We describe a typical cloning procedure, starting with an illustration of how restriction enzymes work in the process.
One of the first restriction enzymes, dubbed EcoR1, was discovered in E. coli and found to cut only within the nucleotide sequence GAATTC (or its mirror image CTTAAG), and specifically between the G and A. The enzyme cuts both DNA strands of the double helix creating small single-stranded fragments at the cut sites that stick out:
Double-stranded DNA before cutting:
Double-stranded DNA after cutting with EcoR1:
Note that the Xs in this example can be A, T, C, or G, corresponding to the basic alphabet ...