Sister chromatid exchange (SCE) is the exchange of genetic material between two identical sister chromatids . It was first discovered by using the Giemsa staining method on a chromatid sister-related complex before chromatid lysis in mitosis . Staining revealed that some segments assigned to sister chromatids were not dyed. Giemsa staining was able to stain due to the presence of a base analogous to bromodeoxyuridine that was introduced into the desired chromatid.
The cause of SCE is not known, but it is required and used as a mutagenicity test for many products. Four to five sister chromatid exchanges per chromosome pair, per mitosis, is in the normal distribution, whereas 14–100 exchanges are not normal and present a threat to the organism. Depending on the cell type, SCEs are elevated in pathologies including Bloom syndrome , in which the recombination rate is ~10–100-fold higher than normal.   Recurrent SCEs may also be related to tumor formation. Sister chromatid exchange is also more frequently observed in B51 (+) Behçet’s disease .
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Mitotic recombination in the budding yeast Saccharomyces cerevisiae is primarily the result of DNA repair processes responding to spontaneous or induced damage that occurs during vegetative development.  } (also reviewed in Bernstein and Bernstein, pp. 220–221  ). For yeast cells to repair damage by homologous recombination, in the same nucleus, a second DNA molecule must be present, having sequence homology with the region to be repaired. In a diploid cell in the G1 phase of the cell cycle, such a molecule is present as a homologous chromosome. However, in the G2 phase of the cell cycle (after DNA replication), a second homologous DNA molecule is also present: the sister chromatid. Evidence indicates that, because of the special close affinity they share, sister chromatids are not only preferred as substrates for recombination repair over distant homologous chromatids, but also for greater DNA damage repair than homologs. has the ability to do. open access
Organisms in diploid natural populations of genomes are highly polymorphic for insertions and deletions . Double-strand breaks (DSBs) formed within such polymorphic regions during meiosis should be repaired by inter- sister chromatid exchange rather than by inter-homolog exchange. A molecular-level study of recombination during budding yeast meiosis showed that DSB-initiated recombination events in regions lacking the corresponding sequences in non-sister homologues are efficiently repaired by inter-sister chromatid recombination. open access This recombination occurs with the same timing as inter-homolog recombination, but at the Holiday junction.with low (2- to 3-fold) yields of combined molecules. This study, and comparable evidence from other organisms (such as Peacock  ) indicates that inter-sister recombination often occurs during meiosis, and up to a third of all recombination events occur between sister chromatids, although mainly Holiday Junction Intermediate is not included.