The proofreading function of DNA polymerase is its ability to detect and replace incorrectly-placed nucleotides during the DNA synthesis process. The proofreading function is carried out by the enzyme’s intrinsic proofreading exonuclease activity. This activity involves the enzyme reading the newly synthesized DNA strand and removing misincorporated nucleotides, thereby ensuring that the correct nucleotides are placed in the right order.
In practical terms, the proofreading function of DNA polymerase helps to reduce errors present in the newly synthesized DNA strand and keeps the overall accuracy of DNA replication high. Recent studies have estimated that the accuracy of proofreading by the polymerase’s exonuclease activity is in the region of 10-8 to 10-9, i.e. less than one error in a billion nucleotides synthesized.
In addition to catching mistakes in the synthesis process, some DNA polymerases also possess 3'-5' exonuclease activity, which allows them to perform a “proofreading” step during which the template strand can be compared to the newly synthesized strand for accuracy. In this way, the proofreading function of DNA polymerase ensures greater accuracy and promotes faithful DNA replication.
For more information, check out this article by the National Human Genome Research Institute:
https://www.genome.gov/26525384/proofreading-and-dna-polymerases/
