Which enzymes are required to unwind DNA and stabilize single strands during replication?

• A. DNA polymerase unwinds DNA; ligase stabilizes single strands; ligase relieves supercoiling.
• B. Primase unwinds DNA; helicase stabilizes strands; ligase relieves coiling.
• C. Helicase unwinds DNA; single-strand binding proteins stabilize single strands; topoisomerase relieves supercoiling ahead of the fork.
• D. Reverse transcriptase unwinds DNA; SSBs stabilize double-stranded DNA; DNA gyrase relieves coiling.

Answer: (C)

Unwinding the double helix at the replication fork is done by helicase, which breaks the hydrogen bonds between base pairs to separate the strands. Once separated, single-strand binding proteins bind to the exposed single strands to keep them from reannealing and to protect them. At the same time, the DNA ahead of the fork becomes over-twisted as the strands are pulled apart, so topoisomerase relieves this supercoiling by making transient breaks and rejoining the DNA, preventing tangling and helping the fork move forward. Together, helicase, single-strand binding proteins, and topoisomerase are the core set needed for unwinding and stabilizing single strands during replication.