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Unformatted text preview: Ch. 11 Recombination- HR is needed to repair DSB from exogenous or endogenous DNA damage, repair ssDNA gaps at replication forks, and maintain telomeres- Crossing over (recombination) occurs between 2 genes during meiosis o Could occur in mitosis after DNA rep b/t nonsister chromatids of homologous chromosomes- Nonhomologous end joining (NHEJ) no homologous template (haploid cells in G1) possible mutations, no repair- Synthesis-dependent strand annealing (SDSA) can only occur after S phase in euk. o DSB resection (5 3 exonuclease) of damaged duplex makes 3 tail on both sides of DSB o Strand invasion (mediated by RecA-binding promotes strand exchange, requires ATP/dATP cofactor) of 3 tail displaces undamaged duplex to form D-loop o DNA synthesis of undamaged template by DNA pol o Strand recapture, annealing by damaged duplex, continue DNA synthesis, ligation- Perfect repair o SDSA using sister chromatid- Gene conversion o Often associated w/ reciprocal crossover of flanking genes o Sometimes incomplete, giving meiotic product with 2 genotypes, with 1/few DNA mismatches (heteroduplex DNA) o SDSA + mismatch repair using either strand- Homologous recombination (after DNA rep) o SDSA but instead of strand recapture o Second end capture of other damaged strand by D-loop, annealing by damaged duplex, continue DNA synthesis, ligation o Holliday junction (HJ), resolution, mismatch repair 4 way symmetry, can move down paired chromatids through branch migration Resolvase acts E-W noncrossover products, or N-S crossover products...
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