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This nucleoprotein filament searches for the homologous duplex DNA in the undamaged sister chromatid.A successful search results in strand invasion, strand exchange and joint molecule formation.DNA synthesis by DNA polymerases generates the genetic information that is required to seal the break.In mammals, KU forms a complex, known as DNAPK, with DNAPK catalytic subunit. It is thought that KU holds the two ends together and facilitates endtoend ligation by the <a href="https://www.ncbi.nlm.nih.gov/pubmed/19955298">sell Sevelamer Carbonate</a> complex of ligase, which usually results in accurate repair of the DSB. Alternatively, binding of the ends by KU can be followed by resection of the free ends by the MRX has been implicated in the joining of ssDNA overhangs at regions where microhomology exists.Recent stud ies mapped HA phosphorylat ion to reg ions flanking a DSB induced by the HO endonuclease, a cleavage event that normally occurs dur ing mating typesw itch ing. Because histone HB, which forms a dimer with HA, was not depleted from this region, the low levels of phosphoHA immediately adjacent to the site of damage are probably not due to nucleosome removal.There are four different classes of chromatinremodelling complexes within this superfamily SWISNF, ISWI, CHD and INO.Biochemical studies have shown that chromatinremodelling complexes use the energy from ATP hydrolysis to induce these changes.They include the mobilization and repositioning of histone octamers in cis, the loss of superhelical torsion of nucleosomal DNA, and the increase in accessibility to nucleosomal DNA for nucleases or proteins involved in transcription.Due to the weak effect of this mutation, it has been difficult to pinpoint the precise function of HA phosphorylation in DSB repair, although given its conservation, a function is likely to exist.The impact of HAX phosphorylat ion has also been examined in mammalian cells.Several other studies in mammalian cells implicate HAX in both NHEJ and HR, and a recent report argues that HAX phosphorylation favours a HR pathway for repair in which sister chromatids are used as a template.Given that the phosphorylation of HA might facilitate the recruitment of DNA repair proteins to the site of damage.R E V I E W S proteins to lesions was not affected.Therefore, it was proposed that HAX phosphorylation promotes the retention and accumulation of DNA repair proteins at sites of damage, without ser ving as the primary recognition site.Although mutations in the yeast HA residues S and T render cells sensitive to DNA damage, it has yet to be established whether these residues become phosphorylated in response to DNA damage.Phosphorylation of S of mammalian histone HB occurs in   response to DNA damage and has been associated with apoptosis.However, in yeast, phosphorylation of HB occurs on S.Although this event has been associated with hydrogen peroxide induced apoptosis, it is unclear whether HB S is phosphorylated in response to DNA damage.Ubiquitylation of HB K and the subsequent methylation of H K are required for efficient checkpoint activation in the presence of DNA damage in yeast.Similarly, HK methylation in mammalian cells is important for the recruitment of BP to sites of DNA damage, although it is not clear if this also contributes to checkpoint activation.

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