In which cell cycle phase does origin licensing occur in eukaryotes?

• A. G1 phase
• B. S phase
• C. G2 phase
• D. G1 phase

Answer: (D)

Origin licensing is the process that marks which replication origins are allowed to initiate DNA synthesis in the upcoming S phase. This setup happens in G1, when CDK activity is low, allowing the origin recognition complex to bind origins and recruit Cdc6 and Cdt1 to load the MCM helicase onto the DNA. The result is a licensed origin, ready to fire when S phase begins. Once replication starts, CDKs and other kinases become active and prevent new licensing, ensuring each origin fires only once per cell cycle. So, licensing occurs in G1 to prepare for replication in S phase.