Practice question

Eukaryotes must replicate each DNA segment exactly once per cell cycle to avoid re-replication which causes double-strand breaks and genomic instability. Origin licensing is restricted to G1 when CDK activity low: ORC, Cdc6, and Cdt1 load double hexamer of MCM2-7 helicase. At S onset, CDK and DDK activate helicase, but re-loading must be blocked. Geminin protein accumulating from S through early mitosis directly binds Cdt1 through extended coiled-coil domain, sterically occluding MCM-interacting surface and preventing second helicase loading. Concurrently, cyclin A-CDK2 phosphorylates Cdt1 and Cdc6, promoting nuclear export and SCF-mediated ubiquitination. APC/C-Cdh1 degrades geminin in late mitosis and G1, resetting system to allow new licensing for next cycle. Depletion of geminin in human cells triggers partial re-replication, ATR checkpoint activation, and apoptosis. Overexpression of geminin and Cdt1 observed in many tumors correlates with aneuploidy and poor prognosis, confirming its role as essential guard of single genome duplication. 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.