Which factors license origins of replication in eukaryotes?

• A. DNA polymerase delta.
• B. ORC and licensing factors.
• C. RNA primase and helicase.
• D. Telomerase.

Answer: (B)

Licensing origins in eukaryotes is about marking where DNA replication can start and ensuring that each origin fires only once per cell cycle. The Origin Recognition Complex (ORC) binds to replication origins and, together with licensing factors, sets up the assembly of the pre-replication complex. Specifically, ORC recruits Cdc6 and Cdt1, which load the MCM2-7 helicase onto DNA as a double hexamer. This loading step, done during G1, licenses the origin so it’s ready to fire in S phase but cannot relicense again until the next cycle. Activation of the licensed origin in S phase then involves other kinases to start unwinding and recruiting the rest of the replication machinery.

The other options don’t perform this licensing role. DNA polymerase delta is a polymerase that extends DNA strands during replication, not the licensing process. RNA primase and helicase participate in fork progression and unwinding, but licensing is about preparing the origin for replication, not the subsequent fork activities. Telomerase extends telomeres, not origin licensing.