and bacteria which live symbiotically with each other in large groups. Contrast.

multinucleate: (of a cell) Having more than one within a single ; i.e. having multiple nuclei occupying the same.

multiomics:

multiple cloning site (MCS):

mutagen: Any physical or chemical agent that (usually ) of an organism and thereby increases the frequency of above natural background levels.

mutagenesis: The process by which the genetic information of an organism is changed, resulting in a. Mutagenesis may occur spontaneously or as a result of exposure to a. In, any laboratory technique by which one or more genetic mutations are deliberately in order to produce a gene, regulatory element, gene product, or  so that the functions of a genetic locus, process, or product can be studied in detail.

mutant: An organism, gene product, or phenotypic resulting from a, of a type that would not be observed naturally in specimens.

mutation: Any permanent change in the of a strand of or, or in the  sequence of a. Mutations play a role in both normal and abnormal biological processes; their natural occurrence is integral to the process of. They can result from errors in, chemical damage, exposure to high-energy radiation, or manipulations by. mechanisms have evolved in many organisms to correct them. By understanding the effect that a mutation has on, it is possible to establish the function of the or sequence in which it occurs.

mutator gene: Any mutant or sequence that increases the spontaneous of one or more other genes or sequences. Mutators are often, or may be mutant such as those that encode  or proteins involved in.

mutein: A, i.e. a protein whose sequence differs from that of the normal because of a.

muton: The smallest unit of a DNA molecule in which a physical or chemical change can result in a (conventionally a single ).

N
n orientation: One of two possible orientations by which a linear DNA fragment can be inserted into a, specifically the one in which the of both fragment and vector have the same orientation. Contrast.

NAD: See.

NADP: See.

nanoinjection:

nascent: In the process of being synthesized; incomplete; not yet fully processed or mature. The term is commonly used to describe of or  which are actively undergoing synthesis during  or, respectively, or sometimes a complete, fully transcribed RNA molecule before any  have been made (e.g.  or ), or a  chain actively undergoing  by a.

ncAA: See.

ncDNA: See.

ncRNA: See.

negative (-) sense strand: See.

negative control:

A type of in which a binds to an  upstream from the coding region and thereby prevents  by. An is necessary to switch on transcription in.

negative supercoiling:

next-generation sequencing (NGS): See.

nick: A break or discontinuity in the of one of a  molecule, i.e. where a  is hydrolyzed but no nucleotides are removed; such a molecule is said to be nicked. A nick is a, where despite the break the DNA molecule is not ultimately broken into multiple fragments, which contrasts with a, where. Nicks may be caused by or by dedicated  known as, which nick DNA at random or specific sites. Nicks are frequently placed by the cell as markers identifying target sites for enzyme activity, including in, , and , and also to release torsional stress from DNA molecules, making them important in manipulating.

nick translation:

nickase:

nicking enzyme:

nicotinamide adenine dinucleotide (NAD):

nicotinamide adenine dinucleotide phosphate:

nitrogenous base:

Any organic compound containing a nitrogen atom that has the chemical properties of a base. A set of – (A),  (G),  (C),  (T), and  (U) – are especially relevant to biology because they are components of, which are the primary  that make up.

non-canonical amino acid (ncAA):

Any, natural or artificial, that is not one of the 20 or 21 encoded by the. There are hundreds of such amino acids, many of which have biological functions and are specified by alternative codes or incorporated into proteins accidentally by. Many of the best known naturally occurring ncAAs occur as intermediates in the metabolic pathways leading to the standard amino acids, while others have been made synthetically in the laboratory.

non-coding DNA (ncDNA): Any segment of that does not a sequence that may ultimately be  and  into a. In most organisms, only a small fraction of the genome consists of protein-coding DNA, though the proportion varies greatly between species. Some non-coding DNA may still be transcribed into functional (as with ) or may serve important developmental or  purposes; other regions (as with so-called "") appear to have no known biological function.

non-coding RNA (ncRNA): Any molecule of that is not ultimately into a. The sequence from which a functional non-coding RNA is  is often referred to as an "RNA gene". Numerous types of non-coding RNAs essential to normal genome function are produced constitutively, including (tRNA),  (rRNA),  (miRNA), and  (siRNA); other non-coding RNAs (sometimes described as "junk RNA") have no known function and are likely the product of spurious transcription.

non-coding strand: See.

nondisjunction: The failure of or to  properly during cell division. Nondisjunction results in daughter cells that are, containing abnormal numbers of one or more specific. It may be caused by any of a variety of factors.

non-homologous end joining (NHEJ):

nonrepetitive sequence: Broadly, any or region of a that does not contain, or in which repeats do not comprise a majority; or any segment of DNA exhibiting the  expected of a unique sequence.

nonsense mutation:

A type of which results in a premature in the   sequence, thereby causing the premature termination of, which results in a truncated, incomplete, and often non-functional.

nonsense suppressor:

nonsynonymous mutation:

A type of in which the of one  base for another results, after  and, in an amino acid sequence that is different from that produced by the original unmutated gene. Because nonsynonymous mutations always result in a biological change in the organism, they are often subject to strong. Contrast.

non-transcribed spacer (NTS): See.

northern blotting: A method in molecular biology used to detect in a sample. Compare ', ', and.

nRNA: See.

N-terminus:

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

nuclear cage:

nuclear DNA: Any molecule contained within the of a eukaryotic cell, most prominently the DNA in. It is sometimes used interchangeably with.

nuclear envelope: A sub-cellular barrier consisting of two concentric that surrounds the  in eukaryotic cells. The nuclear envelope is sometimes simply called the "nuclear membrane", though the structure is actually composed of two distinct membranes, an and an.

nuclear equivalence: The principle that the nuclei of essentially all cells of a mature multicellular organism are genetically identical to each other and to the nucleus of the from which they descended; i.e. they all contain the same genetic information on the same chromosomes, having been replicated from the original zygotic set with extremely high fidelity. Even though all adult have the same set of genes, cells can nonetheless differentiate into distinct  by expressing different subsets of these genes. Though this principle generally holds true, the reality is slightly more complex, as mutations such as,, , and as well as , , and various types of  can all cause different somatic lineages within the same organism to be genetically non-identical.

nuclear export signal (NES):

nuclear lamina: A fibrous network of proteins lining the inner, surface of the, composed of protein filaments similar to those that make up the. It may function as a scaffold for the various contents of the nucleus such as and.

nuclear localization signal (NLS):

An within a which serves as a molecular signal marking the protein for  into the, typically consisting of one or more short motifs containing positively charged amino acids exposed on the mature protein's surface (especially lysines and arginines). Though all proteins are in the cytoplasm, many whose primary biological activities occur inside the nucleus, e.g., require nuclear localization signals identifiable by  in order to cross the. Contrast.

nuclear membrane: See.

nuclear pore:

A complex of that creates an opening in the through which certain molecules and ions are  in order to cross the envelope and thus enter or exit the  (analogous to the  in the ). The nuclear envelope typically has thousands of pores, which selectively regulate the exchange of specific materials between the and the, including nuclear proteins, which are synthesized in the cytoplasm but function in the nucleus, as well as, which are transcribed in the nucleus but must be translated in the cytoplasm.

nuclear protein: Any that is naturally found in or to the cell's  (as opposed to the  or elsewhere).

nuclear RNA (nRNA): Any molecule located within a cell's, whether associated with chromosomes or existing freely in the nucleoplasm, including (snRNA),  (eRNA), and all newly transcribed,  or , prior to their export to the cytosol (hnRNA).

nuclear transfer:

nuclear transport: The mechanisms by which molecules cross the surrounding a cell's nucleus. Though small molecules and ions can cross the membrane freely, the entry and exit of larger molecules is tightly regulated by, so that most such as RNAs and proteins require association with transport factors in order to be  across.

nuclease: Any of a class of capable of cleaving connecting adjacent  in a nucleic acid molecule (the opposite of a ). Nucleases may or   of a duplex molecule, and may cleave randomly or at specific recognition sequences. They are ubiquitous and imperative for normal cellular function, and are also widely employed in laboratory techniques.

nucleic acid: A long, made up of smaller  called  which are chemically linked to one another in a chain. Two specific types of nucleic acid, and, are common to all living organisms, serving to encode the genetic information governing the construction, development, and ordinary processes of all biological systems. This information, contained within the order or, is into , which direct all of the chemical reactions necessary for life.

nucleic acid sequence: The precise order of consecutively linked in a molecule such as  or. Long sequences of nucleotides are the principal means by which biological systems store genetic information, and therefore the accurate,, and of such sequences is of the utmost importance, lest the information be lost or corrupted. Nucleic acid sequences may be equivalently referred to as sequences of nucleotides,, , or, in , , and they correspond directly to sequences of and.

nucleobase:

Any of the five primary or canonical – (A),  (G),  (C),  (T), and  (U) – that form  and, the latter of which are the fundamental building blocks of. The ability of these bases to form via hydrogen bonding, as well as their flat, compact three-dimensional profiles, allows them to "stack" one upon another and leads directly to the long-chain structures of  and. When writing sequences in shorthand notation, the letter N is often used to represent a nucleotide containing a generic or unidentified nucleobase.

nucleoid:

