Protein (more specifically, an endonuclease) produced by bacteria that cleaves DNA at specific sites along its length. Thousands have been found, from many different bacteria; each recognizes a specific nucleotide sequence. In the living bacterial cell, these enzymes destroy the DNA of certain invading viruses (bacteriophages), thus placing a “restriction” on the number of viral strains that can cause infection; the bacterium's own DNA is protected from cleavage by methyl (singlehorzbondCH3) groups, which are added by enzymes at the recognition sites to mask them. In the laboratory, restriction enzymes allow researchers to isolate DNA fragments of interest, such as those that contain genes, and to recombine them with other DNA molecules; for this reason they have become very powerful tools of recombinant DNA biotechnology (see DNA recombination).
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Restriction may also refer to:
The adjectival form, "restricted," may be used in reference to written or unwritten covenants enforcing racial or ethnic segregation or religious segregation — frequently anti-Jewish or more generally anti-Semitic — in housing (see "Restrictive covenant") or in organizations, such as country clubs.