Which enzyme adds new nucleotides to form a complementary strand?

• A. DNA polymerase
• B. RNA polymerase
• C. DNA ligase
• D. Helicase

Answer: (A)

During DNA replication, building a new strand that is complementary to the template requires an enzyme that can add nucleotides and form the backbone. DNA polymerase does this by matching incoming nucleotides to their partners on the template strand (A pairs with T, G with C) and linking them together in a chain. It synthesizes in the 5' to 3' direction and needs a primer to start, plus it checks its work as it goes through proofreading. RNA polymerase makes RNA, not DNA, so it wouldn’t produce a complementary DNA strand. DNA ligase seals fragments after synthesis but doesn’t add new bases to form the strand. Helicase unwinds the DNA but doesn’t synthesize nucleotides either. So the enzyme responsible for adding new nucleotides to form a complementary strand is DNA polymerase.