A sister chromatid is one of the two identical copies of a chromosome formed by DNA replication, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be one-half of the duplicated chromosome. During cell division, the two sister chromatids are separated from each other into two different cells during mitosis or during the second division of meiosis. A pair of sister chromatids is called a dyad, and a full set of sister chromatids is created during the synthesis (S) phase of interphase, when all the chromosomes in a cell are replicated. Sister chromatids are by and large identical since they carry the same alleles, also called variants or versions, of genes because they derive from one original chromosome. Sister chromatids play an important role in maintaining the integrity of the genome by being involved in DNA repair. They are usually spatially close to each other, compared to the homologous chromosome pair, and are attached to each other from the time DNA is duplicated till anaphase, through the action of proteins called cohesins.