Restriction Enzymes

Students are introduced to restriction enzymes and simulate the activity of restriction enzymes with scissors. They are also introduced to restriction maps and asked to make simple predictions based on a map. Restriction enzymes were originally discovered through their ability to break down foreign DNA. Restriction enzymes can distinguish between the DNA normally present in the cell and foreign DNA, such as infecting bacteriophage DNA. They defend the cell from invasion by cutting foreign DNA into pieces and thereby rendering it nonfunctional. Restriction enzymes appear to be made exclusively by prokaryotes. The action of restriction enzymes is introduced and modeled in the activity DNA Scissors. The idea of rejoining restriction fragments and the need for complementarity in the single-stranded “tails” are introduced in the activity DNA Scissors. Restriction enzymes and DNA ligase play starring roles in DNA cloning. The discovery of restriction enzymes gave scientists a way to cut DNA into defined pieces. Every time a given piece of DNA was cut with a given enzyme, the same fragments were produced. These defined pieces could be put back together in new ways. A new phrase was coined to describe a DNA molecule that had been assembled from different starting molecules: recombinant DNA. After restriction digestion, the fragments of DNA are often separated by gel electrophoresis. The chapter also presents answers to exercise questions.