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Achromatid (Greekkhrōmat- 'color' +-id) is one half of a duplicatedchromosome. Before replication, one chromosome is composed of oneDNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chromatids.[1] During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes.[2]
Chromatid pairs are normally genetically identical, and said to behomozygous. However, if mutations occur, they will present slight differences, in which case they areheterozygous. The pairing of chromatids should not be confused with theploidy of an organism, which is the number ofhomologous versions of a chromosome.
Chromatids may be sister or non-sister chromatids. Asister chromatid is either one of the two chromatids of the samechromosome joined together by a commoncentromere. A pair of sister chromatids is called a dyad. Once sister chromatids have separated (during theanaphase of mitosis or the anaphase II ofmeiosis duringsexual reproduction), they are again called chromosomes, each having the same genetic mass as one of the individual chromatids that made up its parent. The DNA sequence of two sister chromatids is completely identical (apart from very rare DNA copying errors).
Sister chromatid exchange (SCE) is the exchange of genetic information between twosister chromatids. SCEs can occur duringmitosis ormeiosis. SCEs appear to primarily reflect DNA recombinationalrepair processes responding toDNA damage (see articleSister chromatid exchange).
Non-sister chromatids, on the other hand, refers to either of the two chromatids of pairedhomologous chromosomes, that is, the pairing of a paternal chromosome and a maternal chromosome. Inchromosomal crossovers, non-sister (homologous) chromatids formchiasmata to exchange genetic material during theprophase I ofmeiosis (SeeHomologous chromosome pair).
