AskDefine | Define chromatid

Dictionary Definition

chromatid n : one of two identical strands into which a chromosome splits during mitosis

User Contributed Dictionary

English

Noun

  1. Either of the two strands of a chromosome that separate during mitosis.

Translations

Extensive Definition

A chromatid is one of two identical copies of DNA making up a chromosome, which are joined at their centromeres, for the process of cell division (mitosis or meiosis). The term is used so long as the centromeres remain in contact. When they separate (during anaphase of mitosis and anaphase 2 of meiosis), the strands are called daughter-chromosomes.
In other words, a chromatid is "one-half of a replicated chromosome". It should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome.

Quantity

In humans, for example, there are normally 23 pairs of homologous chromosomes in each cell (N=23). However, the quantity of chromatids will be a multiple of 23. It can be either 4N, 2N or 1N. The last is only seen in haploid gametes, with only one of each homologous chromosome pair. Such are created in gametogenesis.

4N

In a cell with 4N chromatids, there are 23 chromosome pairs (46 chromosomes), and each chromosome has 2 chromatids. Thus, there are 92 chromatids in each cell (4N). It occurs after the S phase of interphase. (See cell cycle).

2N

Immediately after a mitosis, where a cell has divided in two, but not yet duplicated its DNA in S phase, there are still 23 chromosome pairs (46 chromosomes). However, each chromosome only has one chromatid. Thus there are 46 chromatids (2xN)
Alternatively, a haploid cell with two chromatids per chromosome also has 46 chromatids. However, this doesn't occur naturally in humans.

1N

Immediately after meiosis, each cell, called a gamete, only has half the amount of chromosomes (23 chromosomes). Furthermore, each chromosome only has one chromatid. Thus, there are 23 chromatids (1xN)

Etymology

The term chromatid was proposed by Clarence Erwin McClung (1900) for each of the four threads making up a chromosome-pair during meiosis. It was later used also for mitosis.
The term derives from the Greek chroma (colour); for the derivation of -id, see cytokinesis.

References

chromatid in Catalan: Cromàtida
chromatid in German: Chromatid
chromatid in Spanish: Cromátida
chromatid in French: Chromatide
chromatid in Italian: Cromatidio
chromatid in Hebrew: כרומטידה
chromatid in Kurdish: Kromatîd
chromatid in Dutch: Chromatide
chromatid in Occitan (post 1500): Cromatida
chromatid in Polish: Chromatyda
chromatid in Serbian: Хроматида
chromatid in Swedish: Kromatid
chromatid in Chinese: 染色单体
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