Practice question

DNA double-strand breaks are among most deleterious lesions, potentially generating chromosomal translocations if unrepaired. Detection relies on sensor complex MRN composed of Mre11, Rad50, and Nbs1 that binds broken ends and recruits serine/threonine kinase ATM, ataxia telangiectasia mutated, member of PI3K-like kinase family. ATM exists as inactive dimer; upon recruitment and interaction with Nbs1 C-terminus, it autophosphorylates at serine 1981, dissociates into active monomers, and phosphorylates hundreds of targets including histone variant H2AX at serine 139 forming gamma-H2AX foci that expand megabases around break, p53 at serine 15 stabilizing it, CHK2 at threonine 68 activating it, and Nbs1 itself. ATR, ataxia telangiectasia and Rad3-related, in contrast, is activated by single-stranded DNA coated with RPA and lesions stalling replication forks. ATM activation halts cell cycle via CHK2-Cdc25 axis and initiates repair by homologous recombination or non-homologous end joining. Loss of ATM causes ataxia-telangiectasia disorder with radiosensitivity, highlighting central role in break response and checkpoint signaling. This mechanistic insight is relevant for competitive examinations such as CSIR-NET and GATE, where understanding molecular detail rather than memorization enables accurate interpretation of experimental data and pathway interconnections.