Chromatid
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2008-06-17T01:15:54Z
65.205.95.58
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[[Image:Chromosome.png|thumb|Chromosome.<BR>(1) [[Chromatid]]. One of the two copies of the chromosome after S phase.<BR>(2) [[Centromere]]. The point where the two chromatids touch, and where the microtubules attach.<BR>(3) Short arm<BR>(4) Long arm.]]
A '''chromatid''' is one of two identical copies of DNA making up a [[chromosome]], which are joined at their [[centromere]]s, 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".<ref>[http://biology.about.com/library/glossary/bldefchromatid.htm Chromatid - A Cell Biology Definition<!-- Bot generated title -->]</ref> It should not be confused with the [[ploidy]] of an organism, which is the number of [[Homologous_chromosome|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. N does not refer to haploid or diploid; it refers to the number of chromatids in the cell as a multiple of the haploid number of chromosomes for the organism. For example, because a human haploid germ cell has 23 chromosomes, then "N" refers to a multiple of 23. (e.g. 2N=46 chromatids). 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==
<references/>
{{Chromo}}
{{wiktionarypar|chromatid}}
[[Category:Chromosomes]]
[[Category:Molecular genetics]]
[[Category:Cell biology]]
[[Category:Mitosis]]
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