Nuclease is any of a variety of enzymes that hydrolyze nucleic acids, including DNA and RNA. Nucleases cleave, or separate, phosphodiester bonds between the nucleotides of nucleic acids. In the case of DNA, they can be thought of as a zipper that can separates the rungs of the DNA ladder. Without the use of nucleases, there would be no genetic engineering.
Nucleases that cut bonds at known specifiable sites on a DNA or RNA strand – deoxyribonucleases (DNase) or ribonucleases (RNase) – are called restriction nucleases. There are over 900 of these, mostly derived from bacteria. Exonucleases act only at the end of a nucleic acid, removing a single nucleotide at a time; they may be specific for the 3' or 5' end of the strand. Endonucleases act within the strand; some of them are specific in that they cleave only between particular bases.
A nuclease at work locates its target, known as a recognition sequence, and bonds with it. The nuclease then makes a single cut in each of the sugar phosphates at the ends of its target. This breaks the DNA molecule into fragments. Sometimes the ends are blunt and won't bond with other things; other times, they will. By using nucleases in different ways, scientists are able to recombine DNA, remove harmful genes, and replace single genes on a DNA strand in gene therapy.