An irregularly shaped region within a prokaryotic cell which contains most or all of the cell's genetic material, but is not enclosed by a as in eukaryotes.

nucleolin: The primary of which the eukaryotic is composed, thought to play important roles in  decondensation, transcription of, and  assembly.

nucleolonema: The central region of the, composed of dense, convoluted fibrillar material.

nucleolus: An within the of eukaryotic cells which is composed of proteins, DNA, and RNA and serves as the site of  synthesis.

nucleoplasm:

All of the material enclosed within the of a cell by the, analogous to the enclosed by the main. Like the cytoplasm, the nucleoplasm is composed of a gel-like substance (the ) in which various, , and other are suspended, including nuclear DNA in the form of , the , , and free.

nucleoprotein: Any that is chemically bonded to or conjugated with a. Examples include,, and many.

nucleosidase: Any of a class of which catalyze the decomposition of into their component  and  sugars.

nucleoside: An organic molecule composed of a bonded to a (either  or ). A additionally includes one or more.

nucleosol:

The soluble, liquid portion of the (analogous to the of the ).

nucleosome: The basic structural subunit of used in nuclear DNA such as chromosomes, consisting of a   proteins around which  is wrapped in a manner akin to thread wound around a spool. The technical definition of a nucleosome includes a segment of DNA about 146 base pairs in length which makes 1.67 left-handed turns as it coils around the histone core, as well as a stretch of (generally 38–80 bp) connecting it to an adjacent core particle, though the term is often used to refer to the core particle alone. Long series of nucleosomes are further condensed by association with histone H1 into higher-order structures such as and ultimately. Because the histone–DNA interaction limits access to the DNA molecule by other proteins and RNAs, the precise positioning of nucleosomes along the DNA sequence plays a fundamental role in controlling whether or not genes are and, and hence mechanisms for moving and ejecting nucleosomes have evolved as a means of  the expression of particular loci.

nucleosome-depleted region (NDR): A region of a genome or chromosome in which long segments of DNA are bound by few or no, and thus exposed to manipulation by other proteins and molecules, especially implying that the region is active.

nucleotide:

An organic molecule that serves as the fundamental or subunit of polymers, including  and. Each nucleotide is composed of three connected functional groups: a, a (either  or ), and a single. Though technically distinct, the term "nucleotide" is often used interchangeably with nitrogenous base,, and when referring to the  that make up nucleic acids. Compare.

nucleotide sequence: See.

nucleus:

A large spherical or lobular surrounded by a which functions as the main storage compartment for the genetic material of eukaryotic cells, including the DNA comprising, as well as the site of RNA synthesis during. The vast majority of eukaryotic cells have a single nucleus, though some cells may have, either temporarily or permanently, and in some organisms there exist certain cell types (e.g. mammalian ) which lose their nuclei upon reaching maturity, effectively becoming. The nucleus is one of the defining features of eukaryotes; the cells of prokaryotes such as bacteria lack nuclei entirely.

O
occluding junction:

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

Okazaki fragments: of which are synthesized discontinuously by and later linked together by  to create the  during. Okazaki fragments are the consequence of the unidirectionality of DNA polymerase, which only works in the 5' to 3' direction.

oligogene:

oligomer: Any molecule consisting of a relatively short series of connected or ; e.g. an  is a short series of nucleotides.

oligonucleotide:

A relatively short chain of. In the laboratory, oligonucleotides are commonly used as or  to detect the presence of larger  molecules or assembled into two-dimensional microarrays for   analysis.

oligosaccharide: A molecule consisting of a relatively short chain of connected. Oligosaccharides have important functions in processes such as and. Longer chains are called.

omnipotent suppressor:

oncogene: A that has the potential to cause cancer. In tumor cells, such genes are often and/or  at abnormally high levels.

one gene–one polypeptide:

The hypothesis that there exists a large class of in which each particular gene directs the synthesis of one particular or. Historically it was thought that all genes and proteins might follow this rule by definition, but it is now known that many or most proteins are composites of different polypeptides and therefore the product of multiple genes, and also that some genes do not encode polypeptides at all but instead produce, which are never translated.

opal:

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

open chromatin: See.

open reading frame (ORF): The part of a that has the ability to be from DNA or RNA into protein; any continuous stretch of  that contains a  and a.

operator: A sequence within an, typically located between the sequence and the  of the operon, to which an uninhibited  protein can bind, thereby physically obstructing  from initiating the  of adjacent cistrons.

operon: A functional unit of consisting of a cluster of adjacent which are collectively under the control of a single, along with one or more adjacent  sequences such as  which affect  of the structural genes. The set of genes is transcribed together, usually resulting in a single  molecule, which may then be  together or undergo  to create multiple mRNAs which are translated independently; the result is that the genes contained in the operon are either expressed together or not at all. Regulatory proteins, including, , and , usually bind specifically to the regulatory sequences of a given operon; by some definitions, the genes that code for these regulatory proteins are also considered part of the operon.

operon network:

organelle: A spatially distinct compartment or subunit within a which has a specific, specialized function. Organelles occur in both prokaryotic and eukaryotic cells. In the latter they are often separated from the by being enclosed with their own   (whence the term membrane-bound organelles), though organelles may also be functionally specific areas or structures without a surrounding membrane; some cellular structures which exist partially or entirely outside of the cell membrane, such as  and, are also referred to as organelles. There are numerous types of organelles with a wide variety of functions, including the various compartments of the (e.g. the, , and ), , , , , and , among others. Many organelles are unique to particular cell types or species.

origin of replication (ORI):

A particular location within a DNA molecule at which is initiated. Origins are usually defined by the presence of a particular replicator sequence or by specific chromatin patterns.

osmosis:

osmotic shock:

outron: A sequence near the of a that is removed by a special form of  during. Outrons are located entirely outside of the transcript's, unlike.

overexpression: An abnormally high level of which results in an excessive number of copies of one or more. Overexpression produces a pronounced gene-related.

oxidative phosphorylation:

oxidative stress:

oxygen cascade:

P
p53:

A class of regulatory proteins encoded by the TP53 gene in vertebrates which and in order to protect the genome from mutation and block progression through the  if DNA damage does occur. It is mutated in more than 50% of human cancers, indicating it plays a crucial role in preventing cancer formation.

pachynema:

In, the third of five substages of, following and preceding. During pachynema, the facilitates  between the synapsed, and the  begin to move apart from each other.

palindromic sequence:

A of a double-stranded or  molecule in which the unidirectional sequence (e.g. 5' to 3') of  on one strand matches the sequence in the same direction (e.g. 5' to 3') on the  strand. In other words, a nucleotide sequence is said to be palindromic if it is equal to its own. Palindromic are common in most genomes and are capable of forming.

paracellular transport:

paracrine:

parent cell: The original or ancestral cell from which a given set of descendant cells, known as, have divided by or.

passenger: A DNA fragment of interest designed to be into a 'vehicle' such as a  and then.

passive transport: The movement of a solute across a by traveling down an electrochemical or concentration gradient, using only the energy stored in the gradient and not any energy from external sources. Contrast.

PCR: See.

PCR product: See.

pentose: Any simple sugar or containing five carbon atoms. The compounds and  are both pentose sugars, which, in the form of cyclic five-membered rings, serve as the central structural components of the  and  that make up  and, respectively.

peptidase: See.

peptide: A short chain of monomers linked by covalent. Peptides are the fundamental building blocks of longer chains and hence of.

peptide bond: A covalent chemical bond between the of one and the  of an adjacent amino acid in a  chain, formed by a dehydration reaction catalyzed by, an enzyme within the, during.

pericentriolar material (PCM):

perinuclear space: The space between the and of the.

peripheral membrane protein:

Any of a class of which attach only temporarily to the, either by penetrating the or by attaching to  which are permanently embedded within the membrane. The ability to reversibly interact with membranes makes peripheral membrane proteins important in many different roles, where they commonly function as regulatory of  and. Protein domains often undergo rearrangement, dissociation, or when they interact with the membrane, resulting in the activation of their biological activity. In, peripheral membrane proteins are typically more water-soluble and much easier to isolate from the membrane than.

peroxisome: A small found in many eukaryotic cells which specializes in carrying out oxidative reactions with various enzyme peroxidases and catalase, generally to mitigate damage from reactive oxygen species but also as a participant in various such as  of fatty acids.

persistence: The tendency of a to continue moving in the same direction as previously; that is, even in isotropic environments, there inevitably still exists an inherent bias by which, from instant to instant, cells are more likely not to change direction than to change direction. Averaged over long periods of time, however, this bias is less obvious and cell movements are better described as a. The ability of some viruses to remain present and viable in cells, organisms, or populations for very long periods of time by any of a variety of strategies, including retroviral integration and immune suppression, often in a latent form which replicates very slowly or not at all.

pervasive transcription:

Petri dish: A shallow, transparent plastic or glass dish, usually circular and covered with a lid, which is widely used in biology laboratories to hold solid or liquid for the purpose of. They are particularly useful for, where they provide a flat, sterile surface conducive to colony formation from which scientists can easily isolate and identify particular colonies.

phagocyte: A type of cell which functions as part of the immune system by engulfing and ingesting harmful foreign molecules, bacteria, and dead or dying cells in a process known as.

