Define replication fork and replisome.

• A. The replication fork is the Y-shaped junction where DNA unwinds; the replisome is the multi-protein complex at the fork that coordinates unwinding (helicase) and synthesis.
• B. The replication fork is the site of RNA primer synthesis; the replisome coordinates unwinding and synthesis.
• C. The replisome coordinates immune response at replication.
• D. The replication fork refers to an RNA-DNA hybrid zone during replication.

Answer: (A)

The key idea is how DNA copying happens at the site where two strands separate. The replication fork is the Y-shaped junction where the DNA double helix unwinds so each strand can serve as a template for a new strand. The replisome is the multi-protein machine assembled at that fork that coordinates both unwinding and synthesis. It includes helicase to unwind the strands, primase to lay down RNA primers, DNA polymerases to extend new DNA, and clamp loaders and sliding clamps to keep synthesis efficient. This combination explains why the fork is the unwinding site and the replisome is the coordinated engine driving DNA replication. The other options mix up roles: RNA primer synthesis is part of the replisome’s activity rather than the fork definition, and the replisome does not coordinate immune responses, nor is the fork an RNA–DNA hybrid zone.