A
restriction enzyme or
restriction endonuclease is an
enzyme that cuts
DNA at or near specific recognition
nucleotide sequences known as
restriction sites. Restriction enzymes are commonly classified into three types, which differ in their structure and whether they cut their DNA
substrate at their recognition site, or if the recognition and cleavage sites are separate from one another. To cut DNA, all restriction enzymes make two incisions, once through each
sugar-phosphate backbone (i.e. each strand) of the DNA double helix.