How does DNA replicate?

• A. DNA polymerase unzips the DNA by breaking covalent bonds.
• B. The helicase enzyme breaks hydrogen bonds so the DNA unzips and forms a copy.
• C. Ligase seals the strands after replication.
• D. Primase primes replication and prevents unzipping.

Answer: (B)

The main idea is that DNA replication starts with the double helix being unwound so each strand can serve as a template. This unzipping is done by helicase, which breaks the hydrogen bonds between base pairs, separating the two strands and forming a replication fork.

This is why the statement describing helicase breaking hydrogen bonds to unzip the DNA and form a copy is the best choice. Breaking hydrogen bonds is exactly how the two strands are separated without breaking the covalent bonds in the sugar-phosphate backbone, which would damage the molecule. Once the strands are separated, other enzymes can synthesize new strands along each template.

Consider what the other statements imply: DNA polymerase doesn’t unzip the DNA; it adds nucleotides to build a new strand and relies on the unwound template. It would be incorrect to say it breaks covalent bonds to unzip. Ligase’s job is to seal remaining nicks after synthesis, not to initiate or maintain unzipping. Primase helps start replication by laying down RNA primers, but it doesn’t prevent unzipping or stop the strands from separating.