A sister chromatid refers to identical copies ( chromatids ) formed by DNA replication of a chromosome , in which both copies are joined together by a common centromere . In other words, a sister chromatid can also be referred to as the ‘half’ of a duplicated chromosome. A pair of sister chromatid is called a dyad. A complete set of sister chromatid is created during the synthesis ( SK ) phase of interphase , when all chromosomes in a cell are duplicated. During mitosis or during the second division of meiosis , the two sister chromatid separate from each other into two separate cells.
The paternal (blue) chromosomes and the maternal (pink) chromosomes are homologous chromosomes . After chromosomal DNA replication , the blue chromosome is composed of two identical sister chromatids and the pink chromosome is composed of two identical sister chromatids. In mitosis, sister chromatids separate into daughter cells, but they are now referred to as chromosomes (instead of chromatids), not in the way a child is referred to as a single twin.
Compare sister chromatid to homologous chromosomes , which are two different copies of a chromosome inherited by diploid organisms (such as humans), one from each parent. Sister chromatid are largely alike (since they carry the same allele, also called variants or versions of a gene) because they are derived from a parent chromosome. The one exception is towards the end of meiosis, after crossing over , because sections of each sister chromatid can be exchanged with corresponding sections of homologous chromatids with which they are paired during meiosis. Homologous chromosomes may or may not be identical to each other because they are inherited from different parents.
There is evidence that, in some species, sister chromatids are the preferred template for DNA repair.  Sister chromatid cohesion is essential for the correct distribution of genetic information between daughter cells and for repair of damaged chromosomes. Defects in this process can lead to aneuploidy and cancer, particularly when the checkpoints fail to detect DNA damage or when the mitotic spindles do not function properly when incorrectly attached.
partition of the cage
Mitotic recombination is primarily a consequence of DNA repair processes that respond to spontaneous or induced damage.    Repair of homologous recombination during mitosis is largely limited to interactions between adjacent sister chromatids that are present in a cell after DNA replication but before cell division. Because of the special close affinity they share, sister chromatids are not only preferred as substrates for recombination repair on distant homologous chromatids, but also have greater DNA damage repair capacity than homologs.
Studies of the budding yeast Saccharomyces cerevisiae  indicate that inter-sister recombination frequently occurs during meiosis, and one-third of all recombination events occur between sister chromatids.