Sister chromatid exchange
During prophase I of the first meiotic division, chromosomal crossover occurs. This event happens when homologous chromosomes exchange genetic material. The chromatids of homologous chromosomes pair up forming a chiasma. During a crossover, the chromatids break into segments and are then exchanged with the matching region of the other chromatid. Chromosomal crossover between homologous chromosomes is important because it results in new combinations of genes that are different from either parent, contributing to genetic diversity. However, there are instances wherein an aberrant crossover occurs, such as when there is an exchange of segments between sister chromatids of a chromosome. This event is called sister chromatid exchange. Previously, Giemsa staining technique is used to identify the segments of one dyed sister chromatid that were exchanged with the other undyed sister chromatid.
Sister chromatid exchange occurs between the sister chromatids of a meiotic tetrad or between the sister chromatid of a duplicated somatic chromosome. It can be induced by ultraviolet and ionizing radiation or by mutagenic agents and is observed by autoradiography. This precept is used to test a particular product for potential mutagenic properties.