Practice question

Authentic initiation of DNA synthesis requires coordinated activation of MCM2-7 helicase loaded at licensed origins. Two S-phase kinases perform this: Cdc7-Dbf4 complex called DDK phosphorylates N-terminal tails of MCM2, MCM4 and MCM6 promoting Cdc45 recruitment, while CDK2-Cyclin E and CDK2-Cyclin A phosphorylate Treslin, Ticrr, RecQL4 and TopBP1 creating phospho-binding sites for assembling replication machinery including Cdc45-MCM-GINS active helicase and Pol epsilon recruitment. Entry dataset incorrectly lists p53 as initiating kinase; biologically p53 acts opposite as genome guardian. Upon DNA damage, ATM/ATR stabilizes p53 by phosphorylation disrupting Mdm2 binding, p53 transactivates CDK inhibitor p21 which blocks CDK2-Cyclin E and consequently Rb phosphorylation and origin firing. Thus p53 enforces arrest rather than initiation. In unstressed cycles, rising CDK2 activity after E2F-driven Cyclin E transcription provides permissive signal for origin firing. Therefore genuine replication initiation kinases are DDK and CDK2, while p53 functions to pause cycle for repair, preventing propagation of lesions into S phase. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.