How is replication timing coordinated with the S phase in eukaryotes?

• A. Origins fire randomly during S-phase.
• B. Origins fire during G1 phase with no S-phase regulation.
• C. Origins fire after S-phase ends.
• D. Origins fire during S-phase under regulation by CDKs and DDK, ensuring once-per-S-phase replication.

Answer: (D)

Replication timing is coordinated by a two-step control: licensing origins in G1 and then activating them during S-phase with cell-cycle kinases. In late mitosis to early G1, the origins are "licensed" when the pre-replicative complex loads the MCM helicase onto DNA at potential origins. This licensing sets the stage for replication but does not start it yet. As S-phase begins, S-phase CDKs and the Cdc7-Dbf4 kinase (DDK) become active and phosphorylate key replication factors, including the MCM helicase. This phosphorylation triggers the firing of selected origins so that DNA replication proceeds. Because licensing occurred earlier and firing is strictly regulated during S-phase, each origin can initiate replication only once per cell cycle, producing a coordinated, timely replication program rather than random or repeated initiation.