restriction enzyme

A restriction enzyme is an enzyme that recognizes a specific sequence of double-stranded DNA and cuts the DNA at that site. Restriction enzymes, also called restriction endonucleases, are often referred to as molecular scissors and are a vital tool in genetic engineering. Over 3,000 restriction enzymes have been studied in detail. Some occur naturally in bacteria and archaea and are thought have evolved as a defense against invading viruses; others are made artificially.