restriction enzyme (restriction endonuclease) A type of enzyme that can cleave molecules of foreign DNA at a particular site. Restriction enzymes are produced by many bacteria and protect the cell by cleaving (and therefore destroying) the DNA of invading viruses. The bacterial cell is protected from attack by its own restriction enzymes by modifying the bases of its DNA during replication. Restriction enzymes are widely used in the techniques of genetic engineering (see DNA fingerprinting; DNA library; DNA sequencing; gene cloning; restriction mapping).

restriction enzyme Enzyme used in genetic engineering to cut a molecule of DNA at specific points, in order to insert or remove a piece of DNA. There are many different restriction enzymes; each cuts the DNA at a specific sequence of bases, allowing great precision in genetic engineering.

restriction enzyme An enzyme that causes a molecule of foreign DNA to break. Such enzymes occur in many bacteria, where they destroy viruses. Restriction enzymes are used widely in genetic engineering.