How is replication prevented from re-initiating origins within the same cell cycle in eukaryotes?

• A. Origins are licensed in G1 and fired only once per S-phase, with CDK/DDK activity and licensing factors preventing re-licensing.
• B. Origins fire multiple times per cycle without restriction.
• C. Origins are not regulated by licensing.
• D. Origins re-licensing happens in G2.

Answer: (A)

Replication is controlled by a licensing-and-firing system: origins are prepared for replication in G1 and can fire only once during the upcoming S phase. In G1, the origin recognition complex and partners load the MCM2-7 helicase onto DNA, forming a pre-replicative complex that licenses the origin. This licensing sets the stage for initiation but does not itself start replication. When S-phase begins, S-CDK and DDK activity trigger origin firing and recruit the full replication machinery to synthesize DNA.

Once origins have fired, the cell prevents re-licensing within the same cycle. The high CDK activity present during S and G2 blocks the assembly of new pre-replicative complexes, so new licensing cannot occur. Licensing factors like Cdt1 are inhibited or degraded (and geminin further inhibits Cdt1), and Cdc6 levels are reduced, keeping origins from being re-licensed until the next cycle. Thus, origins are licensed in G1 and fired once in S-phase, with CDK/DDK activity ensuring no re-licensing during the same cell cycle.