Which enzyme relieves strain in the double helix by preventing supercoiling ahead of the replication fork?

Which enzyme relieves strain in the double helix by preventing supercoiling ahead of the replication fork?

• A. Helicase
• B. SSB proteins
• C. DNA ligase
• D. Topoisomerase

Answer: (D)

When the replication fork moves, the DNA ahead of it becomes positively supercoiled as the strands are unwound. This torsional strain can slow or block replication unless it’s relieved. Topoisomerase relieves this strain by making transient breaks in the DNA, allowing the helix to swivel and release supercoils, then resealing the breaks. This action keeps the fork moving smoothly and prevents excessive coiling ahead of it. Helicase is the motor that unwinds the DNA but it does not relieve the torsional strain; SSB proteins stabilize single-stranded DNA to prevent reannealing, not relieve supercoiling; DNA ligase seals nicks after fragments are synthesized, not relieve supercoiling either.

When the replication fork moves, the DNA ahead of it becomes positively supercoiled as the strands are unwound. This torsional strain can slow or block replication unless it’s relieved. Topoisomerase relieves this strain by making transient breaks in the DNA, allowing the helix to swivel and release supercoils, then resealing the breaks. This action keeps the fork moving smoothly and prevents excessive coiling ahead of it.

Helicase is the motor that unwinds the DNA but it does not relieve the torsional strain; SSB proteins stabilize single-stranded DNA to prevent reannealing, not relieve supercoiling; DNA ligase seals nicks after fragments are synthesized, not relieve supercoiling either.