During DNA replication, which problem is alleviated by topoisomerase?

During DNA replication, which problem is alleviated by topoisomerase?

• A. Unwinding the double helix ahead of the fork
• B. Relieving positive supercoiling ahead of the fork to prevent tangling and breakage
• C. Sealing nicks in the sugar-phosphate backbone
• D. Starting RNA primers for DNA synthesis

Answer: (B)

As replication proceeds, helicase unwinds the DNA, and twisting tension builds ahead of the fork. This creates positive supercoils that can hinder unwinding and even cause DNA breaks if not relieved. Topoisomerase relieves this torsional strain by making temporary cuts in the DNA backbone, allowing the strands to pass through and then resealing the breaks. This action specifically addresses the buildup of positive supercoiling ahead of the fork, preventing tangling and breakage. Unwinding ahead of the fork is the job of helicase, not topoisomerase. Sealing nicks in the sugar-phosphate backbone is done by DNA ligase, and starting RNA primers for DNA synthesis is done by primase.

As replication proceeds, helicase unwinds the DNA, and twisting tension builds ahead of the fork. This creates positive supercoils that can hinder unwinding and even cause DNA breaks if not relieved. Topoisomerase relieves this torsional strain by making temporary cuts in the DNA backbone, allowing the strands to pass through and then resealing the breaks. This action specifically addresses the buildup of positive supercoiling ahead of the fork, preventing tangling and breakage.

Unwinding ahead of the fork is the job of helicase, not topoisomerase. Sealing nicks in the sugar-phosphate backbone is done by DNA ligase, and starting RNA primers for DNA synthesis is done by primase.