DNA and RNA are polymers made by linking together smaller units called nucleotides. Nucleases are enzymes that break the chemical bonds, called phosphodiester bonds, that hold the nucleotides of DNA or RNA polymers together. Enzymes that cleave the phosphodiester bonds of DNA are called deoxyribonucleases, and enzymes that cleave the phosphodiester bonds of RNA are called ribonucleases.

Nucleases can be divided into two classes, exonucleases and endonucleases, based on the positions of the cleaved bonds within the DNA or RNA polymers. The exonucleases are involved in trimming the ends of RNA and DNA polymers, cleaving the last phosphodiester bond in a chain. This cleavage results in the removal of a single nucleotide from the polymer. If the enzyme removes nucleotides from the 3′ ("three prime") end, it is referred to as a 3′ exonuclease. If cleavage is at the 5′ end, the enzyme is called a 5′ exonuclease. The endonucleases cleave phosphodiester...