Restriction endonucleases

Restriction endonucleases recognise specific base pairs within DNA sequences where they exhibit enzymatic activity through the cleaving of DNA into smaller fragments. They are found in bacteria and it is thought that their original biological role was to protect the DNA of the bacteria within which they were contained from being infected by viral DNA by cleaving it before it could cause damage. Different restriction endonucleases are specific to different sequences (they can recognise sequence anywhere from 4-8 base pairs in length) and this results in a variety of fragment lengths being produced depending upon which restriction enzyme is used. Restriction endonucleases cleave both strands of double stranded DNA by cutting a nucleotide between a 3'-OH group and a phosphate (the phosphodiester bond found in DNA) and always produce fragments with a 5' phosphate and a 3'-OH. Some restriction endonucleases cut at the symmetry centre of the sequence they are specific for and produce what is know as a 'blunt end', ie. it has no overhang of single stranded DNA. Others produce what we call 'sticky or cohesive ends' in which a single stranded overhang is produced. These cohesive strands are what give restriction endonucleases their role in producing recombinant DNA as any two peices of DNA (including those from different genomes) that have been cut with the same restriction endonuclease will have the same 'sticky ends' and therefore be complementary to each other, enabling complementary base pair binging under the right conditions .

Also see Restriction enzyme