Which exonuclease activity is responsible for proofreading mispaired bases during DNA replication?

• A. 5' to 3' exonuclease
• B. 5' to 3' polymerase
• C. 3' to 5' polymerase
• D. 3' to 5' exonuclease

Answer: (D)

During DNA replication, correct base pairing is checked by the polymerase itself. The proofreading is done by its 3' to 5' exonuclease activity, which removes incorrectly paired nucleotides from the 3' end of the growing DNA strand. When a mispair occurs, this exonuclease trims back the mismatched bases, creating a proper 3' end, and then the polymerase resumes synthesis by adding the correct nucleotides in the 5' to 3' direction. This step is what keeps replication highly accurate and reduces mutation rates. The other options don’t perform this correction at the growing end: removing nucleotides from the 5' end (5' to 3' exonuclease) is involved in primer removal and processing, not proofreading of the nascent strand; a 3' to 5' polymerase direction isn’t the exonuclease activity responsible for removing mispaired bases.