What is the role of topoisomerase during DNA replication?

• A. Synthesizes RNA primers
• B. Joins Okazaki fragments
• C. Adds nucleotides to the growing strand
• D. Relieves torsional stress by introducing transient breaks to prevent supercoiling ahead of the fork

Answer: (D)

Replication fork unwinding creates torsional stress ahead of the fork as the DNA becomes overwound. Topoisomerase relieves this stress by briefly cutting one or both strands of the DNA backbone, letting the helix untwist or strands pass through, and then resealing the breaks. This prevents accumulating supercoils from stalling replication and keeps the fork moving forward. In bacteria, the type II topoisomerase DNA gyrase introduces negative supercoils to help relieve positive supercoiling ahead of the fork. In eukaryotes, topoisomerase I makes single-strand breaks to relieve torsion.

Primase makes RNA primers for DNA synthesis, not relief of torsion. DNA ligase seals nicks between Okazaki fragments after synthesis, not untangling supercoils. DNA polymerase adds nucleotides to grow the new strand, not relieving torsional stress.