Which enzyme relieves supercoiling by transiently breaking the DNA during replication?

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

Answer: (A)

When DNA is being copied, the two strands must be separated, and this unwinding creates torsional strain ahead of the replication fork, leading to supercoiling. Topoisomerases fix this by making a controlled, temporary cut in the DNA backbone. One type cuts a single strand to rotate the helix and then reseals the break, relieving tension. Another type cuts both strands to pass another DNA segment through before resealing, which also relieves supercoiling. The break is transient, so the DNA stays intact after the molecule relaxes. This action is essential to keep the fork moving smoothly.

The other enzymes have different roles: helicase unwinds the DNA but doesn’t relieve torsional strain by cutting DNA; DNA polymerase builds the new strand; DNA ligase seals nicks after fragments are joined.