Most mistakes made during DNA replication are directly corrected by DNA polymerase.
DNA replication is a highly accurate process, but mistakes can occasionally occur, such as when a DNA polymerase inserts an incorrect nucleotide. Uncorrected mistakes may sometimes lead to serious consequences, such as cancer. Most of the mistakes made during DNA replication are promptly corrected by DNA polymerase. DNA polymerase proofreads the base that has just been added by checking whether the newly-added base has paired correctly with the base in the template strand, as shown below. If it is the correct base, the next nucleotide is added. If an incorrect base has been added, the enzyme makes a cut at the phosphodiester bond and releases the incorrect nucleotide. This is performed by the exonuclease action of DNA polymerase. Once the incorrect nucleotide has been removed, replication continue.