It is no exaggeration to say that genetic engineering has been made possible by the discovery and isolation of enzymes that recognize specific nucleotide sequences in DNA and can cleave the DNA at a precise point within the sequence. These enzymes are known as Type II restriction endonucleases, and they allow us to cut DNA molecules in a reproducible fashion. This precision cutting is essential for many of the procedures of genetic manipulation. Circular plasmid molecules must be cut at a unique site, to open them prior to the insertion of foreign DNA for cloning. For the isolation of genes, all molecules of the genomic DNA must be cut identically, so that each gives the same set of restriction fragments. With luck, one of these fragments will contain the desired gene, and further restriction of the isolated fragment may allow recovery of a smaller fragment containing little more than the gene.
