Which helicase complex unwinds DNA at replication forks in eukaryotes?

• A. The CMG complex (Cdc45–MCM2-7–GINS) unwinds DNA at replication forks in eukaryotes.
• B. DnaB helicase is the primary eukaryotic helicase at forks.
• C. MCM2-7 helicase alone is sufficient to unwind DNA.
• D. PCNA acts as the helicase at replication forks.

Answer: (C)

At replication forks in eukaryotes, the unwinding of DNA is performed by the CMG helicase complex, which is made up of Cdc45, MCM2-7, and GINS. The MCM2-7 component provides the core motor activity, but it only becomes an active helicase when it associates with Cdc45 and GINS. This assembly forms the CMG complex that travels with the fork and unwinds the DNA ahead of the replication machinery. So, while MCM2-7 is essential, it requires Cdc45 and GINS to function as the active helicase. DnaB is the prokaryotic helicase, and PCNA is a sliding clamp that increases processivity but does not unwind DNA.