phagocytosis: The process by which foreign molecules, cells, and small particulate matter is engulfed and ingested via by specialized cells known as (a class which includes  and neutrophils).

phagosome: A large, intracellular, membrane-bound formed as a result of and containing whatever previously extracellular material was engulfed during that process.

pharmacogenomics: The study of the role played by the in the body's response to pharmaceutical drugs, combining the fields of pharmacology and.

phenome: The complete set of that are or can be expressed by a, cell, tissue, organism, or species; the sum of all of its manifest chemical, morphological, and behavioral characteristics or traits.

phenomic lag: A delay in the of a genetic  owing to the time required for the manifestation of changes in the affected biochemical pathways.

phenotype: The composite of the observable morphological, physiological, and behavioral of an organism that result from the of the organism's  as well as the influence of environmental factors and the interactions between the two.

phenotypic switching:

phosphatase: Any of a class of that catalyze the hydrolytic cleavage of a phosphoric acid monoester into a ion and an alcohol, e.g. the removal of a phosphate group from a  via the breaking of the  connecting the phosphate to a  or  sugar or to another phosphate, a process termed. The is performed by.

phosphate:

phosphate backbone:

The linear chain of alternating phosphate and sugar compounds that results from the linking of consecutive in the same of a  molecule, and which serves as the structural framework of the nucleic acid. Each individual strand is held together by a repeating series of connecting each  to the  or  sugars of two adjacent nucleotides. These bonds are created by and broken by.

phosphodiester bond: A pair of ester bonds linking a molecule with the two pentose rings of consecutive on the same strand of a. Each phosphate forms a covalent bond with the of one pentose and the  of the adjacent pentose; the repeated series of such bonds that holds together a long chain of nucleotides in  and  molecules is known as the.

phospholipid: Any of a subclass of consisting of a central alcohol (usually glycerol) covalently bonded to three functional groups: a negatively charged group, and two long  chains. This arrangement results in a highly amphipathic molecule which in aqueous solutions tends to aggregate with other, similar molecules in a lamellar or micellar conformation with the phosphate "heads" oriented outward, exposing them to the solution, and the  fatty acid "tails" oriented inward, minimizing their interactions with water and other polar compounds. Phospholipids are the major structural in almost all biological  except the membranes of some plant cells and, where  dominate instead.

phospholipid bilayer: See.

phosphorylation: The attachment of a ion,, to another molecule or ion or to a by covalent bonding. Phosphorylation and the inverse reaction,, are essential steps in numerous biochemical pathways, including in the production of (ATP) (as in ); in the metabolism of  and the synthesis of ; and in the  of amino acid residues in many proteins. which catalyze phosphorylation reactions are known as ; those that catalyze dephosphorylation are known as.

piRNA: See.

pitch:

Piwi-interacting RNA (piRNA):

plasma membrane: See.

plasmid: Any small molecule that is physically separated from the larger body of and can  independently. Plasmids are most commonly found as small, circular, molecules in prokaryotes such as bacteria, though they are also sometimes present in archaea and eukaryotes.

plasmid partitioning: The process by which which have been inside a  are distributed equally between  during.

plasmid-mediated resistance:

plastid: Any of a class of found in the cells of some eukaryotes such as plants and algae which are hypothesized to have evolved from endosymbiotic cyanobacteria; examples include chloroplasts, chromoplasts, and leucoplasts. Plastids retain their own which replicate independently of the host cell's genome. Many contain photosynthetic pigments which allow them to perform, while others have been retained for their ability to synthesize unique chemical compounds.

pleomorphism: Variability in the size, shape, or of cells and/or their nuclei, particularly as observed in and, where morphological variation is frequently  of a cellular abnormality such as disease or. In microbiology, the ability of some microorganisms such as certain bacteria and viruses to alter their morphology, metabolism, or mode of reproduction in response to changes in their environment.

plithotaxis:

ploidy: The number of complete sets of in a cell, and hence the number of possible present within the cell at any given  locus.

pluripotency:

plus-strand: See.

point mutation: A by which a single base is changed,, or  from a  of DNA or RNA.

poly(A) tail:

polyadenylation: The addition of a series of multiple, known as a, to the of a , typically a. A class of, polyadenylation serves different purposes in different cell types and organisms. In eukaryotes, the addition of a poly(A) tail is an important step in the processing of a raw transcript into a mature mRNA, ready for export to the cytoplasm where occurs; in many bacteria, polyadenylation has the opposite function, instead promoting the RNA's degradation.

polyclonal: Describing cells, proteins, or molecules descended or derived from more than one (i.e. from more than one genome or genetic lineage) or made in response to more than one unique stimulus. are often described as polyclonal if they have been produced or raised against multiple distinct or multiple variants of the same antigen, such that they can recognize more than one unique. Contrast.

polylinker: See.

polymer: A composed of multiple repeating or ; a chain or aggregation of many individual molecules of the same compound or class of compound. The formation of polymers is known as and generally only occurs when  sites are present and the concentration of monomers is sufficiently high. Many of the major classes of are polymers, including  and.

polymerase: Any of a class of which catalyze the synthesis of molecules, especially  polymers, typically by encouraging the  of free  to those of an existing. and are essential for  and, respectively.

polymerase chain reaction (PCR): Any of a wide variety of molecular biology methods involving the rapid production of millions or billions of copies of a specific, allowing scientists to selectively fragments of a very small sample to a quantity large enough to study in detail. In its simplest form, PCR generally involves the incubation of a target DNA sample of known or unknown sequence with a reaction mixture consisting of, a heat-stable , and free  (dNTPs), all of which are supplied in excess. This mixture is then alternately heated and cooled to pre-determined temperatures for pre-determined lengths of time according to a specified pattern which is repeated for many cycles, typically in a which automatically controls the required temperature variations. In each cycle, the most basic of which includes a phase,  phase, and  phase, the copies synthesized in the previous cycle are used as  for synthesis in the next cycle, causing a chain reaction that results in the exponential growth of the total number of copies in the reaction mixture. Amplification by PCR has become a standard technique in virtually all molecular biology laboratories.

polymerization: The process by which are created from their constituent ; a chemical reaction or series of reactions in which monomeric are covalently linked together into a polymeric chain or three-dimensional structure; e.g. the polymerization of a  chain by linking consecutive, a reaction catalyzed by a.

polymorphism:

polypeptide: A long, continuous, and unbranched chain of  linked by covalent, typically longer than a. generally consist of one or more polypeptides or arranged in a biologically functional way.

polyploid: (of a cell or organism) Having more than two homologous copies of each ; i.e. any level that is greater than. Polyploidy may occur as a normal condition of chromosomes in certain cells or even entire organisms, or it may result from errors in or mutations causing the duplication of the entire chromosome set.

polysaccharide: A linear or branched chain of. Examples include and cellulose.

polysome:

A complex of a molecule and two or more which act to  the mRNA transcript into a.

polysomy: The condition of a cell or organism having at least one more copy of a particular than is normal for its level, e.g. a  organism with three copies of a given chromosome is said to show. Every polysomy is a type of.

polytene chromosome:

position effect: Any effect on the or functionality of a or sequence that is a consequence of its location or position within a  or other DNA molecule. A sequence's precise location relative to other sequences and structures tends to strongly influence its activity and other properties, because different on the same molecule can have substantially different  and physical/chemical environments, which may also change over time. For example, the of a gene located very close to a,, or  is often  or entirely prevented because the proteins that make up these structures block access to the DNA by , while the same gene is transcribed at a much higher rate when located in. Proximity to, , and other , as well as to regions of frequent by , can also directly affect expression; being located near the end of a chromosomal arm or to common  points may affect when  occurs and the likelihood of. Position effects are a major focus of research in the field of.

positional cloning:

A strategy for identifying and a based on knowledge of its  or position alone and with little or no information about its  or function, in contrast to. This method usually begins by comparing the genomes of individuals expressing a of unknown provenance (often a ) and identifying  shared between them. Regions defined by markers flanking one or more are cloned, and the genes located between the markers can then be identified by any of a variety of means, e.g. by  the region and looking for, by comparing the sequence and expression patterns of the region in  and  individuals, or by testing the ability of the putative gene to  a mutant phenotype.

positive (+) sense strand: See.

positive control:

positive supercoiling:

post-transcriptional modification:

post-translational modification:

potency:

precursor cell:

Pribnow box:

primary transcript: The unprocessed, single-stranded molecule produced by the of a  sequence as it exists before  such as  convert it into a mature RNA product such as an,, or. A precursor mRNA or pre-mRNA, for example, is a type of primary transcript that becomes a mature mRNA ready for after processing.

primase: Any of a class of enzymes that catalyze the synthesis of short, ~10-base which by complementing the  during  are used as  by  to initiate the synthesis of.

primer: A short,, typically 5–100 bases in length, which "primes" or initiates nucleic acid synthesis by to a complementary sequence on a  and thereby provides an existing  from which a  can  the new strand. Natural systems exclusively use primers to initiate  and, whereas the  syntheses performed in many laboratory techniques such as  often use  primers. In modern laboratories, primers are carefully designed, often in "forward" and "reverse" pairs, to complement specific and unique sequences within, with consideration given to their and  temperatures, and then purchased from commercial suppliers which create oligonucleotides on demand by.

