What happens if a DNA polymerase makes a mistake eg inserts an incorrect base during DNA replication?
If a base has been paired incrroectly, the DNA molecule will adopt a slightly unusual shape (since the bases will either be two purines or two pyrimidines, so the pair will either be too large or too small). A region (subunit) of the polymerase enzyme soon after the polymerase catalytic site will recognise the strange shape and cut the base back off using its endonuclease function. The polymerase can then try again to insert the correct base and move on.
