Practice question

Coordination between DNA replication and mitosis is enforced by G2/M checkpoint which prevents nuclear envelope breakdown and chromosome condensation while replication forks still active or lesions persist. Sensor proteins ATRIP recognizes RPA-coated single-stranded DNA at stalled forks, activating ATR kinase, while MRN complex recruits ATM at double-strand breaks. These apical kinases phosphorylate mediator Claspin and adaptor proteins that activate Chk1 and Chk2 kinases. Chk1 phosphorylates phosphatases Cdc25B and Cdc25C at serine 216 creating binding site for 14-3-3 proteins sequestering them in cytoplasm, away from nuclear cyclin B-CDK1 target, preserving inhibitory phosphorylation placed by Wee1. Additionally p53 transcriptional program elevates p21, 14-3-3 sigma, and Gadd45 that bind and inhibit cyclin B-CDK1. Upon completion of replication and successful lesion repair via homologous recombination, checkpoint signaling silences, PP2A reverses phosphorylations, Cdc25 translocates into nucleus and dephosphorylates CDK1, driving explosive activation and mitotic entry after genome duplicated. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.