primer dimer (PD):

primer walking:

priming: The initiation of synthesis by the or  of one or more  to a complementary sequence within a.

probe: Any reagent used to make a single measurement in a biochemical assay such as a gene expression experiment. Molecules which have a specific affinity for one or more other molecules may be used to probe for the presence of those other molecules in samples of unknown composition. Probes are often or otherwise used as to indicate whether or not a specific chemical reaction is taking place. See also.

probe-set: A collection of two or more designed to measure a single molecular species, such as a collection of designed to  to various parts of the  transcripts generated from a single gene.

process molecular gene concept:

prometaphase: The second stage of in, following and preceding, during which the  disintegrates, the  inside form  around their ,  emerging from the poles of the  reach the nuclear space and attach to the kinetochores, and  associated with the microtubules begin to push the chromosomes toward the center of the cell.

promoter: A sequence or region of DNA, usually 100–1,000 base pairs long, to which bind in order to recruit to the sequence and initiate the  of one or more genes. Promoters are located of the genes they transcribe, near the transcription start site.

promotion: See.

prophase: The first stage of in both and, occurring after  and before, during which the DNA of the chromosomes is  into , the  disintegrates,  move to opposite ends of the cell, and the  forms.

protease:

Any of a class of which catalyze, i.e. the decomposition of proteins into smaller polypeptides or individual amino acids, by cleaving via hydrolysis. Proteases are ubiquitous components of numerous, and therefore it is often necessary to inhibit them in order for laboratory techniques involving protein activity to be effective.

protein: A composed of one or more  of  linked by. Proteins are the three-dimensional structures created when these chains into specific higher-order arrangements following, and it is this folded structure which determines a protein's chemical activity and hence its biological function. Ubiquitous and fundamental in all living organisms, proteins are the primary means by which the activities of life are performed, participating in the vast majority of the biochemical reactions that occur inside and outside of cells. They are often classified according to the type(s) of reaction(s) they facilitate or catalyze, by the chemical substrate(s) they act upon, or by their functional role in cellular activity; e.g. as, , , , or links within.

protein complex: An assembly or aggregate of multiple held together by intermolecular forces, especially one with a particular biological function. Complexes may include many of the same protein or all different proteins. Numerous cellular activities, including,, and , rely on protein complexes.

protein folding:

protein sorting:

protein targeting:

proteinogenic amino acid: Any of the 20 canonical which are encoded by the and incorporated into  and ultimately  during. The term may also be inclusive of an additional two amino acids encoded by non-standard which can be incorporated by special translation mechanisms.

proteolysis: The decomposition of into their component or individual  by  the  linking the amino acids together via hydrolysis. Proteolysis is an important reaction used not only for degrading and inactivating proteins but sometimes also to activate them by removing amino acid residues which inhibit their activity. It is usually catalyzed by known as.

proteome: The entire set of that is or can be by a particular, cell, tissue, or species at a particular time (such as during a single lifespan or during a specific developmental stage) or under particular conditions (such as when compromised by a certain disease).

proteomics:

proton motive force:

protoplasm: The biological contents enclosed within a space, variously referring to the, or the cytoplasm and considered collectively, and sometimes exclusive of.

protoplast: A plant, fungal, or bacterial cell which has had its removed by mechanical, chemical, or enzymatic means; or the complete contents (the ) of an intact cell excluding the cell wall.

pulsatile secretion:

purine:

A double-ringed heterocyclic organic compound which, along with, is one of two molecules from which all (including the used in  and ) are derived. (A) and (G) are classified as purines. The letter R is sometimes used to indicate a generic purine; e.g. in a nucleotide sequence read, R may be used to indicate that either purine nucleobase, A or G, can be substituted at the indicated position.

putative gene: A specific suspected to be a functional based on the identification of its. The gene is said to be "putative" in the sense that no function has yet been described for its.

pyknosis:

The irreversible condensation of inside the as the cell undergoes  or, resulting in a compact mass which  strongly and is conspicuous under a microscope. It is followed by.

pyrimidine:

A single-ringed heterocyclic organic compound which, along with, is one of two molecules from which all (including the used in  and ) are derived. (C), (T), and  (U) are classified as pyrimidines. The letter Y is sometimes used to indicate a generic pyrimidine; e.g. in a nucleotide sequence read, Y may be used to indicate that either pyrimidine nucleobase – C, T, or U – can be substituted at the indicated position.

pyrimidine dimer: A type of caused by photochemical damage to or, whereby exposure to ultraviolet (UV) radiation induces the formation of covalent bonds between   occupying adjacent positions in the same polynucleotide, which in turn may cause local conformational changes in  and prevent  with the opposite strand. In DNA, the dimerization reaction occurs between neighboring and  residues (T−T, C−C, or T−C); it can also occur between cytosine and  residues in. Pyrimidine dimers are usually quickly corrected by, but uncorrected lesions can inhibit or arrest polymerase activity during or.

pyruvic acid:

Q
quantitative PCR (qPCR):

quiescent culture: A in which there is little or no active cell growth or replication but in which the cells nonetheless continue to survive, as observed with some cultures.

R
random walk: A popular description of the path followed by a or particle when there is no bias in movement, i.e. when the direction of movement at any given instant is not influenced by the direction of movement in the preceding instant. The essential randomness of cell movement in a uniform environment is only apparent over long periods of time, however; in the short term, cells can and do exhibit.

rDNA: An abbreviation of. An abbreviation of.

reading frame: A way of dividing the in a or  molecule into a set of consecutive, non-overlapping, which is how the sequence is interpreted or "read" by  and  during. In coding DNA, each triplet is referred to as a and corresponds to a particular  to be added to the nascent peptide during translation. In general, only one reading frame (the so-called ) in a given section of a nucleic acid can be used to make functional proteins, but there are exceptions in a few organisms. A results in a shift in the normal reading frame and affects all downstream codons.

real-time PCR (rtPCR): See.

reassociation kinetics: The measurement and manipulation of the rate of of strands of, generally by heating and denaturing a  molecule into  and then observing their rehybridization at a cooler temperature. Because the G+C requires more energy to anneal than the base pair A+T, the rate of reannealing between two strands depends partly on their, and it is therefore possible to predict or estimate the sequence of the duplex molecule by the time it takes to fully hybridize. Reassociation kinetics is studied with : fragments reannealing at low C0t values tend to have highly, while higher C0t values imply more unique sequences.

receptor: A protein which initiates a cellular response to an external stimulus or by binding a specific extracellular, often a dedicated signaling molecule. Numerous types of receptors exist which serve an enormous variety of functions. Cell-surface receptors, such as those that bind acetylcholine and insulin, are embedded within the with their  exposed to the extracellular space; intracellular receptors, including many  receptors, are located in the cytoplasm, where they bind ligands that have diffused across the membrane and into the cell.

reciprocal translocation: A type of by which there is a reciprocal exchange of chromosome segments between two or more non-. When the exchange of material is evenly balanced, reciprocal translocations are usually harmless.

recombinant DNA (rDNA): Any molecule in which laboratory methods of have brought together genetic material from multiple sources, thereby creating a  that would not otherwise be found in a naturally occurring genome. Because DNA molecules from all organisms share the same basic chemical structure and properties, DNA sequences from any species, or even sequences created de novo by, may be incorporated into recombinant DNA molecules. Recombinant DNA technology is widely used in.

recombinase:

recombination: See ', ', and.

recombinator: Any that increases the likelihood of in nearby regions of the genome, e.g. the Chi sequence in certain species of bacteria.

recon: The smallest unit of a DNA molecule capable of undergoing, i.e. a pair of consecutive nucleotides, adjacent to each other in.

regulon: A group of non-contiguous which are as a unit, generally by virtue of having their  controlled by the same regulatory element or set of elements, e.g. the same  or. The term is most commonly used with prokaryotes, where a regulon may consist of genes from multiple.

repeat:

repetitive DNA:

A region or fragment of DNA consisting largely or entirely of.

replacement mutation: See.

replication: The process by which certain biological molecules, notably the and, produce copies of themselves. A technique used to estimate technical and biological variation in experiments for statistical analysis of data. Replicates may be technical replicates, such as dye swaps or repeated array, or biological replicates, biological samples from separate experiments which are used to test the effects of the same experimental treatment.

replication eye:

The eye-shaped structure that forms when a pair of, each growing away from the, separates the strands of the double helix during.

replication fork:

The point at which the paired strands of a molecule are separated by during, breaking the hydrogen bonds between the complementary strands and thereby forming a structure with two branching single strands of DNA. Once unpaired, these strands serve as from which  synthesizes the  and. As replication proceeds, helicase moves along the DNA and continues to separate the strands, causing the replication fork to move as well. A pair of replication forks forms when helicases work in opposite directions from a single, creating a.

replication rate: The speed at which are incorporated into an elongating chain by during ; or more generally the speed at which any chromosome, genome, cell, or organism makes a complete, independently functional copy of itself.

replicator: Any fragment or region of DNA that contains a. Any molecule or structure capable of ; namely,, but also crystals of many minerals, e.g. kaolinite.

replicon: Any molecule or region of or that  from a single.

