Which enzymes relieve torsional stress ahead of the replication fork?

• A. DNA topoisomerases (Topo I and II in eukaryotes; Topo IV in bacteria)
• B. DNA helicases
• C. DNA ligases
• D. DNA polymerases

Answer: (A)

Unwinding the DNA at the replication fork creates torsional stress ahead of the fork as the DNA becomes overwound. This positive supercoiling would stall replication unless it’s relaxed. Topoisomerases solve this by making transient cuts in the DNA backbone to allow the helix to untwist and then reseal the breaks. In eukaryotes, topoisomerase I makes single-strand breaks to relieve torsion, while topoisomerase II makes double-strand breaks to pass another segment through and reseal. In bacteria, the equivalent relief comes from topoisomerases, including Topo IV, with other type II enzymes like gyrase helping by introducing negative supercoils to counteract positive supercoiling. So the enzymes that relieve torsional stress ahead of the fork are topoisomerases. Helicases separate strands but don’t relieve torsion directly and can even contribute to torque; ligases seal nicks without relaxing supercoiling, and polymerases synthesize DNA rather than manage DNA topology.