Restriction Enzymes and Plasmids

The first major breakthrough on the road to genetic engineering came with work done on restriction endonucleases by Herbert Boyer of the University of California at San Francisco. As defined by Karl Drlica in Understanding DNA and Gene Cloning: A Guide for the Curious, restriction endonucleases "are a group of enzymes [a special type of protein] that . . . occur naturally in a large number of different bacterial species, serving as part of the natural defense mechanism that protects bacterial cells against invasion by foreign DNA molecules such as those contained in viruses."15

When, for example, a virus attacks a single-celled bacterium, restriction endonucleases are unleashed and go to work, cutting the invading DNA into small, nonthreatening pieces. "Crucial to this protective device is the ability of the nuclease to discriminate between its own DNA and the invading DNA; otherwise the cell would destroy...