Which enzyme unwinds the DNA double helix during replication?

• A. Ligase
• B. Primase
• C. DNA polymerase III
• D. Helicase

Answer: (D)

Unwinding the DNA double helix during replication is the step that exposes the template strands for copying. This job is done by helicase, an enzyme that uses energy from ATP hydrolysis to break the hydrogen bonds between the base pairs and move along the DNA to separate the strands, creating a replication fork. As the strands separate, single-stranded binding proteins stabilize them to prevent re-annealing. The other enzymes have different roles: ligase seals the gaps between Okazaki fragments on the lagging strand; primase lays down an RNA primer to provide a starting point for DNA synthesis; and DNA polymerase III adds nucleotides to build the new DNA but relies on helicase to keep the strands open and cannot unwind the helix by itself.