Chromatid

Schematic representation of a metaphase chromosome, consisting of two chromatids - the one on the right is labeled 1. At the centromere (2) the two chromatids are linked. The short (3) and long arms (4) of the right chromatid are also marked.

Chromatid (the chromatid, rarely: the chromatids ; plural: chromatids) denotes a part of the chromosomes of the eukaryotes . A chromatid consists of a DNA -Doppelstrang and the associated chromatin - proteins . A chromosome consists of one or two chromatids, depending on which cell cycle phase a cell is in, whether after or before nuclear division.

background

Before a eukaryotic cell can divide, it usually has to grow and duplicate its genetic make-up ( replication ). The chromosomes, which contain the vast majority of the DNA and thus the genetic material, are located in the cell nucleus . After the genome has been duplicated , the cell nucleus divides ( mitosis ), in which identical copies of the chromosomes are distributed to the daughter nuclei. As a rule, cells divide shortly afterwards , with each daughter cell receiving a cell nucleus.

Duplication and separation of the chromatids

Cell cycle scheme.
I = interphase, M = mitosis

Metaphase chromosomes from a human female lymphocyte cell , staining with the fluorescent dye chromomycin A3 . The chromosomes are partially on top of each other. Each metaphase chromosome is made up of two chromatids, which are separated lengthways by a dark gap.

Chromosomes after a nuclear division (mitosis) initially consist of a chromatid and are therefore identical to it. If the cell does not divide further, i.e. leaves the cell cycle ( G ₀ phase ; see figure on the left), this state remains. If the cell tries to divide further, it first grows ( G ₁ phase ) before the DNA is doubled ( S phase ). At the end of the duplication, the DNA double strand of each chromosome is present twice. The two DNA double strands are packaged separately and therefore form separate chromatids.

At the beginning of mitosis, in prophase , the chromosomes condense (see article Mitosis , section Condensation ) so that they can be seen separately from one another under the microscope . During the condensation, the threads of the two sister chromatids are also untangled so that they then lie next to each other. Here and also in the subsequent metaphase , a chromosome still consists of two chromatids that are attached to each other on the centromere . In good microscopic specimens, the two chromatids of a chromosome are recognizable as units separated along the longitudinal axis (figure on the right), but this is not the case with all specimens. In the subsequent anaphase , the two chromatids of a chromosome are separated: the spindle apparatus attaches to the centromere and pulls the chromatids in opposite directions, so that each daughter nucleus that is created has one. A chromosome, which had already doubled to two chromatids before mitosis, has become two chromosomes, each with one chromatid again, after mitosis.

Special cases

• Special forms of nucleus division take place during the maturation of the germ cells , the meiosis . In the first meiotic nuclear division, the sister chromatids of a chromosome are not separated. This separation only takes place in the subsequent second meiotic division. In meiosis, the identical sister chromatids lie along their entire length in close spatial proximity to the homologous chromosome of the other parent. In this phase, sister chromatids can be exchanged during meiosis , in which the sister chromatids break off at the same level and are exchanged for corresponding parts of a homologous chromosome and reattached ( crossing-over , recombination ).
• In the case of non-disjunction , an incorrect distribution of the chromatids during mitosis or meiosis II, there is no separation of the sister chromatids. Usually only one of the chromosomes is affected here.
• Polytene chromosomes , which occur, for example, in the salivary glands of flies and mosquitoes , are formed from many, sometimes over a thousand, chromatids arranged in parallel.

Word origin

The term chromatid was introduced by Clarence Erwin McClung to differentiate it from chromosome 1900 , initially for the chromatids of bivalent cells in meiotic cells.

literature

• Bruce Alberts , Alexander Johnson, Julian Lewis, Martin Raff , Keith Roberts, Peter Walter (2002): Molecular biology of the cell , 4th ed. Publisher: Garland Science, New York. On-line

Individual evidence

1. ^ Y Urata, SJ Parmelee, DA Agard and JW Sedat (1995): A three-dimensional structural dissection of Drosophila polytene chromosomes. Journal of Cell Biology 131: 279-295.
2. ↑ CE MCCLUNG in Kansas Univ. QA IX. 78 (1900). The term 'chromosome' being..restricted to the units of the division figures, there remains no name for the parts composing these when they are compound, as in tetrads and diads ... I should like, therefore, to propose the term ' chromatid 'for each of these. Quoted from: Oxford English Dictionary, 2nd edition 1989, entry Chromatid . Online (registration required).

Wiktionary: Chromatid  - explanations of meanings, word origins, synonyms, translations