replisome: The entire complex of molecular machinery that carries out the process of, including all proteins, nucleic acids, and other molecules which participate at an active.

reporter: An -compliant term to describe a reagent used to make a single measurement in a gene expression experiment. MIAME defines it as "the nucleotide sequence present in a particular location on the array". A reporter may be a segment of single-stranded DNA that is covalently attached to the array surface. See also.

reporter gene:

repression: See.

repressor: A that inhibits the of one or more  by binding to the  and blocking the attachment of  to the, thus preventing. This process is known as or.

rescue: The restoration of a defective cell or tissue to a healthy or normal condition, or the or recovery of a mutant gene to its normal functionality, especially in the context of experimental genetics, where an experiment (e.g. a drug,, or gene transfer) resulting in such a restoration is said to rescue the normal.

residue: An individual or of a larger polymeric ; e.g. a  is composed of  residues, and a  or  is composed of  residues.

response element: A short sequence of DNA within a region that is able to bind specific in order to   of specific genes.

restitution: The spontaneous rejoining of an experimentally broken which restores the original configuration.

restitution nucleus: A containing twice the expected number of chromosomes owing to an error in cell division, especially an unreduced, product of  resulting from the failure of the first or second meiotic division.

restriction cloning: The use of and the specific for them in order to   molecules.

restriction enzyme:

An or  that recognizes and cleaves a nucleic acid molecule into  at or near specific recognition sequences known as  by breaking the  of the nucleic acid. Restriction enzymes are naturally occurring in many organisms, but are also routinely used for artificial modification of DNA in laboratory techniques such as.

restriction fragment: Any DNA fragment that results from the cutting of a DNA strand by a at one or more.

restriction fragment length polymorphism (RFLP):

restriction map: A diagram of known within a known DNA sequence, such as a, obtained by systematically exposing the sequence to various and then comparing the lengths of the resulting , a technique known as restriction mapping. See also.

restriction site:

A short, specific of nucleotides (typically 4 to 8 bases in length) that is reliably recognized by a particular. Because restriction enzymes usually bind as homodimers, restriction sites are generally spanning both strands of a  molecule. Restriction cleave the  between two nucleotides within the recognized sequence itself, but other types of restriction enzymes make their cuts at one end of the sequence or at a nearby sequence.

reverse genetics: An experimental approach in in which a researcher starts with a known and attempts to determine its function or its effect on phenotype by any of a variety of laboratory techniques, commonly by deliberately mutating the gene's nucleic acid sequence or by repressing or silencing its expression and then  the mutated organisms for obvious changes in phenotype. When the gene of interest is the only one in the genome whose expression has been manipulated, any observed phenotypic changes are assumed to be influenced by it. This is the opposite of, in which a known phenotype is linked to one or more unknown genes.

reverse transcriptase (RT): An enzyme capable of synthesizing a molecule from an template, a process termed.

reverse transcription: The synthesis of a molecule from an, the opposite of ordinary. This process, mediated by the enzyme, is used by many viruses to their genomes, as well as by  and in eukaryotic cells.

ribonuclease (RNase): Any of a class of enzymes which catalyze the hydrolytic cleavage of in  molecules, thus severing polymeric  of  into smaller components. Compare.

ribonucleic acid (RNA): A polymeric molecule composed of a series of which incorporate a set of four :  (A),  (G),  (C), and  (U). Unlike, RNA is more often found as a folded onto itself, rather than a paired double strand. Various types of RNA molecules serve in a wide variety of essential biological roles, including, , , and , as well as functioning as signaling molecules and, in certain viral genomes, as the primary genetic material itself.

ribonucleoprotein (RNP): A that is a complex of one or more molecules and one or more. Examples include and the enzyme ribonuclease P.

ribonucleotide: A containing as its pentose sugar component, and the monomeric subunit of  (RNA) molecules. Ribonucleotides canonically incorporate any of four : (A),  (G),  (C), and  (U). Compare.

ribonucleotide reductase (RNR):

An enzyme which catalyzes the formation of via the reductive dehydroxylation of, specifically by removing the 2' hydroxyl group from the ring of  (rNDPs). RNR plays a critical role in regulating the overall rate of DNA synthesis such that the ratio of DNA to cell mass is kept constant during cell division and.

ribose: A sugar which, as D-ribose in its pentose ring form, is one of three primary components of the from which  (RNA) molecules are built. Ribose differs from its structural analog only at the 2' carbon, where ribose has an attached hydroxyl group that deoxyribose lacks.

ribosomal DNA (rDNA): A DNA sequence that codes for (rRNA). In many eukaryotic genomes, rDNA occupies large, highly conserved regions of multiple chromosomes and is rich in both and.

ribosomal RNA (rRNA): A type of which is found in and is the primary constituent of, binding to ribosomal proteins to form the and. It is ribosomal RNA which enables ribosomes to perform protein synthesis by working as a that catalyzes the set of reactions comprising. Ribosomal RNA is transcribed from the corresponding (rDNA), and is the predominant form of RNA found in most cells, effecting the translation of all encoded proteins despite never being translated itself.

ribosome: A macromolecular complex made of both RNA and protein which serves as the site of. Ribosomes have two, each of which consists of one or more strands of bound to various ribosomal proteins: the, which reads the messages encoded in molecules, and the , which links  in sequence to form a  chain. Ribosomes are fundamental and ubiquitous in all cell types and are used by all known forms of life.

riboswitch: A sequence within a  that can bind a small effector molecule, preventing or disrupting  and thereby acting as a switch that regulates the mRNA's.

ribozyme: An molecule with enzymatic activity, i.e. one that is capable of catalyzing one or more specific biochemical reactions, similar to. Ribozymes function in numerous capacities, including in as part of the large subunit.

RNA: See.

RNA gene: A that codes for any of the various types of (e.g.  and ).

RNA interference (RNAi):

RNA polymerase:

Any of a class of that synthesize  molecules from a  template. RNA polymerases are essential for and are found in all living organisms and many viruses. They build long single-stranded polymers called by adding  one at a time in the -to- direction, relying on the  provided by the  strand to transcribe the nucleotide sequence faithfully.

RNA splicing:

RNA-induced silencing complex (RISC): A complex which works to endogenous and exogenous genes by participating in various  pathways at the transcriptional and translational levels. RISC can bind both and  fragments and then cleave them or use them as guides to target complementary mRNAs for degradation.

RNase: See.

Robertsonian translocation (ROB): A type of by which at or near the  of two   cause a reciprocal exchange of segments that gives rise to one large  chromosome (composed of the ) and one extremely small chromosome (composed of the ), the latter of which is often subsequently lost from the cell with little effect because it contains very few genes. The resulting shows one fewer than the expected total number of chromosomes, because two previously distinct chromosomes have essentially fused together. of Robertsonian translocations are generally not associated with any phenotypic abnormalities, but do have an increased risk of generating meiotically unbalanced.

rolling circle replication (RCR):

rough endoplasmic reticulum (RER):

rRNA: See.

rtPCR: An abbreviation of real-time polymerase chain reaction, synonymous with. An abbreviation of.

S
S phase:

The phase of the during which is, occurring after the  phase and before the  phase.

samesense mutation: See.

Sanger sequencing: A method of based on the in vitro of a DNA  sequence, during which fluorochrome-labeled, chain-terminating dideoxynucleotides are randomly incorporated in the elongating strand; the resulting fragments are then sorted by size with, and the particular fluorochrome terminating each of the size-sorted fragments is detected by laser chromatography, thus revealing the  of the original DNA template through the order of the fluorochrome labels as one reads from small-sized fragments to large-sized fragments. Though Sanger sequencing has been replaced in some contexts by methods, it remains widely used for its ability to produce relatively long sequence reads (500+ ) and its very low error rate.

saturation hybridization: An nucleic acid reaction in which one polynucleotide component (either  or ) is supplied in great excess relative to the other, causing all complementary sequences in the other polynucleotide to pair with the excess sequences and form hybrid  molecules.

scaffolding:

scRNA: See.

second-generation sequencing: See.

selectable marker:

selfish genetic element:

Any genetic material (e.g. a or any other DNA sequence) which can enhance its own and/or transmission into subsequent generations at the expense of other genes in the genome, even if doing so has no positive effect or even a net negative effect on the  of the genome as a whole. Selfish elements usually work by producing self-acting which repeatedly  their own  into other parts of the genome, independently of normal  (as with ); by facilitating the uneven swapping of chromosome segments during  events (as with ); or by disrupting the normally equal redistribution of replicated material during  or  such that the probability that the selfish element is present in a given  is greater than the normal 50 percent (as with ).

semiconservative replication: The standard mode of that occurs in all living cells, in which each of the two parental of the original  molecule are used as, with  replicating each strand separately and simultaneously in  directions. The result is that each of the two double-stranded daughter molecules is composed of one of the original parental strands and one newly synthesized complementary strand, such that each daughter molecule conserves the precise sequence of information (indeed the very same atoms) from one-half of the original molecule. Contrast ' and '.

sense: A distinction made between the individual of a molecule in order to easily and specifically identify each strand. The two strands are distinguished as sense and antisense or, equivalently, the ' and the '. It is the antisense/template strand which is actually used as the template for ; the sense/coding strand merely resembles the sequence of on the RNA transcript, which makes it possible to determine from the DNA sequence alone the expected amino acid sequence of any protein  from the RNA transcript. Which strand is which is relative only to a particular RNA transcript and not to the entire DNA molecule; that is, either strand can function as the sense/coding or antisense/template strand.

