Nucleases are enzymes of the class hydrolase that cleave nucleic acids. They are most often specific in action, with ribonucleic acids (RNA) only acted upon by ribonucleases and deoxyribonucleic acids (DNA) acted upon only by doxyribonucleases.
More specific than nucleases are exonucleases and endonucleases. These are nucleic enzymes that cleave nucleotides one at a time from one end of a polynucleotide chain.
Exonucleases may also be called "restriction enzymes" because they split the DNA molecules only at recognized subunits. Type I nucleases split the DNA randomly, while Type II and III split the DNA at a predictable location, making them crucial for genetic engineering. Exonucleases are Type II enzymes.
Exonucleases are also called phosphoesterases, and hydrolyze phosphodiester bonds from the 3' or 5' terminus of polynucleotide molecules. This serves a very specific purpose. In a strand of DNA, nucleotides – adenine, cytosine, guanine, and thymine – are held together in strands by phosphodiesters. When the polynucleotide chain grows during synthesis, it grows in a specific order down a chain of bases and sugars. Sugars are marked, in biology, with a '. In a specific strand of nucleotides, the sugars are numbered: 1', 2', etc. A polynucleotide chain always grows during synthesis from the 5' to the 3' terminus.
Because exonucleases break the bond at the 3' or 5' terminus, the entire nucleotide can be removed from start to finish and separated out from the DNA, to be modified, replaced, or borrowed by the researcher as necessary.
Web Resources On Exonuclease
Properties of Exonucleases and Endonucleases
Functional cooperation between exonucleases and endonucleases
Book Resources On Exonuclease
Endo-Exonucleases: Actions in the Life and Death of Cells by Murray J. Fraser
Genomes by T.A. Brown