DNA exonuclease

DNA polymerases all synthesize new DNA using a template and make the new DNA in a 5 3 direction (new nucleotides are added to the 3 end). In addition to making DNA, some of the DNA polymerases can also hydrolyze it. An exonuclease works only on the ends of the DNA (or RNA), and like everything else about DNA, exonuclease activity has a direction too. The 3 5 exonuclease activity removes nucleotides from the 3 end (by hydrolyzing the phosophodi-ester bond). Since the chain grows in the 5 3 direction, polymerases that have a 3 5 exonuclease activity can look back over their work... 

The bacterial DNA exonucleases which have been purified and examined in detail are with two exceptions derived from E. coli and E. coli infected with bacteriophages all show a strong specificity for the secondary structure of their polydeoxynucleotide substrate. 

Some of the bases in DNA and RNA can be chemically modified via methylation. Enzymes, similar to proteases, called exo- and endo-nucleases can cleave RNA and DNA. Exonucleases cleave nucleic acids from the ends. Endonucleases recognize specific sequences of duplex DNA and cleave at a specific site within... 

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