sense codon: Any that specifies an, as opposed to a, which does not specify any particular amino acid but instead signals the end of translation.

sequence: See.

sequence logo: In, a graphical representation of the of or  at each position within a nucleic acid or protein. Sequence logos are created by many sequences and used to depict  as well as the degree of  within the pool of aligned sequences.

sequence-tagged site (STS): Any DNA that occurs exactly once within a particular, and whose and nucleotide sequence are known with confidence.

sequencing: The determination of the order or of in a  molecule, or of  in a, by any means. Sequences are usually written as a linear string of letters which conveniently summarizes much of the atomic-level structure of the molecule.

sex chromosome: See.

sex linkage:

Shine–Dalgarno sequence: In many prokaryotic, the AGGAGGU, located 6–8 bases upstream of the translation, which functions as a binding site for the by complementing a sequence in the.

short arm:

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

short interspersed nuclear element (SINE):

short tandem repeat (STR): See.

shotgun sequencing:

signal transduction:

silencer: A sequence or region of DNA that can be bound by a, thereby blocking the of a nearby gene.

silencing:

silent allele: An that does not produce a detectable. Compare.

silent mutation: A type of which does not have an observable effect on the organism's. Though the term "silent mutation" is often used interchangeably with, synonymous mutations are not always silent, nor vice versa. which result in a different but one with similar functionality (e.g. leucine instead of isoleucine) are also often classified as silent, since such mutations usually do not significantly affect protein function.

simple sequence repeat (SSR): See.

single-nucleotide polymorphism (SNP): Any of a single which occurs at a specific position within a  and with measurable frequency within a population; for example, at a specific base position in a DNA sequence, the majority of the individuals in a population may have a  (C), while in a minority of individuals, the same position may be occupied by an  (A). SNPs are usually defined with respect to a "standard" reference genome; an individual human genome differs from the reference human genome at an average of 4 to 5 million positions, most of which consist of SNPs and short. See also.

single-strand break (SSB): The loss of continuity of the in one of a. See also '; contrast '.

single-stranded: Composed of a single, unpaired molecule, i.e. one linear of  sharing a single, as opposed to a  of two such strands joined by base pairing. See also ' and '.

single-stranded DNA (ssDNA): Any molecule that consists of a single nucleotide polymer or, as opposed to a pair of complementary strands held together by hydrogen bonds. In most circumstances, DNA is more stable and more common in double-stranded form, but high temperatures, low concentrations of dissolved salts, and very high or low pH can cause double-stranded molecules to decompose into two single-stranded molecules in a process known as ; this reaction is exploited by naturally occurring enzymes such as those involved in  as well as by laboratory techniques such as.

siRNA: See.

sister chromatids: A pair of identical copies produced as the result of the of a, particularly when both copies are joined together by a common ; the pair of sister chromatids is called a dyad. The two sister chromatids are ultimately separated from each other into two different cells during or.

site-directed mutagenesis:

small conditional RNA (scRNA): A class of small molecules engineered so as to change conformation conditionally in response to cognate molecular inputs, often with the goal of controlling pathways ' or '.

small interfering RNA (siRNA):

small nuclear RNA (snRNA):

small nucleolar RNA (snoRNA): A class of small molecules whose primary function is to direct the chemical modification of other RNAs, mainly (tRNA),  (snRNA), and especially  (rRNA) as a part of  synthesis in the. SnoRNAs contain sequences that complement sequences within these target RNAs and guide  complexes to them, which can then catalyze specific nucleoside modifications, typically  or.

small temporal RNA (stRNA): A subclass of, originally described in nematodes, which regulate the timing of developmental events by binding to complementary sequences in the of and inhibiting their. In contrast to, which serve similar purposes, stRNAs bind to their target mRNAs after the initiation of translation and without affecting mRNA stability, which makes it possible for the target mRNAs to resume translation at a later time.

smooth endoplasmic reticulum (SER):

snoRNA: See.

snRNA: See.

solenoid fiber:

soluble RNA (sRNA): See.

somatic cell:

Any biological cell forming the body of an organism, or, in multicellular organisms, any cell other than a,, or undifferentiated. Somatic cells are theoretically distinct from cells of the, meaning the they have undergone can never be transmitted to the organism's descendants, though in practice exceptions do exist.

somatic cell nuclear transfer (SCNT):

somatic crossover: See.

Southern blotting: A method used for detecting a specific in  samples. The technique combines separation of DNA fragments by, transfer of the DNA to a synthetic membrane, and subsequent identification of target fragments with radio- or fluorescent.

spacer:

Any sequence or region of separating neighboring, whether or not. The term is used in particular to refer to the non-coding regions between the many repeated copies of the genes. See also.

spatially-restricted gene expression: The of one or more genes only within a specific anatomical region or tissue, often in response to a paracrine signal. The boundary between the jurisdictions of two spatially restricted genes may generate a sharp gradient there, as with striping patterns.

spectral karyotype (SKY):

spindle apparatus:

spliceosome:

splicing: See.

split-gene:

sRNA: See.

ssDNA: See.

ssRNA: See.

stable uncharacterized transcript (SUT):

standard genetic code: The used by the vast majority of living organisms for  into. In this system, of the 64 possible permutations of that can be made from the four, 61 code for one of the 20, and the remaining three code for. For example, the codon CAG codes for the amino acid glutamine and the codon is a stop codon. The standard genetic code is described as or redundant because some amino acids can be coded for by more than one different codon.

start codon: The first by a  from a mature  transcript, used as a signal to initiate  synthesis. In the, the start codon always codes for the same, methionine, in eukaryotes and for a modified methionine in prokaryotes. The most common start codon is the triplet AUG. Contrast.

statistical genetics: A branch of genetics concerned with the development of statistical methods for drawing inferences from genetic data. The theories and methodologies of statistical genetics often support research in,, and.

stem cell: Any biological cell which has not yet into a specialized cell type and which can divide through to produce more stem cells.

stem-loop:

sticky end: A term used to describe the end of a molecule where one is longer than the other by one or more, creating a single-stranded "overhang" of unpaired bases, in contrast to a so-called "", where no such overhang exists because the terminal nucleobases on each strand are  with each other. Blunt ends and sticky ends are relevant when multiple DNA molecules, e.g. in, because many  cut DNA in a way that leaves behind terminal overhangs in the digested fragments. These sticky-ended molecules ligate much more readily with other sticky-ended molecules having overhangs, allowing scientists to ensure that specific DNA fragments are ligated together in specific places.

stop codon:

A that signals the termination of protein synthesis during of a  transcript. In the, three different stop codons are used to dissociate from the growing amino acid chain, thereby ending translation: UAG (nicknamed "amber"), UAA ("ochre"), and UGA ("opal"). Contrast.

strand: An individual chain of comprising a polymer, existing either singly (in which case the nucleic acid molecule is said to be single-stranded) or  in a  (in which case it is said to be double-stranded).

stringency: The effect of conditions such as temperature and pH upon the degree of that is required for a reaction to occur between two single-stranded nucleic acid molecules. In the most stringent conditions, only exact complements can successfully hybridize; as stringency decreases, an increasing number of can be tolerated by the two hybridizing strands.

stRNA: See.

structural gene: A that codes for any protein or RNA other than a. Structural gene products include,, and certain.

structural protein: A which as its primary function contributes to the mechanical shape and structure of,, or (e.g. collagen and actin), as distinguished from proteins which serve some other purpose, such as. This distinction is not well-defined, however, as many proteins have both structural and non-structural roles.

subcellular localization: The subdivision of the interior of a cell into functionally distinct spaces or (e.g. ) and the localization or delegation of particular cellular functions and activities to these particular spaces; or the determination by any of various laboratory methods (e.g. fluorescent ) of the precise location(s) within a cell where a specific molecule has occupancy, or at which a specific activity occurs.

submetacentric: (of a linear or chromosome fragment) Having a positioned close to but not exactly in the middle of the chromosome, resulting in  arms of slightly different lengths. Compare.

substitution: A type of in which a single and its attached  is replaced by a different nucleotide.

substrate: A chemical compound or molecule upon which a particular directly acts, often but not necessarily binding the molecule by forming one or more chemical bonds. See also. The substance, biotic or abiotic, upon which an organism grows or lives, or by which it is supported; e.g. a particular used in. See also.

substratum: A solid surface to which a cell or organism adheres or by which it is supported, or over which it moves. See also.

subunit: A single unit of a multi-unit compound or molecular aggregate; e.g. a from which a larger is composed (as with  in ), or an individual  chain in a multi-chain, or an entire protein which participates alongside other proteins as part of a.

supercoiling:

suppression: See.

suspension culture: A type of in which individual or aggregates of cells are suspended in a gently agitated liquid. Many prokaryotic and eukaryotic cell types readily proliferate in suspension cultures, but they are particularly useful for culturing non-adherent cell lines such as hematopoietic cells, plant cells, and insect cells. Compare.

swivel point:

symporter: Any of a class of which facilitate the transport of two or more different molecules across the membrane at the same time and in the same direction; e.g. glucose and sodium ions. Contrast ' and '.

synapsis:

synaptonemal complex: A complex of scaffolding proteins that mediates and between the chromatids of homologous chromosomes during  of.

syncytium:

A cell, i.e. a cell containing more than one or, in the broadest sense, more than one  (a meaning which is equated with ). Syncytia may form as a result of between uninucleate cells, migration of a nucleus from one cell to another, or multiple nuclear divisions without accompanying  (forming a ). The term may also refer to cells which are interconnected by specialized membranes with as in some neuromuscular cell types.

syndesis: The of chromosomes during.

synezis: The aggregation of into a dense knot that adheres to one side of the, commonly observed during in certain organisms.

synonymous mutation:

A type of in which the of one  base for another results, after  and, in an amino acid sequence which is identical to the original unmutated sequence. This is possible because of the of the, which allows different  to code for the same amino acid. Though synonymous mutations are often considered, this is not always the case; a synonymous mutation may affect the efficiency or accuracy of , , , or any other process by which genes are , and thus become effectively non-silent. Contrast.

synteny:

synthesis phase: See.

