Before replication, one chromosome is composed of one DNA molecule. Following S phase of interphase, each chromosome now composed of two DNA double-helix molecules (in other words, DNA replication increases the amount of DNA but does not increase the number of chromosomes.) The two identical copies—each forming one half of the replicated chromosome—are called chromatids, and these chromatids eventually separate longitudinally to become individual chromosomes during the later stages of cell division.
They are normally genetically identical ("homozygous") but may have slight differences due to mutations, in which case they are heterozygous. It should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome.
Sister vs. non-sister chromatids
Technically, chromatids may be sister or non-sister chromatids. In general, a sister chromatid is either of the two chromatids of the same chromosome joined together by a common centromere. When sister chromatids separate (during anaphase of mitosis and anaphase II of meiosis), the strands are called daughter chromosomes (although having the same genetic mass as the individual chromatids that made up its parent, the daughter "molecules" are still referred to as chromosomes much in the way that one child is not referred to as a single twin).
A non-sister chromatid, on the other hand, refers to either of the two chromatids of any of the paired homologous chromosomes (a paternal chromosome and a maternal chromosome). Non-sister (homologous) chromatids form chiasma(ta) to exchange genetic material during meiosis (i.e. Prophase I).
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