DNA Proofreading: Ensuring Accuracy in Replication
- Imagine copying a 6-billion-letter book by hand.
- Even a single typo could change the meaning of a sentence.
- DNA replication faces a similar challenge: copying billions of base pairs with near-perfect accuracy.
Think of DNA polymerase III as a meticulous editor, scanning each word (or nucleotide) for errors and correcting them on the spot.
The Role of DNA Polymerase III in Proofreading
- DNA polymerase III is the primary enzyme responsible for synthesizing new DNA strands.
- But it does more than just add nucleotides, it also proofreads its work.
How Proofreading Works
- Mismatch Detection
- DNA polymerase III checks each newly added nucleotide.
- If the base pairing is incorrect (e.g., adenine paired with cytosine instead of thymine), the hydrogen bonds between the bases are unstable, signaling an error.
- Exonuclease Activity
- DNA polymerase III has a built-in exonuclease function that removes nucleotides from the 3′ end of the growing strand.
- When a mismatch is detected, the enzyme pauses, reverses its direction, and removes the incorrect nucleotide.
- Correction and Resumption
