What is the significance of 3' to 5' exonuclease activity in DNA polymerases?

• A. It enables rapid synthesis.
• B. It enables proofreading and removal of misincorporated nucleotides.
• C. It creates RNA primers.
• D. It seals DNA nicks.

Answer: (B)

The main idea is proofreading during DNA replication. DNA polymerases have a built-in 3' to 5' exonuclease activity that serves as a quick corrective tool when a wrong nucleotide is added. If the polymerase puts down an incorrect base, the enzyme can pause and switch to its proofreading site, where nucleotides are removed from the 3' end toward the 5' end of the growing strand. This excision clears the mispaired nucleotide (and sometimes a few preceding ones) so the correct base can be laid down again, and synthesis can continue. This proofreading step greatly increases replication accuracy, preventing mutations from misincorporations. It’s a distinct function from primer creation or later steps like RNA primer removal and nick sealing, which involve different activities and enzymes.