Which enzyme unwinds the DNA double helix during replication?

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

Answer: (B)

Unwinding the DNA double helix is the first step in replication. Helicase uses energy from ATP to break the hydrogen bonds between base pairs, splitting the two strands apart at the replication fork so the templates become accessible for copying. Without this opening, DNA polymerases wouldn’t have a template to read. Other enzymes have different roles: primase lays down an RNA primer to start synthesis, DNA polymerase builds the new DNA strand, and ligase seals the gaps between fragments on the lagging strand. Because the key job here is to separate the strands, helicase is the enzyme that does this unwinding.