<P> Chromatids may be sister or non-sister chromatids . A sister chromatid is either one of the two chromatids of the same chromosome joined together by a common centromere . A pair of sister chromatids is called a dyad . Once sister chromatids have separated (during the anaphase of mitosis or the anaphase II of meiosis during sexual reproduction), they are again called chromosomes . Although having the same genetic mass as the individual chromatids that made up its parent, the daughter "molecules" are called chromosomes in a similar way that one child of a pair of twins is not referred to as a single twin . The DNA sequence of two sister chromatids is completely identical (apart from very rare DNA copying errors). </P> <P> Sister chromatid exchange (SCE) is the exchange of genetic information between two sister chromatids . SCEs can occur during mitosis or meiosis . SCEs appear to primarily reflect DNA recombinational repair processes responding to DNA damage (see articles Sister chromatids and Sister chromatid exchange). </P> <P> Non-sister chromatids, on the other hand, refers to either of the two chromatids of paired homologous chromosomes, that is, the pairing of a paternal chromosome and a maternal chromosome . In chromosomal crossovers, non-sister (homologous) chromatids form chiasmata to exchange genetic material during the prophase I of meiosis (See Homologous recombination). </P>

Before prophase i chromatid arms in a chromosome are