Chromatid

Achromatid(Greekkhrōmat-'color' +-id) is one half of a duplicatedchromosome.Before replication, one chromosome is composed of oneDNAmolecule. In replication, the DNA molecule is copied, and the two molecules are known as chromatids.During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes.

In the diagram, (1) refers to a chromatid: 1-half of two identical threadlike strands of a replicatedchromosome.During cell division, the identical copies (called a "sister chromatid pair") are joined at the region called thecentromere(2). Once the paired sister chromatids have separated from one another (in theanaphaseofmitosis) each is known as a daughter chromosome. The short arm of the right chromatid (3), and the long arm of the right chromatid (4), are also marked.

Schematickaryogramof the human chromosomes, showing their usual state in the G₀and G₁phase of the cell cycle. At top center it also shows the chromosome 3 pair inmetaphase(annotated as "Meta." ), which takes place after having undergoneDNA synthesiswhich occurs in theS phase(annotated as S) of the cell cycle. During metaphase, each chromosome is duplicated intosister chromatids.

Further information:Karyotype

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 theploidyof an organism, which is the number ofhomologous versionsof a chromosome.

Sister chromatids

Main article:Sister chromatids

Condensation and resolution of human sister chromatids in early mitosis

Chromatids may be sister or non-sister chromatids. Asister chromatidis either one of the two chromatids of the samechromosomejoined together by a commoncentromere.A pair of sister chromatids is called a dyad. Once sister chromatids have separated (during theanaphaseof mitosis or the anaphase II ofmeiosisduringsexual 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 duringmitosisormeiosis.SCEs appear to primarily reflect DNA recombinationalrepairprocesses 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 formchiasmatato exchange genetic material during theprophase Iofmeiosis(SeeHomologous chromosome pair).

See also

• Kinetochore