Which enzyme creates small nicks ahead of the replication fork to relieve tension created by helicase?

Which enzyme creates small nicks ahead of the replication fork to relieve tension created by helicase?

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

Answer: (A)

When the DNA double helix is unwound by helicase, torsional stress builds up ahead of the replication fork as positive supercoils. To keep the fork moving, the cell must relieve that tension. Topoisomerase does exactly this by making transient breaks in the DNA backbone to allow the strands to pass or rotate and then resealing the breaks. Type I topoisomerases cut one strand, easing supercoiling, while Type II topoisomerases cut both strands to pass a segment through, also relieving tension (and, in bacteria, can introduce negative supercoils as needed). This activity directly addresses the need to relieve torsion ahead of the fork, enabling smooth replication progression. DNA polymerase builds new DNA, helicase creates the tension by unwinding, and ligase seals nicks after synthesis, not ahead-of-fork torsional relief.

When the DNA double helix is unwound by helicase, torsional stress builds up ahead of the replication fork as positive supercoils. To keep the fork moving, the cell must relieve that tension. Topoisomerase does exactly this by making transient breaks in the DNA backbone to allow the strands to pass or rotate and then resealing the breaks. Type I topoisomerases cut one strand, easing supercoiling, while Type II topoisomerases cut both strands to pass a segment through, also relieving tension (and, in bacteria, can introduce negative supercoils as needed). This activity directly addresses the need to relieve torsion ahead of the fork, enabling smooth replication progression. DNA polymerase builds new DNA, helicase creates the tension by unwinding, and ligase seals nicks after synthesis, not ahead-of-fork torsional relief.