The discovery of the mode of action of the class of bacterial enzymes known as restriction endonucleases provided the major breakthrough in opening up the field of genetic engineering. In vivo, these enzymes are involved in recognizing and cutting up foreign DNA entering the cell; their most likely role is thus protecting the bacteria against phage infection. The property that is relevant to us is that these enzymes recognize specific DNA sequences. The enzymes used in DNA manipulations are in fact known as Class II restriction endonucleases; these enzymes cut the DNA within the recognition sequence at a defined point. Treatment of a DNA sample with such enzymes will thus result in each molecule being cut at the same positions and thereby lead to the formation of reproducible fragments.
