During DNA replication, which enzyme is primarily responsible for proofreading newly synthesized DNA by removing misincorporated nucleotides via $3\prime \to 5\prime$ exonuclease activity?

• A. DNA ligase
• B. Primase
• C. DNA helicase
• D. DNA polymerase III (in bacteria)

Answer: (D)

Verified step by step guidance

1. Understand that during DNA replication, the enzyme responsible for synthesizing new DNA strands also has a proofreading function to ensure accuracy.
2. Recall that proofreading involves the removal of incorrectly paired nucleotides, which is done by an exonuclease activity that works in the 3\' to 5\' direction.
3. Identify that DNA polymerase III in bacteria has this 3\' to 5\' exonuclease activity, allowing it to remove misincorporated nucleotides immediately after they are added.
4. Differentiate DNA polymerase III from other enzymes like DNA ligase (which joins DNA fragments), primase (which synthesizes RNA primers), and DNA helicase (which unwinds the DNA helix), none of which have proofreading exonuclease activity.
5. Conclude that the enzyme primarily responsible for proofreading newly synthesized DNA by removing misincorporated nucleotides via 3\' to 5\' exonuclease activity is DNA polymerase III.