In the sequence of steps of DNA replication, which enzyme adds complementary DNA nucleotides during replication?

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

Answer: (A)

The main idea here is identifying the enzyme that actually builds the new DNA strand by adding nucleotides complementary to the template. DNA polymerase does exactly that: it catalyzes the addition of deoxynucleoside triphosphates to the growing DNA chain, using the template strand to guide which nucleotide pairs with each position (A with T, G with C) and extending the chain in the 5' to 3' direction. It requires a primer to start because it cannot begin a new strand from scratch; primase provides that RNA primer to give a starting 3' hydroxyl group for the first nucleotide to attach.

For context, helicase unwinds the double helix, primase lays down the primer, and ligase seals gaps between fragments. But the step that adds the complementary DNA nucleotides and builds the new strand is the job of DNA polymerase.