Double-strand breaks (DSBs) result from exposure to ionizing radiation, x-rays, from enzymatic cleavage, or during replication of a single-strand break. DSBs are especially problematic because of the absence of a normal strand to serve as a template. Arrest of the cell cycle to facilitate repair of the DSB is mediated by p53. DNA damage-response proteins, including ataxia-telangiectasia mutated (ATM) and ataxia-telangiectasia related (ATR), are recruited. There are two logically alternate pathways for double-strand break repair, homologous recombination or nonhomologous end joining, which are reviewed elsewhere (16,17).
In homologous recombination, 5'^3' exonuclease activity results in a single-strand overhang that will facilitate a homology search. Replication A protein (RPA) along with RAD51-related proteins are assembled, and the homologous sequence on the second, intact chromosome, is identified. By a complex process of strand pairing, formation of a Holliday junction structure, branch migration, and resolution of the Holliday junction by resolvases, the DNA break is repaired. In the alternate pathway, the broken ends of the DNA are recognized by the Ku70/Ku80 complex together with DNA dependent-protein kinase, and the break is repaired by the action of XRCC4-ligase 4. In nonhomologous end joining, there is no template to guide the repair, and the result is less precise, with the loss or gain of a few nucleotides.
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