In which direction can DNA polymerase I proofread while laying down a new nucleotide strand?

In which direction can DNA polymerase I proofread while laying down a new nucleotide strand?

• A. 5' → 3'
• B. Both directions
• C. Only during primer synthesis
• D. 3' → 5'

Answer: (D)

DNA polymerases add new nucleotides in the 5’ to 3’ direction, but their proofreading uses a 3’ to 5’ exonuclease activity. This means the enzyme can backtrack from the 3’ end of the growing strand and remove mispaired nucleotides in the 3’→5’ direction, correcting errors before continuing synthesis. After the incorrect nucleotides are excised, synthesis resumes in the correct 5’→3’ direction. So proofreading occurs moving from the growing 3’ end toward the 5’ direction, not in the direction of synthesis.

DNA polymerases add new nucleotides in the 5’ to 3’ direction, but their proofreading uses a 3’ to 5’ exonuclease activity. This means the enzyme can backtrack from the 3’ end of the growing strand and remove mispaired nucleotides in the 3’→5’ direction, correcting errors before continuing synthesis. After the incorrect nucleotides are excised, synthesis resumes in the correct 5’→3’ direction. So proofreading occurs moving from the growing 3’ end toward the 5’ direction, not in the direction of synthesis.