Which enzyme relieves torsional strain ahead of the replication fork by introducing negative supercoils?

• A. Topoisomerase
• B. Ligase
• C. Helicase
• D. Gyrase

Answer: (D)

During DNA replication, unwinding by helicase creates positive supercoils ahead of the replication fork, generating torsional strain that can impede progress. To relieve this strain in bacteria, a type II topoisomerase called DNA gyrase introduces negative supercoils, using energy from ATP, to counteract the positive supercoiling and keep the fork moving. This action specifically reduces torsional stress ahead of the fork, enabling continuous unwinding.

Topoisomerase I would typically relax negative supercoils rather than introduce them, helicase is the unwinding enzyme that creates torsion, and ligase seals nicks after synthesis without altering supercoiling. So the enzyme that best fits the description is DNA gyrase.