Polymerase Chain Reaction

The polymerase chain reaction, PCR, is a laboratory technique that makes millions to billions of copies of a targeted stretch of DNA from a tiny starting sample by repeatedly cycling the sample through a defined set of temperatures: a high temperature step separates the DNA's two strands, a lower temperature step lets short primer sequences bind the target region, and an intermediate temperature step lets a heat stable DNA polymerase enzyme copy it. The American biochemist Kary Mullis conceived the technique in 1983 while working at Cetus Corporation in Emeryville, California, and first demonstrated it successfully on 16 December 1983; the invention earned him a share of the 1993 Nobel Prize in Chemistry. The technique only became practical to automate once Taq polymerase, a DNA copying enzyme isolated from the heat tolerant bacterium Thermus aquaticus, replaced the ordinary enzymes that fell apart at the temperature needed to separate the DNA strands; that discovery is what let a single machine, the thermal cycler, run the whole reaction unattended. PCR is now foundational across molecular biology, genetic testing, forensic science and medical diagnostics.