Which factor ensures that DNA replication occurs only once per cell cycle?
Guaranteeing exactly one round of DNA replication per cycle requires blocking re-loading of MCM helicases onto origins after they have fired. Two mechanisms converge. First, CDK2 phosphorylates licensing factors Cdc6 marking for SCF degradation and nuclear export, and Cdt1 phosphorylation marking for CRL4-Cdt2 ubiquitination triggered by PCNA at replication forks. Geminin provides second safeguard independent of phosphorylation. Absent in G1 because APC/C-Cdh1 ubiquitinates it, geminin begins accumulation from early S reaching high levels through G2 and M. It binds with high affinity to coiled-coil domain of Cdt1, occluding interaction surface for MCM2-7 recruitment to origin recognition complex, thereby preventing formation of new pre-replicative complexes at fired origins. In M phase, APC/C-Cdc20 and later APC/C-Cdh1 degrade geminin, releasing Cdt1 for licensing only when CDK activity low in next G1. Loss of geminin in human cells induces rereplication detectable as >4N DNA content, induction of DNA damage response via ATR-Chk1-p53, and apoptosis, while overexpression arrests cells at G1/S, underscoring essential role of geminin-Cdt1 balance maintaining diploid stability.
Ref: McGarry & Kirschner, Geminin Prevents Rereplication, Cell 1998; Arias & Walter, Mechanism of Cdt1 Regulation Preventing Rereplication.