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Chromosome

A chromosome is a deoxyribonucleic acid (DNA) molecule with part or all of the genetic material (genome) of an organism. Most eukaryotic chromosomes include packaging proteins which, aided by chaperone proteins, bind to and condense the DNA molecule this prevents it from becoming an unmanageable tangle.[1][2] Chromosomes are normally visible under a light microscope [3] Before this happens, every chromosome is copied once (S phase), and the new copy is joined to the original by a centromere, this results either in an X-shaped structure (pictured here) if the centromere is located in the middle of the chromosome, or a two-arm structure if the centromere is located near one of the ends. The original chromosome and the copy are now called sister chromatids. During metaphase, the X-shape structure is called a metaphase chromosome. In this highly condensed form, chromosomes are easiest to distinguish and study.[4] In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.[5] Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause the progression of cancer.

Etymology[edit source] The word chromosome (/ˈkroʊməˌsoʊm, -ˌzoʊm/[6][7]) comes from the Greek χρῶμα (chroma, "color") and σῶμα (soma, "body"), describing their strong staining by particular dyes.[8] The term was coined by the German scientist von Waldeyer-Hartz,[9] referring to the term chromatin. The term chromatin was introduced by Walther Flemming, who discovered cell division. Some of the early karyological terms have become outdated.[10][11] For example, Chromatin (Flemming 1880) and Chromosome (Waldeyer 1888), both ascribe color to a non-colored state.[12]

History of discovery[edit source] The German scientists Schleiden,[4] Virchow, and Bütschli were among the first scientists who recognized the structures now familiar as chromosomes.[13] In a series of experiments beginning in the mid-1880s, Theodor Boveri gave the definitive demonstration that chromosomes are the vectors of heredity. His two principles, or postulates, were the continuity of chromosomes and the individuality of chromosomes.[citation needed][further explanation needed] It is the second of these principles that were so original.[citation needed] Wilhelm Roux suggested that each chromosome carries a different genetic configuration, and Boveri was able to test and confirm this hypothesis. He was aided by the rediscovery at the start of the 1900s of Gregor Mendel's earlier work, In his famous textbook The Cell in Development and Heredity, Wilson linked together the independent work of Boveri and Sutton (both around 1902) by naming the chromosome theory of inheritance the Boveri–Sutton chromosome theory Ernst Mayr remarks that the theory was hotly contested by some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt, and T.H. Morgan. Eventually, complete proof came from chromosome maps in Morgan's own lab.[16] The number of human chromosomes was published in 1923 by Theophilus Painter. By inspection through the microscope, he counted 24 pairs, which would mean 48 chromosomes. His error was copied by others and it was not until 1956 that the true number, 46, was determined by Indonesia-born cytogeneticist Joe Hin Tjio.[17]

Prokaryotes[edit source] The prokaryotes bacteria and archaea typically have a single circular chromosome, but many variations exist.[18] The chromosomes of most bacteria, which some authors prefer to call genophores, can range in size from only 130,000 base pairs in the endosymbiotic bacteria Candidatus Hodgkinia cicadicola[19] and Candidatus Tremblaya princeps,[20] to more than 14,000,000 base pairs in the soil-dwelling bacterium Sorangium cellulosum.[21] Spirochaetes of the genus Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi, the cause of Lyme disease, containing a single linear chromosome.[22]

structure in sequences[edit source] Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a one-point (the origin of replication) from which replication starts, whereas some archaea contain multiple replication origins.[23] The genes in prokaryotes are often organized in operons and do not usually contain introns, unlike eukaryotes.

DNA packaging[edit source] Prokaryotes do not possess nuclei. Instead, their DNA is organized into a structure called the nucleoid.[24][25] The nucleoid is a distinct structure and occupies a defined region of the bacterial cell. This structure is, however, dynamic and is maintained and remodeled by the actions of a range of histone-like proteins, which associate with the bacterial chromosome.[26] In archaea, the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes.[27][28] Certain bacteria also contain plasmids or other extrachromosomal DNA. These are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer.[4] In prokaryotes (see nucleoids) and viruses,[29] the DNA is often densely packed and organized; in the case of archaea, by homologs to eukaryotic histones, and in the case of bacteria, by histone-like proteins.

Eukaryotes[edit source] Organization of DNA in a eukaryotic cell. See also: Eukaryotic chromosome fine structure Chromosomes in eukaryotes are composed of chromatin fiber. Chromatin fiber is made of nucleosomes (histone octamers with part of a DNA strand attached to and wrapped around it). Chromatin contains the vast majority of DNA and a small amount That is inherited maternally can be found in the mitochondria. Chromatin is present in most cells, with a few exceptions. Such as red blood cells. During cell division, chromatin condenses further to form microscopically visible chromosomes. The structure of chromosomes varies through the cell cycle. During cellular, division chromosomes are replicated, divided, and passed to their daughter cells so as to ensure the genetic diversity and survival of their progeny. The major structures in DNA compaction are DNA, the nucleosome, the 10 nm "beads-on-a-string" fiber, the 30 nm fiber and the metaphase chromosome. Eukaryotes (cells with nuclei such as those found in plants, fungi, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere. Under most circumstances, these arms are not visible as such. In addition, most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have an additional small circular or linear cytoplasmic chromosomes.

