Exonuclease activities of polymerase

Occasionally DNA polymerase, in error, adds a nucleotide to the 3 -OH terminus that cannot hydrogen-bond to the corresponding base in the template strand. Such a nucleotide would clearly alter the information content of the daughter DNA molecule, and mechanisms exist for correcting such incorporation errors. 

Pol I also has a 5 3 endonuclease activity. An endonucleolytic cut is made between two base pairs that follow a 5 -P-terminated segment of unpaired bases, as shown below. 

This type of endonucleolytic activity is unimportant in normal replication but is important in excision repair. 

Several substrates and products alter activity of the polymerase 15 3 exonuclease. The 5 terminus attacked by the 

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