T
tandem repeat: A pattern within a in which one or more are repeated and the repetitions are directly adjacent (i.e. tandem) to each other. An example is ATGACATGACATGAC, in which the sequence ATGAC is repeated three times.

TATA box:

A highly conserved sequence containing a of repeating  and  base pairs that is commonly found in  of genes in archaea and eukaryotes. The TATA box often serves as the site of initiation of or as a binding site for.

taxis:

TCA: See.

telestability: Structural destabilization of the at a  that is relatively distant from the site of binding of a.

telocentric: (of a linear or chromosome fragment) Having a positioned at the terminal end of the chromosome (near or within the ), resulting in only a single arm. Compare.

telomere: A region of at each end of a linear  which protects the end of the chromosome from deterioration and from fusion with other chromosomes. Since each round of results in the shortening of the chromosome, telomeres act as disposable buffers which are sacrificed to perpetual truncation instead of nearby genes; telomeres can also be lengthened by the enzyme telomerase.

telomeric silencing: The of of genes in regions adjacent to. Telomeres also appear to reduce the accessibility of subtelomeric to modification by DNA.

telophase: The final stage of cell division in both and, occurring after and before or simultaneously with, during which a nuclear membrane is synthesized around each set of ,  are reassembled, and the  is disassembled. Following cytokinesis, the new daughter cells resume.

template strand:

The of a molecule which is used as a template for RNA synthesis during. The sequence of the template strand is to the resulting RNA transcript. Contrast '; see also '.

terminalization: In cytology, the progressive shift of from their original to more distal positions as proceeds through  and.

termination codon: See.

terminator: A DNA or its RNA which signals the termination of  by triggering processes that ultimately arrest the activity of  or otherwise cause the release of the RNA  from the transcriptional complex. Terminator sequences are usually found near the ends of the of  and. They generally function after being themselves transcribed into the nascent RNA strand, whereupon the part of the strand containing the sequence either directly interacts with the transcriptional complex or forms a such as a  which signals the recruitment of enzymes that promote its disassembly.

tetramer: A molecular aggregate consisting of four. The term is often used to refer to composed of four proteins, e.g. haemoglobin, or to individual proteins composed of four. Compare ', ', and.

three-prime end: See.

three-prime untranslated region: See.

thymidine:

One of the four standard used in molecules, consisting of a   with its N9 nitrogen  to the C1 carbon of a  sugar. The prefix deoxy- is commonly omitted, since there are no ribonucleoside analogs of thymidine used in RNA, where it is replaced with instead.

thymine:

A used as one of the four standard nucleobases in  molecules. Thymine forms a with. In, thymine is not used at all, and is instead replaced with.

thymine dimer: See.

tissue:

tissue culture:

tissue-specific gene expression: Gene function and which is restricted to a particular tissue or cell type. Tissue-specific expression is usually the result of an which is activated only in the proper cell type.

tonicity:

tonoplast: See.

topoisomerase: Any of a class of enzymes which catalyze changes in the topological state of a molecule by  or  the  of one or both strands, relaxing the torsional stress inherent in the  and unwinding or untangling the paired strands before  the nicks. This process is usually necessary prior to and. Topoisomerases thereby convert DNA between its and, linked and unlinked, and knotted and unknotted forms without changing the sequence or overall chemical composition, such that the substrate and product molecules are structural isomers, differing only in their shape and their, , and/or.

totipotency: A state of in which a cell or nucleus fully retains the ability to into all of the cell types represented in the adult organism, or to give rise to all of these cell types upon  into an appropriate cytoplasm (as in ). Such cells or nuclei are said to be totipotent. The that serves as the progenitor cell for sexually reproducing multicellular organisms is the archetypal totipotent cell; almost all of the cells into which it ultimately differentiates are not totipotent, though some cells such as  remain totipotent or  throughout the organism's life.

tracer: A molecule or a specific atom within a molecule that has been chemically or radioactively so that it can easily be tracked or followed through a biochemical process or located in a cell or tissue.

trailer sequence: See.

trans: On the opposite side; across from; from a different molecule. Contrast.

trans-acting: Affecting a or sequence on a different nucleic acid molecule or. A or sequence within a particular DNA molecule such as a  is said to be trans-acting if it or its  influence or act upon other sequences located relatively far away or on an entirely different molecule or chromosome. For example, a acts "in trans" if it binds to or interacts with a sequence located on any strand or molecule different from the one on which it is encoded. Contrast.

transcribed spacer: A sequence that is transcribed and thus included in the primary  (as opposed to a ) but subsequently excised and discarded during the maturation of functional RNAs of the.

transcript: A product of ; that is, any molecule which has been synthesized by using a complementary  molecule as a. When transcription is completed, transcripts separate from the DNA and become independent. Particularly in eukaryotes, multiple are usually necessary for raw transcripts to be converted into stable and persistent molecules, which are then described as mature, though not all transcribed RNAs undergo maturation. Many transcripts are accidental, spurious, incomplete, or defective; others are able to perform their functions immediately and without modification, such as certain.

transcript of unknown function (TUF):

transcriptase: See.

transcription: The first step in the process of, in which an molecule, known as a, is synthesized by enzymes called using a  or other  sequence as a. Transcription is a critical and fundamental process in all living organisms and is necessary in order to make use of the information encoded within a. All classes of RNA must be transcribed before they can exert their effects upon a cell, though only (mRNA) must proceed to  before a functional  can be produced, whereas the many types of  fulfill their duties without being translated. Transcription is also not always beneficial for a cell: when it occurs at the wrong time or at a, or when or infectious pathogens utilize the host's transcription machinery, the resulting transcripts (not to mention the waste of valuable energy and resources) are often harmful to the host cell or genome.

transcription factor (TF): Any that controls the rate of of genetic information from  to  by binding to a specific  and  or  the recruitment of  to nearby. Transcription factors can effectively turn "on" and "off" specific genes in order to make sure they are at the right times and in the right places; for this reason, they are a fundamental and ubiquitous mechanism of.

transcription start site (TSS):

The specific location within a at which begins, defined by the specific nucleotide or codon corresponding to the first ribonucleotide(s) to be assembled in the  transcript (which is not necessarily the same as the  to be ). This site is usually considered the beginning of the and is the reference point for numbering the individual nucleotides within a gene. Nucleotides of the start site are assigned negative numbers and those  are assigned positive numbers, which are used to indicate the positions of nearby sequences or structures relative to the TSS. For example, the for RNA polymerase might be a short sequence immediately upstream of the TSS, from approximately -80 to -5, whereas an  within the coding region might be defined as the sequence starting at nucleotide +207 and ending at nucleotide +793.

transcription unit: The segment of DNA between the and the termination site of, containing the for one or more. All genes within a transcription unit are transcribed together into a single transcript during a single transcription event; the resulting RNA may subsequently be cleaved into separate RNAs, or may be  as a unit and then cleaved into separate polypeptides.

transcriptional bursting: The intermittent nature of and mechanisms. Both processes occur in "bursts" or "pulses", with periods of gene activity separated by irregular intervals.

transcriptome: The entire set of molecules (often referring to all types of RNA but sometimes exclusively to ) that is or can be by a particular, cell, population of cells, or species at a particular time or under particular conditions. The transcriptome is distinct from the and the.

transcriptomics:

transductant: A cell which has undergone and been successfully transduced.

transduction: The transfer of genetic material between cells by a virus or viral vector, either naturally or artificially.

transfectant: A cell which has undergone and been successfully transfected.

transfection: The deliberate experimental introduction of exogenous into a cell or embryo. In the broadest sense the term may refer to any such transfer and is sometimes used interchangeably with, though some applications restrict the usage of transfection to the introduction of naked or purified non-viral or  into cultured eukaryotic cells (especially animal cells) resulting in the subsequent incorporation of the foreign DNA into the host  or the non-hereditary modification of  by the foreign RNA. As a contrast to both standard non-viral transformation and, transfection has also occasionally been used to refer to the uptake of purified viral nucleic acids by bacteria or plant cells without the aid of a viral vector.

transfer RNA (tRNA):

