Nucleases are enzymes that cleave, or separate, phosphodiester bonds between the nucleotides of nucleic acids. You might imagine them as a zipper that can separate 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 are called restriction nucleases. There are over 900 of these, mostly derived from bacteria. An endonuclease cleaves bonds inside the DNA/RNA strand; an exonuclease cleaves them from the ends.
A nuclease at work locates its target; also known as a recognition sequence, this is the spot on DNA that the nuclease will bond with. It makes a single cut in each of the sugar phosphates at the ends of its target. This will break the DNA molecule into fragments. Sometimes the ends are blunt and won't bond with other things; other times, they will. Again, it depends on the nuclease you use.
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.
Web Resources On Nuclease
Nucleases: DNase and RNase
Restriction Enzymes Glossary
Book Resources On Nuclease
Nuclease Methods and Protocols by Catherine H. Schein
Molecular Biology of Nucleases by Nawin C. Mishra