Restriction enzymes cut DNA at specific sites based on the sequence of bases along the strand at the cut site. These enzymes were first identified and studied in strains of the bacteria E. Coli in the 1950’s and 60’s. The term restriction was used to describe them because their activity restricted the growth of viruses that infect E. coli.
Restriction enzymes are nucleases - enzymes that cut nucleic acid polymers (i.e. DNA and RNA). There are two types of nuclease: endonuclease and exonuclease. Endonucleases make cuts within a DNA polymer. Exonucleases remove individual nucleotides* from the end of a strand. Restriction enzymes are a type of endonuclease - they cut at specific sites in the middle of DNA strands.