Which enzyme unwinds the DNA double helix to start replication?

• A. DNA ligase
• B. DNA polymerase
• C. DNA helicase
• D. Primase

Answer: (C)

Unwinding the DNA double helix to start replication is done by DNA helicase. This enzyme binds at the replication origin and uses energy from ATP hydrolysis to break the hydrogen bonds between paired bases, separating the two strands. As helicase moves, it creates a replication fork and exposes single-stranded templates for the DNA polymerases that follow. Without this unwinding step, the strands stay together and replication cannot begin. After the strands are separated, primase lays down a short RNA primer to provide a starting point for DNA polymerase, which then synthesizes the new DNA, and later DNA ligase seals any remaining nicks.