What is the function of topoisomerase during DNA replication?

• A. Unwinds DNA helicase
• B. Synthesizes RNA primers
• C. Binds to replication origins
• D. Relieves supercoiling and torsional strain ahead of the replication fork

Answer: (D)

During DNA replication, the double helix is unwound by helicase, which creates tension ahead of the replication fork in the form of supercoiling. Topoisomerase is the enzyme that relieves this torsional strain, allowing the fork to progress smoothly. It does this by making transient cuts in the DNA backbone and then rejoining the strands, either by single-strand breaks (relaxing tension) or double-strand breaks (untangling) depending on the type. This prevents the DNA from becoming overly twisted and tangled as replication proceeds.

This is why the function described here—relieving supercoiling and torsional strain ahead of the replication fork—is the correct one. Unwinding the DNA is the job of helicase, RNA primers are synthesized by primase, and replication origins are recognized by initiator proteins; none of these are the primary action of topoisomerase.