* 3'-to-5' Exonuclease Activity: DNA polymerase has a secondary active site called an exonuclease site. This site can break the phosphodiester bond between nucleotides at the 3' end of a newly synthesized DNA strand.
* Error Detection: If DNA polymerase inserts an incorrect nucleotide, it will cause a mismatch with the template strand. This mismatch is detected by the exonuclease site.
* Removal and Replacement: The exonuclease site removes the incorrect nucleotide. Then, DNA polymerase can insert the correct nucleotide in its place.
This proofreading activity significantly reduces the error rate of DNA replication, making it remarkably accurate.
It's important to note that while proofreading is very effective, it's not perfect. There are still rare occasions when errors slip through. This is why organisms have other mechanisms for repairing DNA damage, like mismatch repair.
