Which DNA replication enzyme eases the tension of coiled DNA by introducing nicks and cuts into the DNA strand?

Which DNA replication enzyme eases the tension of coiled DNA by introducing nicks and cuts into the DNA strand?

• A. Topoisomerase
• B. Helicase
• C. Primase
• D. Ligase

Answer: (A)

During DNA replication, the unwinding action of helicase ahead of the fork twists the DNA and creates torsional strain. To prevent this from stalling replication, the cell uses topoisomerase to relieve the tension. It does this by making a controlled cut in the DNA backbone—either a single-strand nick (type I) or a double-strand break (type II)—allowing the DNA to unwind or pass another segment through the break, and then resealing the cut. This temporary cleavage is essential to keep the DNA from becoming unbearably supercoiled as replication proceeds. For contrast, helicase unwinds the helix without cutting, primase lays down RNA primers to start synthesis, and ligase seals remaining nicks after replication. So the enzyme that introduces nicks and cuts to relieve torsional stress is topoisomerase.

During DNA replication, the unwinding action of helicase ahead of the fork twists the DNA and creates torsional strain. To prevent this from stalling replication, the cell uses topoisomerase to relieve the tension. It does this by making a controlled cut in the DNA backbone—either a single-strand nick (type I) or a double-strand break (type II)—allowing the DNA to unwind or pass another segment through the break, and then resealing the cut. This temporary cleavage is essential to keep the DNA from becoming unbearably supercoiled as replication proceeds. For contrast, helicase unwinds the helix without cutting, primase lays down RNA primers to start synthesis, and ligase seals remaining nicks after replication. So the enzyme that introduces nicks and cuts to relieve torsional stress is topoisomerase.