Which enzyme is part of the replication machinery and resolves torsion by cutting a strand?

Which enzyme is part of the replication machinery and resolves torsion by cutting a strand?

• A. Helicase
• B. DNA ligase
• C. Topoisomerase
• D. Primase

Answer: (C)

Torsional stress during DNA replication is relieved by topoisomerase, which transiently cuts a strand to allow the DNA to unwind and relieve supercoils, then reseals the break. As helicase unwinds the double helix, positive supercoils build ahead of the fork; topoisomerase relieves this by making a temporary cut, letting strands rotate, and then rejoining them. This action is essential to keep replication moving. The other enzymes have different roles: helicase unwinds but doesn’t create a controlled cut to relieve torsion, primase synthesizes RNA primers, and DNA ligase seals nicks after fragments. In some organisms a related enzyme (DNA gyrase) performs this role as a type II topoisomerase.

Torsional stress during DNA replication is relieved by topoisomerase, which transiently cuts a strand to allow the DNA to unwind and relieve supercoils, then reseals the break. As helicase unwinds the double helix, positive supercoils build ahead of the fork; topoisomerase relieves this by making a temporary cut, letting strands rotate, and then rejoining them. This action is essential to keep replication moving. The other enzymes have different roles: helicase unwinds but doesn’t create a controlled cut to relieve torsion, primase synthesizes RNA primers, and DNA ligase seals nicks after fragments. In some organisms a related enzyme (DNA gyrase) performs this role as a type II topoisomerase.