Chromosome

Diagram of a replicated and condensed metaphase eukaryotic chromosome. (1) Chromatid – one of the two identical parts of the chromosome after S phase. (2) Centromere – the point where the two chromatids touch. (3) Short (p) arm. (4) Long (q) arm.

A chromosome (from ancient Greek: χρωμόσωμα, chromosoma, chroma means color, soma means body) is a DNA molecule with part or all of the genetic material ( genome) of an organism.

Chromosomes are normally visible under a light microscope only when the cell is undergoing the metaphase of cell division. Before this happens, every chromosome is copied once ( S phase), and the copy is joined to the original by a centromere, resulting in an X-shaped structure. The original chromosome and the copy are now called sister chromatids. During metaphase, when a chromosome is in its most condensed state, the X-shape structure is called a metaphase chromosome. In this highly condensed form chromosomes are easiest to distinguish and study. [1]

Chromosomes vary widely between different organisms. Some species such as certain bacteria, which lack histones, 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. [1] In prokaryotes (see nucleoids) and viruses, [2] the DNA is often densely packed and organized; in the case of archaea, by homology to eukaryotic histones, and in the case of bacteria, by histone-like proteins.

DNA condensation of the duplicated chromosomes during cell division ( mitosis or meiosis) results either in a four-arm structure (pictured to the right) 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. 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 and die, or it may unexpectedly evade apoptosis, leading to the progression of cancer.

Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. However, others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation.

History of discovery

Walter Sutton (left) and Theodor Boveri (right) independently developed the chromosome theory of inheritance in 1902.

The word chromosome ( m/ [3] [4]) comes from the Greek χρῶμα (chroma, "colour") and σῶμα (soma, "body"), describing their strong staining by particular dyes. [5]

Schleiden, [1] Virchow and Bütschli were among the first scientists who recognized the structures now familiar as chromosomes. [6] The term was coined by von Waldeyer-Hartz, [7] referring to the term chromatin, which was introduced by Walther Flemming.

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 were the continuity of chromosomes and the individuality of chromosomes.[ citation needed][ further explanation needed] It is the second of these principles that was so original.[ citation needed] Wilhelm Roux suggested that each chromosome carries a different genetic load. Boveri was able to test and confirm this hypothesis. Aided by the rediscovery at the start of the 1900s of Gregor Mendel's earlier work, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the chromosomes. Boveri influenced two generations of American cytologists: Edmund Beecher Wilson, Nettie Stevens, Walter Sutton and Theophilus Painter were all influenced by Boveri (Wilson, Stevens, and Painter actually worked with him). [8]

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 (the names are sometimes reversed). [9] Ernst Mayr remarks that the theory was hotly contested by some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt and T.H. Morgan, all of a rather dogmatic turn of mind. Eventually, complete proof came from chromosome maps in Morgan's own lab. [10]

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. [11]

Other Languages
Afrikaans: Chromosoom
Alemannisch: Chromosom
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asturianu: Cromosoma
azərbaycanca: Xromosom
تۆرکجه: کوروموزوم
Bân-lâm-gú: Jiám-sek-thé
беларуская: Храмасома
български: Хромозома
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čeština: Chromozom
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فارسی: کروموزوم
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hrvatski: Kromosomi
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italiano: Cromosoma
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oʻzbekcha/ўзбекча: Xromosomalar
ਪੰਜਾਬੀ: ਗੁਣਸੂਤਰ
پنجابی: کروموسوم
polski: Chromosom
português: Cromossomo
română: Cromozom
русский: Хромосома
Scots: Chromosome
shqip: Kromozomi
Simple English: Chromosome
slovenčina: Chromozóm
slovenščina: Kromosom
کوردی: کرۆمۆسۆم
српски / srpski: Хромозом
srpskohrvatski / српскохрватски: Kromosom
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ייִדיש: כראמאזאם
Yorùbá: Krómósómù
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中文: 染色体