Sister chromatid exchange


Sister chromatid exchange is the exchange of genetic material between two identical sister chromatids.
It was first discovered by using the Giemsa staining method on one chromatid belonging to the sister chromatid complex before anaphase in mitosis. The staining revealed that few segments were passed to the sister chromatid which were not dyed.
The Giemsa staining was able to stain due to the presence of bromodeoxyuridine analogous base which was introduced to the desired chromatid.
The reason for the is not known but it is required and used as a mutagenic testing of many products. Four to five sister chromatid exchanges per chromosome pair, per mitosis is in the normal distribution, while 14-100 exchanges is not normal and presents a danger to the organism. SCE is elevated in pathologies including Bloom syndrome, having recombination rates ~10-100 times above normal, depending on cell type. Frequent SCEs may also be related to formation of tumors.
Sister chromatid exchange has also been observed more frequently in B51 Behçet's disease.

Mitosis

recombination in the budding yeast Saccharomyces cerevisiae is primarily a result of DNA repair processes responding to spontaneous or induced damages that occur during vegetative growth.

Meiosis

The genomes of diploid organisms in natural populations are highly polymorphic for insertions and deletions. During meiosis double-strand breaks that form within such polymorphic regions must be repaired by inter-sister chromatid exchange, rather than by inter-homolog exchange. A molecular-level study of recombination during budding yeast meiosis has shown that recombination events initiated by DSBs in regions that lack corresponding sequences in the non-sister homolog are efficiently repaired by inter-sister chromatid recombination. This recombination occurs with the same timing as inter-homolog recombination, but with reduced yields of Holliday junction joint molecules. This study, and comparable evidence from other organisms, indicates that inter-sister recombination occurs frequently during meiosis, and up to one-third of all recombination events occur between sister chromatids, although mainly by a pathway that does not involve Holliday junction intermediates.