Restriction enzymes are bacterial proteins that recognize specific DNA sequences and cut DNA at or near the recognition site. These enzymes are widely used in molecular genetics for analyzing DNA and creating recombinant DNA molecules.

Biological Function and Historical Background

Restriction enzymes apparently evolved as a primitive immune system in bacteria. If viruses enter a bacterial cell containing restriction enzymes, the viral DNA is fragmented. Destruction of the viral DNA prevents destruction of the bacterial cell by the virus. The term "restriction" derives from the phenomenon in which bacterial viruses are restricted from replicating in certain strains of bacteria by enzymes that cleave the viral DNA, but leave the bacterial DNA untouched. In bacteria, restriction enzymes form a system with modification enzymes that methylate the bacterial DNA. Methylation of DNA at the recognition sequence typically protects the microbe from cleaving its own DNA.

Since the 1970s...