Interphase chromatin[edit source] During interphase (the period of the cell cycle where the cell is not dividing), two types of chromatin can be distinguished: •	Euchromatin, which consists of DNA that is active(, e.g., being expressed as protein). •	Heterochromatin, which consists of mostly inactive DNA. It seems to serve structural purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types: o	Constitutive heterochromatin, which is never expressed. It is located around the centromere and usually contains repetitive sequences. o	Facultative heterochromatin, which is sometimes expressed.

Metaphase chromatin and division[edit source] See also: mitosis and meiosis Human chromosomes during metaphase In the early stages of mitosis or meiosis (cell division), the chromatin double helix becomes more and more condensed. They cease to function as accessible genetic material (transcription stops) and become a compact, transportable form. This compact form makes the individual chromosomes visible. They form the classic four-arm structure. A pair of sister chromatids attached to each other at the centromere. The shorter arms are called p arms (from the French petit, small) and the longer arms are called q arms (q follows p in the Latin alphabet; q-g "grande"; alternatively it is sometimes said q is short for queue meaning tail in French[30]). This is the only natural context in which individual chromosomes are visible with an optical microscope. During mitosis, microtubules grow from centrosomes located at opposite ends of the cell and also attach to the centromere at specialized structures called kinetochores, one of which is present on each sister chromatid. A special DNA base sequence in the region of the kinetochores provides longer-lasting attachment in this region. The microtubules then pull the chromatids apart toward the centrosomes, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and DNA can again be transcribed. In spite of their appearance, chromosomes are structurally highly condensed, which enables these giant DNA structures to be contained within a cell nucleus.

Human chromosomes[edit source]

Chromosomes in humans can be divided into two types: autosomes (body chromosome(s)) and allosomes (sex chromosome(s)). Certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes. The autosomes contain the rest of the genetic hereditary information. Both types act in the same way during cell division. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell.

Number in various organisms[edit source] Main article: List of organisms by chromosome count

In eukaryotes[edit source] These tables give the total number of chromosomes (including sex chromosomes) in a cell nucleus. For example, most eukaryotes are diploid, like humans who have 22 different types of autosomes. Each present as two homologous pairs, and two sex chromosomes. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosome types, such as bread wheat, which is hexaploid and has six copies of seven different chromosome types – 42 chromosomes in total.

Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes (see the table). Other eukaryotic chromosomes(,i.e., mitochondrial and plasmid-like small chromosomes,)are much more variable in number, and there may be thousands of copies per cell.

In prokaryotes[edit source]

Prokaryotic species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies.[62] For example, Buchnera, a symbiont of aphids, has multiple copies of its chromosome, ranging from 10–400 copies per cell.[63] However, in some large bacteria, such as Epulopiscium fishelsoni up to 100,000 copies of the chromosome can be present.[64] Plasmids and plasmid-like small chromosomes are, as in eukaryotes, highly variable in copy number. The number of plasmids in the cell is almost entirely determined by the rate of division of the plasmid – fast division causes a high copy number.

Karyotype[edit source] Main article: Karyotype Karyogram of a human male The karyotype illustrated the number of chromosomes and how many chromosomes look under a microscope. The technique of determining the karyotype is usually called karyotyping. Cells can be locked part-way through division (in metaphase) in vitro (in a reaction vial) with colchicine. These cells are then stained, photographed, and arranged into a karyogram, with the set of chromosomes arranged. Autosomes are arranged in order of length, and sex chromosomes (here X/Y) are at the end. Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes). These are XX in females and XY in males.

Fluorescent in Situ Hybridization FISH is a method used to identify a specific location on chromosomes with a high degree of complementarity. FISH is very useful to medicine since it can tell us whether chromosome location is mutated or not.

Reference : https://en.wikipedia.org/wiki/Karyotype http://www.abnova.com/support/technologies.asp?switchfunctionid={F4D881FA-9F54-46D4-9A64-FEA6F0667799}&gclid=Cj0KCQiAq97uBRCwARIsADTziyaNNhYMWxoZ4Nhm5ZvOAf-3TnQ2WkAPj7nGJyGnkoUHkRu4dJ0sck0aAm1BEALw_wcB

https://www.mechanobio.info/genome-regulation/what-is-chromatin-heterochromatin-and-euchromatin/