Final answer:
Geminin is a protein that inhibits CDT1 to prevent DNA re-replication after the S phase, contributing to the regulation of the cell cycle at the G2 checkpoint to ensure the integrity of DNA replication before mitosis.
Step-by-step explanation:
Geminin is a protein that plays a critical role in DNA replication, specifically it helps prevent re-replication by inhibiting the action of CDT1, which is necessary for loading the mini-chromosome maintenance complex (MCM) onto DNA. This inhibition is crucial to ensure that once the DNA has been replicated during the S phase, it is not re-replicated before the cell enters mitosis. Geminin's activity is regulated by the cell cycle and is typically present during the S, G2, and early M phases, being degraded during the metaphase to anaphase transition. Its function is particularly important at the G2 checkpoint, where it contributes to ensuring all chromosomes have been properly replicated and that the replicated DNA is undamaged. If these conditions are not met, the cell cycle is halted allowing for DNA repair or completion of DNA replication, preventing the cell from entering mitosis with genomic errors.