nuclease, any enzyme that cleaves nucleic acids by breaking phosphodiester bonds between nucleotide molecules. Nucleases act via hydrolysis, using water to break bonds, and thus belong to the class of enzymes known as hydrolases. They serve a vital role in certain basic biological processes, namely DNA replication, RNA processing, and the initiation of DNA repair and recombination. They also are used in research for the purification of nucleic acids and proteins. Nucleases occur naturally in microorganisms and in both animals and plants.
There are two primary types of nucleases: exonucleases and endonucleases. Exonucleases operate selectively by degrading from the end of the nucleotide chain, whereas endonucleases cleave in the middle of chains. There are various subtypes of exonucleases, examples being the 17 known exonucleases in the bacterium Escherichia coli.
Many types of endonucleases are known as well, among them the restriction enzymes, which split only those DNA molecules in which they recognize particular subunits. Some split the target DNA molecule at random sites (Type I), but others split the molecule only at the recognition site (Type II) or at a fixed distance from the recognition site (Type III). Type II and III restriction enzymes are powerful tools in the elucidation of the sequence of bases in DNA molecules. They play a fundamental role in the field of recombinant DNA technology, or genetic engineering.
Other examples of endonucleases include some types of ribonuclease (RNAse), which act only upon RNA, and deoxyribonuclease (DNAse), which act only upon DNA. Aberrant activity of certain DNAse molecules, particularly defects that result in incomplete degradation of nucleic acid, is associated with autoimmune disease.