Which enzyme adds new nucleotides to the newly forming strand of DNA during replication?

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

Answer: (A)

DNA polymerase is the enzyme that adds new nucleotides to the growing DNA strand. It uses the existing strand as a template and extends the chain by forming phosphodiester bonds, always adding to a 3' end so synthesis proceeds in the 5' to 3' direction. A short RNA primer provided by primase gives DNA polymerase a starting 3' OH to extend from, because polymerase cannot start a strand from scratch. DNA polymerase also proofreads as it goes to correct mistakes.

In contrast, RNA polymerase makes RNA and is used for transcription, not DNA replication. Primase makes the RNA primer it’s named for, which is necessary only to start synthesis. Ligase seals the gaps between Okazaki fragments on the lagging strand after synthesis is complete. So the enzyme that actually incorporates the new nucleotides into the DNA backbone is DNA polymerase.