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Many Applications from a Simple Idea

In April, 1983, Kary Mullis took a drive on a moonlit California mountain road and changed the course of molecular biology. During that drive, he conceived the Polymerase Chain Reaction (PCR).

PCR is an in vitro method for enzymatically syn­thesizing defined sequences of DNA (Figure 1). The reaction uses two oligonucleotide primers that hybridize to opposite strands and flank the target DNA sequence that is to be amplified. The elonga­tion of the primers is catalyzed by a heat-stable DNA polymerase (such as Taq DNA Polymerase)1.
A repetitive series of cycles involving template de­naturation, primer annealing, and extension of the annealed primers by the polymerase results in expo­nential accumulation of a specific DNA fragment.

The ends of the fragment are defined by the 5' ends of the primers2. Because the primer extension products synthesized in a given cycle can serve as a template in the next cycle, the number of target DNA copies approximately doubles every cycle; thus, 20 cycles of PCR yield about a million copies (220) of the target DNA.

The PCR technique is now so pervasive in molecular biology that it is difficult to think of life without it. Because of PCR, “insufficient nucleic acid” is no longer a limitation in molecular biology research and many medical diagnostic procedures. More importantly, innovative researchers have continually updated the definition of “PCR applications”, in­creasing the usefulness and scope of the technique.