A restriction enzyme, also known as a restriction endonuclease, is a protein produced by bacteria that cleaves DNA sequences at specific sites, producing DNA fragments with a known sequence at each end. These enzymes are found in bacteria and archaea and provide a defense mechanism against invading viruses. Restriction enzymes are one class of the restriction modification system, where the restriction enzymes selectively cut up foreign DNA in a process called restriction digestion, while host DNA is protected by a modification enzyme that modifies the prokaryotic DNA and blocks cleavage.

Restriction enzymes are important tools for genetic engineering and are used to manipulate DNA for different scientific applications. They can be isolated from bacterial cells and used in the laboratory to manipulate fragments of DNA, such as those that contain genes. There are at least three thousand restriction enzymes, and each one of these enzymes cuts a specific DNA sequence and doesnt discriminate as to where the DNA comes from.

Restriction enzymes can be classified into four types, designated I, II, III, and IV, which differ primarily in structure, cleavage site, specificity, and cofactors. Type I and III enzymes are similar in that both restriction and methylase activities are carried out by one large enzyme complex, in contrast to the type II system, in which the restriction enzyme is independent of its methylase. Type IV restriction enzymes cleave only methylated DNA and show weak sequence specificity.

In summary, restriction enzymes are proteins produced by bacteria that cleave DNA sequences at specific sites, producing DNA fragments with a known sequence at each end. They are important tools for genetic engineering and are used to manipulate DNA for different scientific applications.