In the last two decades, tremendous advances have been made in these areas. One of the most basic requirements of manipulating DNA is the ability to cleave it into fragments and rejoin these fragments. To accomplish this, biologists have utilized naturally occurring enzymes. These enzymes are called restriction endonucleases or restriction enzymes .
Restriction enzymes occur naturally in bacteria and are used by the bacteria as a defense mechanism.
Restriction enzymes recognize specific sequences of DNA and cut at those sequences. Hundreds of restriction enzymes have been discovered and purified and they can be used to cut DNA into many different fragments. One important and much used restriction enzyme is EcoRI, which gets its name from its source bacterium E. coli . 5' EcoRI has two identical subunits that cleave each DNA strand between the G and A. Combinations of different restriction enzymes will cut DNA into various combinations of fragments. This allows researchers to cut DNA in very specific ways by using restriction enzymes that recognize specific sites.