Restriction Enzymes -

Discovery of a variety of restriction enzymes is one reason for the rapid development of recombinant DNA technology. The ability to join DNA molecules from different sources grew out of research on the restriction enzymes (also called restriction endonucleases) produced in bacteria as part of the bacterial defences against invading foreign DNAs. The DNA of the host bacterium is concomitantly protected by a chemical modification (methylation) that prevents the enzymal attack.

Restriction enzymes are a group of enzymes that recognize a specific nucleotide sequence in DNA, often four or six base pairs long, and cut both strands of DNA within the recognition site. They are site specific. Different restriction enzymes found in different organisms recognize different nucleotide sequences and therefore cut DNA at different cleavage sites.

Sometimes restriction enzymes cleave both DNA strands at precisely opposite points on the two strands, yielding blunt ended fragments. Most restriction enzymes recognize only one short base sequence in a DNA molecule and make two single strand breaks, one in each strand, generating 3'-OH and 5' -P groups at each position. The sequences recognized by restriction enzymes are often palindromes (i.e., inverted repetition sequences) which have symmetry of form.

Restriction enzymes fall into two groups according to the manner in which they cleave DNA. Some enzymes recognize a specific nucleotide pair sequence and then cleave the DNA at a nonspecific site away from that recognition site. On the other hand, some enzymes are sequence specific and thus the number of cuts they make in a particular DNA molecule or population of molecules is dependent on the number of times the particular sequence is present in the DNA.

These numbers are large but still small compared to the number of sugar phosphate bonds in an organism. Of special interest are the smaller DNA molecules, such as viral or plasmid DNA, which may have only 1-10 sites of cutting (or even none) for particular enzymes. Plasmids having a single site for a particular enzyme are especially valuable.