Which enzyme unwinds the DNA double helix by breaking hydrogen bonds between strands?

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

Answer: (D)

The main idea is that unwinding the DNA double helix requires breaking the hydrogen bonds that hold base pairs together, and the enzyme that does this is helicase. Helicase uses energy from ATP hydrolysis to move along the DNA and pry the two strands apart, creating the replication fork and exposing single-stranded templates for DNA synthesis. This action is distinct from other enzymes: topoisomerase relieves torsional strain ahead of the fork without breaking base-pair hydrogen bonds, ligase seals nicks in the DNA backbone, and primase synthesizes short RNA primers to start synthesis. So, helicase is the enzyme that directly breaks the hydrogen bonds between strands to unwind DNA.