Restriction enzyme

A restriction enzyme, restriction endonuclease, REase, ENase or restrictase is an enzyme that cleaves DNA into fragments at or near specific recognition sites within molecules known as restriction sites. Restriction enzymes are one class of the broader endonuclease group of enzymes. Restriction enzymes are commonly classified into five 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. These enzymes are found in bacteria and archaea and provide a defense mechanism against invading viruses. Inside a prokaryote, the restriction enzymes selectively cut up foreign DNA in a process called restriction digestion; meanwhile, host DNA is protected by a modification enzyme (a methyltransferase) that modifies the prokaryotic DNA and blocks cleavage. Together, these two processes form the restriction modification system. More than 3,600 restriction endonucleases are known which represent over 250 different specificities. Over 3,000 of these have been studied in detail, and more than 800 of these are available commercially. These enzymes are routinely used for DNA modification in laboratories, and they are a vital tool in molecular cloning. The term restriction enzyme originated from the studies of phage λ, a virus that infects bacteria, and the phenomenon of host-controlled restriction and modification of such bacterial phage or bacteriophage. The phenomenon was first identified in work done in the laboratories of Salvador Luria, Jean Weigle and Giuseppe Bertani in the early 1950s. It was found that, for a bacteriophage λ that can grow well in one strain of Escherichia coli, for example E. coli C, when grown in another strain, for example E. coli K, its yields can drop significantly, by as much as 3-5 orders of magnitude. The host cell, in this example E.