Which enzyme is responsible for unzipping the double helix at the start of DNA replication?

Which enzyme is responsible for unzipping the double helix at the start of DNA replication?

• A. Reads the DNA sequence to synthesize RNA
• B. Joins Okazaki fragments
• C. Responsible for unzipping/separating double helix during first step of DNA replication
• D. Synthesizes nucleotides

Answer: (C)

Unwinding the DNA double helix to start replication is carried out by helicase, an enzyme that uses energy from ATP to break the hydrogen bonds between base pairs. This separation creates the replication fork and exposes each DNA strand as a template for copying. After the strands are opened, primase lays down RNA primers so DNA polymerases can begin synthesis, with the leading strand copied continuously and the lagging strand copied in fragments that are later joined. The enzyme that reads the DNA sequence to synthesize RNA is primase, not the unwinding step. The enzyme that joins Okazaki fragments is ligase, which comes into play after synthesis. Saying that an enzyme simply “synthesizes nucleotides” is misleading in this context, since the main copying step is carried out by polymerases that add nucleotides to a growing DNA strand, while the unzipping action is specific to helicase.

Unwinding the DNA double helix to start replication is carried out by helicase, an enzyme that uses energy from ATP to break the hydrogen bonds between base pairs. This separation creates the replication fork and exposes each DNA strand as a template for copying. After the strands are opened, primase lays down RNA primers so DNA polymerases can begin synthesis, with the leading strand copied continuously and the lagging strand copied in fragments that are later joined. The enzyme that reads the DNA sequence to synthesize RNA is primase, not the unwinding step. The enzyme that joins Okazaki fragments is ligase, which comes into play after synthesis. Saying that an enzyme simply “synthesizes nucleotides” is misleading in this context, since the main copying step is carried out by polymerases that add nucleotides to a growing DNA strand, while the unzipping action is specific to helicase.