What regulates origin licensing and firing in eukaryotic S-phase?

• A. Licensing is restricted to the G2 phase
• B. Licensing occurs in G1 via ORC, Cdc6, and Cdt1 loading MCM; firing is regulated by S-phase CDKs and DDK kinases
• C. Licensing and firing both occur during S-phase
• D. Licensing occurs in G1 and firing occurs in G1

Answer: (B)

The main idea tested is how eukaryotic origins are prepared and then activated in a way that prevents re-licensing, ensuring replication happens once per cycle. In G1, origin licensing occurs: the Origin Recognition Complex binds origins and, with Cdc6 and Cdt1, loads the MCM2-7 helicase onto DNA to form the pre-replication complex. This marks the origin as ready to fire, but keeps it dormant until S-phase. Then, during S-phase, firing is triggered by kinases—S-phase CDKs and DDK—that phosphorylate components of the licensed complex and promote recruitment of the rest of the replication machinery (like Cdc45 and GINS) to initiate DNA synthesis. The elevated CDK activity in S-phase also prevents any new licensing, so each origin fires only once per cycle. This description matches the regulation implied by the correct choice: licensing in G1 via ORC, Cdc6, and Cdt1 loading MCM; firing regulated by S-phase CDKs and DDK kinases.