Restriction Enzymes

Restriction enzymes were important in the RFLP forensic DNA-typing procedure. Many different bacteria make special enzymes called restriction endonucleases, or restriction enzymes. There are many different restriction enzymes, but they all do one thing: They cut the doublestranded DNA molecule at a specific place. Strictly speaking, the enzyme itself does not “cut” DNA; it catalyzes the hydrolysis of DNA. Hydrolysis means that a molecule is separated into two parts and a molecule of water (H2O) is added in the process. But, as a kind of shorthand, molecular biologists often talk about restriction enzymes “cutting” DNA.

A restriction enzyme cuts DNA at a specific place along the chain because it recognizes a short base sequence of four to seven bases. The restriction enzyme HaeIII, for example, recognizes the sequence . . . GGCC . . . and EcoR1 recognizes the sequence . . . GAATTC. Four additional examples of restriction endonucleases are illustrated in the figure. The heavy arrows in the figure show the cut points in double-stranded DNA at the recognition sequence. The earliest forensic DNA-typing method involved the use of restriction enzymes.