Everything about Chromatids totally explained
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 shouldn't 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.
Further Information
Get more info on 'Chromatids'.
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