Exonucleases are enzymes (found as individual enzymes, or as parts of larger enzyme complexes) that cleave nucleotides one at a time from an end of a polynucleotide chain. These enzymes hydrolyze phosphodiester bonds from either the 3' or 5' terminus of a polynucleotide molecule.
DNA polymerase I contains a 5' exonuclease that clips off the RNA primer contained immediately upstream from the site of DNA synthesis in a 5' --> 3' manner. Pol I then synthesizes DNA nucleotides in place of the RNA primer it had just removed. DNA polymerase I also has 3' to 5' exonuclease activity which is used in editing and proofreading DNA for errors.
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