Genetics and Molecular Biology Practice Exam

Proofreading during DNA replication is performed by DNA polymerase, which has a 3' to 5' exonuclease activity. As the polymerase adds nucleotides, it checks that each base pairs correctly. If a mismatch slips in, the enzyme backs up and excises the incorrect nucleotide from the growing strand, creating a new correct 3' end. It then resumes synthesis, inserting the proper nucleotide and continuing along the template. This built-in proofreading greatly reduces replication errors. The other options don’t perform this immediate, base-by-base correction: ligase seals nicks after synthesis, RNA polymerase is responsible for creating RNA primers, and nucleases can cut nucleic acids but aren’t the specialized proofreading function during replication.