This enzyme proofreads the growing strand of DNA. If mistakes are detected, this same enzyme removes the incorrect nucleotide and replaces it.

This enzyme proofreads the growing strand of DNA. If mistakes are detected, this same enzyme removes the incorrect nucleotide and replaces it.

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

Answer: (A)

The main idea here is how DNA replication stays accurate through proofreading by the polymerase enzyme. As DNA polymerase builds the new strand, it checks that each incoming base pairs correctly with the template. If a mismatch slips in, the enzyme pauses and uses its 3′ to 5′ exonuclease activity to remove the incorrect nucleotide from the growing end. It then resumes synthesis by inserting the correct nucleotide and continuing along the strand. This built-in proofreading dramatically lowers the error rate during replication. DNA polymerase is the enzyme that carries out this proofreading and replacement, making it the best answer. RNA polymerase, by contrast, synthesizes RNA from a DNA template and isn’t responsible for proof-reading the DNA during replication. Ligase’s role is to seal nicks in the backbone after fragments are joined; it doesn’t remove and replace mismatched bases. Nuclease enzymes can degrade nucleic acids, but the specific proofreading and immediate correction of mispaired nucleotides on the growing DNA strand is a property of DNA polymerase.

The main idea here is how DNA replication stays accurate through proofreading by the polymerase enzyme. As DNA polymerase builds the new strand, it checks that each incoming base pairs correctly with the template. If a mismatch slips in, the enzyme pauses and uses its 3′ to 5′ exonuclease activity to remove the incorrect nucleotide from the growing end. It then resumes synthesis by inserting the correct nucleotide and continuing along the strand. This built-in proofreading dramatically lowers the error rate during replication.

DNA polymerase is the enzyme that carries out this proofreading and replacement, making it the best answer. RNA polymerase, by contrast, synthesizes RNA from a DNA template and isn’t responsible for proof-reading the DNA during replication. Ligase’s role is to seal nicks in the backbone after fragments are joined; it doesn’t remove and replace mismatched bases. Nuclease enzymes can degrade nucleic acids, but the specific proofreading and immediate correction of mispaired nucleotides on the growing DNA strand is a property of DNA polymerase.