Chromatid

The term chromatid ( the chromatid, rare: the chromatid; plural: chromatids ) refers to a portion of the chromosomes of eukaryotes. A chromatid consists of a double-stranded DNA and chromatin associated proteins. Depending on which cell cycle phase a cell is, there is a chromosome from one or two chromatids.

Background

Before a eukaryotic cell can divide, it must usually grow and their genetic double ( replication). The chromosomes that contain the vast majority of the DNA and thus the genetic material, are located in the nucleus. After genome duplication, there is a division of the cell nucleus (mitosis ), are distributed in identical copies of chromosomes to the daughter nuclei. Usually occurs shortly after cell division in which each daughter cell receives one nucleus.

Duplication and separation of chromatids

Chromosomes after nuclear division (mitosis ) consist initially of one chromatid, are thus identical with this. If the cell is not further shares, ie from the cell cycle is eliminated ( G0 phase, as shown at left ), this state is retained. Aiming the cell division to another, followed by a first growth ( G1 phase ) before the DNA was doubled (S- phase). At the end of duplication, therefore, is of the double-stranded DNA of each chromosome twice before. Both DNA double strands are separately packaged and thus form separate chromatids. These identical sister chromatids at meiosis lie along its entire length in close spatial proximity to the homologous chromosome of the other parent. In this phase it may come at meiosis in a sister chromatid exchange, in which the sister chromatids break at the same level and replaced with corresponding parts of a homologous chromosome and re- added ( Crossing Over, recombination).

At the onset of mitosis, in prophase, the chromosomes condense ( see Article mitosis, condensing section ) so that they are separated from each other to identify microscope. During the condensation, the threads of the two sister chromatids are disentangled, so that they then are adjacent. Here and in the subsequent metaphase is a chromosome that is still composed of two chromatids, which hang together at the centromere. In good microscopic preparations are the two chromatids of a chromosome as separate units along the longitudinal axis visible (pictured right), but this is not at all the case preparations. In the ensuing anaphase the two chromatids of a chromosome are separated: The spindle apparatus is at the centromere and pulls the chromatids in opposite direction, so that each resulting daughter nucleus receives one. Of a chromosome that has doubled before mitosis two chromatids, characterized two chromosomes with again each have become one chromatid after mitosis.

Special cases

• Special forms of nuclear division take place during the maturation of germ cells, meiosis. In the first meiotic nuclear division, the sister chromatids of a chromosome are not separated. This separation takes place until the subsequent second meiotic division.
• In non- disjunction, a faulty distribution of chromatids during mitosis or meiosis II, there will be no separation of sister chromatids. In general, here is concerned only one of the chromosomes.
• Polytene chromosomes, which occur for example in the salivary glands of flies and mosquitoes, are formed from many, sometimes over a thousand, parallel juxtaposed chromatids.

Word origin

The term was introduced as distinct from chromatid chromosome in 1900 by Clarence Erwin McClung, initially for the chromatids of bivalents in meiotic cells.