The polymerase chain reaction is a technique for quickly "cloning" a particular piece of DNA in the test tube (rather than in living cells like E. coli). Thanks to this procedure, one can make virtually unlimited copies of a single DNA molecule even though it is initially present in a mixture containing many different DNA molecules.
|Discussion of DNA synthesis|
Using automated equipment, each cycle of replication can be completed in less than 5 minutes. After 30 cycles, what began as a single molecule of DNA has been amplified into more than a billion copies (230 = 1.02 x 109).
With PCR, it is routinely possible to amplify enough DNA from a single hair follicle for DNA typing. Some workers have successfully amplified DNA from a single sperm cell. The PCR technique has even made it possible to analyze DNA from microscope slides of tissue preserved years before. However, the great sensitivity of PCR makes contamination by extraneous DNA a constant problem.
|View an animation of the PCR.|
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Mullis, K. B., "The Unusual Origin of the Polymerase Chain Reaction", Scientific American, April 1990. The author tells how he worked out the principles of the PCR (which won him a Nobel Prize) while driving at night to his weekend retreat in the California mountains.