A special class of molecule, typically 76 to 90 in length, that serves as a physical adapter allowing  transcripts to be  into sequences of  during protein synthesis. Each tRNA contains a specific triplet corresponding to an amino acid that is covalently attached to the tRNA's opposite end; as translation proceeds, tRNAs are recruited to the, where each mRNA  is paired with a tRNA containing the complementary anticodon. Depending on the organism, cells may employ as many as 41 distinct tRNAs with unique anticodons; because of within the, several tRNAs containing different anticodons carry the same amino acid.

transferase: Any of a class of which the chemical transfer of a functional group or substituent from one molecule to another. For example, acetyltransferases catalyze the movement of an acetyl group in a process known as ; methyltransferases catalyze the movement of one or more methyl groups in a process known as.

transfer-messenger RNA (tmRNA): A type of RNA molecule in some bacteria which has dual -like and -like properties, allowing it to simultaneously perform a number of different functions during.

transformant: A cell or organism which has taken up extracellular DNA by and which can express genes encoded by it.

transformation:

transgene: Any or other segment of genetic material that has been isolated from one organism and then transferred either naturally or by any of a variety of techniques into another organism, especially one of a different species. Transgenes are usually introduced into the second organism's. They are commonly used to study gene function or to confer an advantage not otherwise available in the unaltered organism.

transition: A in which a nucleotide is substituted for another purine ( ↔ ) or a  nucleotide is substituted for another pyrimidine ( ↔ ). Contrast.

translation: The second step in the process of, in which the transcript produced during is read by a  to produce a functional.

translatome: The entire set of molecules that are by a particular, cell, tissue, or species at a particular time or under particular conditions. Like the, it is often used as a proxy for quantifying levels of , though the transcriptome also includes many RNA molecules that are never translated.

translocation: A type of chromosomal abnormality caused by the structural rearrangement of large sections of one or more. There are two main types: and.

transmembrane protein: See.

transmission genetics: The branch of genetics that studies the mechanisms involved in the transfer of genes from parents to offspring.

transport protein:

Any which functions by permitting the movement of particular molecules, proteins, or other substances across a, either or  and in either or both directions (by which they may be further subclassified into, , and ). and are examples of transport proteins.

transporter: See.

transposable element (TE):

transposase: Any of a class of self-acting capable of to the flanking sequences of the  which encodes them and catalyzing its movement to another part of the genome, typically by an / mechanism or a replicative mechanism, in a process known as.

transposition: The process by which a nucleic acid sequence known as a changes its position within a, either by and re-inserting itself at a different  (cut-and-paste) or by  itself and inserting into another locus without moving the original element from its original locus (copy-paste). These reactions are catalyzed by an enzyme known as a which is encoded by a gene within the transposable element itself; thus the element's products are self-acting and can autonomously direct their own replication. Transposed sequences may re-insert at random loci or at sequence-specific targets, either on the same DNA molecule or on different molecules.

trans-splicing:

transvectant: A cell which has undergone and been successfully transvected.

transvection:

transversion: A in which a nucleotide is substituted for a  nucleotide, or vice versa (e.g.  ↔  or  ↔ ). Contrast.

tricarboxylic acid cycle (TCA): See.

triglyceride:

Any of a class of chemical compounds which are ester derivatives of glycerol, consisting of a glycerol backbone connected to any three substituents via ester bonds. Triglycerides are one of three major classes of esters formed by fatty acids in biological systems, along with and cholesteryl esters. They are the primary constituent of tissue in vertebrates.

trimer: A molecular aggregate consisting of three. The term is often used to refer to composed of three proteins, e.g. many membrane porins, or to individual proteins composed of three. Compare ', ', and.

trinucleotide repeat: Any sequence in which an individual nucleotide is many times, whether in a gene or non-coding sequence. At most some degree of repetition is normal and harmless, but mutations which cause specific triplets (especially those of the form n) to increase in  above the normal range are highly unstable and responsible for a variety of.

triplet: A unit of three successive in a or  molecule. A triplet within a coding sequence that codes for a specific amino acid is known as a.

trisomy: A type of in which a cell or organism has three copies of a particular  instead of the normal two.

tRNA: See.

tRNA-ligase: See.

tropism:

The directional growth or movement of a cell or organism in response to a stimulus, e.g. light, heat, the pull of gravity, or the presence of a particular chemical, such that the response is dependent on the direction of the stimulus (as opposed to a non-directional ). Positive tropism is growth or movement toward the stimulus; negative tropism is away from the stimulus. See also ' and '.

turgor pressure:

twisting number:

U
ubiquitin:

ubiquitination: The of a biomolecule (often another protein) by covalently attaching a protein to it, thus making it identifiable to molecules capable of recognizing ubiquitin.

umber: See.

uncharged tRNA: A without an attached. Contrast.

underwinding: See.

unequal crossing over:

uniparental inheritance:

uniporter: A type of which catalyzes the movement of a single, specific solute or chemical species across a lipid membrane in either direction. Contrast ' and '.

unique DNA:

A class of DNA determined by to be present only once in the analyzed genome, as opposed to. Most structural genes and their introns are unique.

unstable mutation: A with a high frequency of.

untranslated region (UTR): Any sequence which is transcribed along with a, and thus included within a, but which is not ultimately during protein synthesis. A typical mRNA transcript includes two such regions: one immediately upstream of the coding sequence, known as the (5'-UTR), and one downstream of the coding sequence, known as the  (3'-UTR). These regions are not removed during (unlike ) and are usually considered distinct from the  and the  (both of which are later additions to a primary transcript and not themselves products of transcription). UTRs are a consequence of the fact that transcription usually begins considerably upstream of the of the coding sequence and terminates long after the  has been transcribed, whereas translation is more precise. They often include motifs with regulatory functions.

upregulation:

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

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

upstream activating sequence (UAS):

uracil: A used as one of the four standard nucleobases in  molecules. Uracil forms a with. In, uracil is not used at all, and is instead replaced with.

uridine: One of the four standard used in molecules, consisting of a   with its N9 nitrogen  to the C1 carbon of a  sugar. In, uridine is replaced with.

V
vacuole: Any of a class of, fluid-filled present in many eukaryotic cells as well as bacteria, often large and conspicuous under the microscope and serving any of a huge variety of functions, including acting as a resizable reservoir for the storage of water,, toxins, or foreign material; maintaining cellular and ; supporting immune functions; housing symbiotic bacteria; and assisting in the  of old cellular components.

variable number tandem repeat (VNTR):

variegation: Variation or irregularity in a particular, especially a conspicuous visible such as color or pigmentation, occurring simultaneously in different parts of the same individual organism due to any of a variety of causes, such as, , activity, , or infection by pathogens.

variome:

vector: Any molecule used as a vehicle to artificially transport foreign genetic material into another cell, where it can be and/or. Vectors are typically engineered sequences consisting of an  (often a ) and a longer "backbone" sequence containing an, a, and a. Vectors are widely used in molecular biology laboratories to isolate,, or the insert in the target cell.

vectorization:

vegetal cell: See.

vesicle: Any space completely enclosed by its own, which is separate though usually derived from other membranes (often the ) either by or by mechanical disruption such as. The term is applied to many different structures but especially to the small, roughly spherical created during  and, as well as to  and various other small intracellular or extracellular organelles.

W
Warburg effect:

western blotting:

whole genome sequencing (WGS): The process of the entirety or near-entirety of the DNA sequences comprising an organism's with a single procedure or experiment, generally inclusive of all  and  (e.g. ) DNA.

wild type (WT):

The of the typical form of a species as it occurs in nature; a product of the standard "normal" at a given, as opposed to that produced by a non-standard  allele.

wobble base pairing:

writhing number:

X
X chromosome: One of two present in organisms which use the XY sex-determination system, and the only sex chromosome in the X0 system. The X chromosome is found in both males and females and typically contains much more content than its counterpart, the.

X-inactivation: The process by which one of the two copies of the is silenced by being irreversibly condensed into transcriptionally inactive in the cells of female therian mammals. A form of, X-inactivation prevents females from producing twice as many from genes on the X chromosome as males, who only have one copy of the X chromosome. Which X chromosome is inactivated is randomly determined in the early embryo, making it possible for cell lineages with different inactive Xs to exist in the same organism.

X-linked trait:

Y
Y chromosome: One of two present in organisms which use the XY sex-determination system. The Y chromosome is found only in males and is typically much smaller than its counterpart, the.

Y fork: See.

yeast artificial chromosome (YAC):

Z
Z-DNA:

zinc finger:

zygonema:

In, the second of five substages of, following and preceding. During zygonema, occurs, physically binding  to each other, and the cell's  divides into two daughter centrosomes, each containing a single.

zygosity: The degree to which multiple copies of a,, or have the same genetic sequence; e.g. in a diploid organism with two complete copies of its genome (one maternal and one paternal), the degree of similarity of the present in each copy. Individuals carrying two different alleles for a particular gene are said to be ' for that gene; individuals carrying two identical alleles are said to be ' for that gene. Zygosity may also be considered collectively for a group of genes, or for the entire set of genes and genetic comprising the genome.

zygote: A type of eukaryotic cell formed as the direct result of a fertilization event between two. In multicellular organisms, the zygote is the earliest developmental stage.