Which enzyme unwinds the DNA strand during replication?

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

Answer: (C)

During DNA replication, the double helix must be unwound to expose bases for copying. This unwinding is performed by helicase, a motor protein that uses energy from ATP hydrolysis to break the hydrogen bonds between base pairs and separate the two strands. As helicase moves along the DNA, it creates a replication fork and provides single-stranded templates for the polymerases to copy. DNA polymerase then synthesizes the new strands by adding nucleotides complementary to the templates, but it does not unwind the DNA itself. Ligase seals the gaps between fragments on the lagging strand, and primase lays down RNA primers to start synthesis. So the enzyme responsible for unwinding the DNA strands is helicase.