Topoisomerase primarily does what during replication?

Topoisomerase primarily does what during replication?

• A. Initiates synthesis by laying down RNA primers.
• B. Synthesizes the leading strand.
• C. Stabilizes single-stranded DNA.
• D. Corrects overwinding ahead of replication forks by breaking, swiveling and rejoining DNA strands.

Answer: (D)

During replication, the DNA helix ahead of the moving fork becomes overwound as it is unwound to expose the template. Topoisomerases relieve this torsional strain by making transient breaks in the backbone and then rejoining the ends. They can cut one strand and swivel to release the twist, or cut both strands to pass another duplex through and reseal, which either relieves supercoils or helps decatenate tangled DNA. This action directly prevents the buildup of overwinding that would stall the fork, allowing replication to continue smoothly. That’s why it’s described as correcting overwinding ahead of replication forks by breaking, swiveling, and rejoining DNA strands. The other options involve RNA primer synthesis, DNA strand synthesis, or stabilizing single-stranded DNA, which are performed by primase, DNA polymerase, and single-stranded DNA–binding proteins, respectively, not topoisomerase.

During replication, the DNA helix ahead of the moving fork becomes overwound as it is unwound to expose the template. Topoisomerases relieve this torsional strain by making transient breaks in the backbone and then rejoining the ends. They can cut one strand and swivel to release the twist, or cut both strands to pass another duplex through and reseal, which either relieves supercoils or helps decatenate tangled DNA. This action directly prevents the buildup of overwinding that would stall the fork, allowing replication to continue smoothly. That’s why it’s described as correcting overwinding ahead of replication forks by breaking, swiveling, and rejoining DNA strands. The other options involve RNA primer synthesis, DNA strand synthesis, or stabilizing single-stranded DNA, which are performed by primase, DNA polymerase, and single-stranded DNA–binding proteins, respectively, not topoisomerase.