Which enzyme unzips the double helix to form the replication fork?

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

Answer: (A)

Unzipping the DNA to form the replication fork is performed by helicase. This enzyme binds to the DNA and uses energy from ATP to break the hydrogen bonds between complementary bases, effectively separating the two strands so each can serve as a template for copying. As helicase moves, it creates the replication fork structure where the strands are single-stranded and ready for DNA polymerases to synthesize new strands.

Other enzymes have different roles: topoisomerase relieves the twisting force that builds up ahead of the fork, preventing supercoiling; primase lays down RNA primers to provide starting points for DNA synthesis; ligase seals nicks in the backbone to join fragments on the lagging strand.