Exonuclease activities of DNA Polymerases

An exonuclese is an enzyme that removes nucleotides. Most DNA Polymerases, whose primary function is to add nucleotides in the 5'-3' direction, also have exonuclease activities. (A) 3'-5' exonuclease activity performs a 'proof reading' function by removing a nucleotide that has just been added incorrectly. (B) 5'-3' exonuclease activity displaces nucleotides ahead of the addition by polymerase activity. This is useful for example in removing RNA primers.

Figure © 2012 TA Brown, Introduction to Genetics (1st ed.); additional text © 2012 by Steven M. Carr