bacteria that cleaves DNA at specific sites along its length.
Thousands have been found, from many different bacteria; each recognizes a specific nucleotide sequence. In the living bacterial cell, these enzyme s destroy the DNA of certain invading virus es ( bacteriophage s), thus placing a "restriction" on the number of viral strains that can cause infection; the bacterium's own DNA is protected from cleavage by methyl (―CH 3 ) groups, which are added by enzymes at the recognition sites to mask them. In the laboratory, restriction enzymes allow researchers to isolate DNA fragments of interest, such as those that contain gene s, and to recombine them with other DNA molecules; for this reason they have become very powerful tools of recombinant DNA biotechnology (see DNA recombination ).