DNA polymerase
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DNA polymerase
- 3 prime to 5 prime exonuclease activity, meaning it can excise an incorrect nucleotide and replace it with the correct one during replication.
- The mismatch repair pathway identifies and fixes the replication errors that escape DNA polymerase exonuclease activity.
- It only addresses single-strand breaks in newly replicated DNA; It cannot repair DNA damage.
- These proofreading and repair mechanisms increase the fidelity of DNA replication to only one mistake per 10^9 bases added!
DNA polymerase Proofreading
- Notice that the last nucleotide on the daughter strand is a mismatch. Thymine pairs with adenine, not cytosine.
- DNA polymerase and show that it excises the mismatched base, cytosine, in the three prime to five prime direction. This is the opposite direction of DNA replication, which proceeds in the five prime to three prime direction.
- Once is excises the incorrect base, it can continue synthesizing DNA in the 5 prime to 3 prime direction.
- In prokaryotes, DNA polymerase I and DNA polymerase III both have 3 prime to 5 prime exonuclease activity and can both fill the resulting gap. DNA polymerase III normally elongates daughter strands during prokaryotic replication. DNA polymerase I excises RNA primers and replaces them with DNA.
- In eukaryotes, DNA polymerase delta and epsilon can both excise mismatched nucleotides and fill the gap. Delta on the lagging strand and epsilon on the leading strand.