Thirty rounds of heating and cooling turn one DNA fragment into a billion
Start with a single stretch of DNA, warm it, cool it and warm it again, and each lap roughly doubles the number of copies. Run thirty laps and the arithmetic reaches about a billion. That repetitive trick, the polymerase chain reaction, earned Kary Mullis a share of the 1993 Nobel Prize in Chemistry.
Mullis, an American biochemist, invented the polymerase chain reaction, usually shortened to PCR, in 1983 while working at Cetus Corporation. He shared the Nobel with Michael Smith, a biochemist who had developed other key methods for handling DNA. The point of the technique is simple: most DNA samples are far too small to study, so PCR makes huge numbers of copies of one chosen stretch of genetic code.
Each cycle has three stages. First the mixture is heated to roughly 94 to 98 degrees Celsius for up to half a minute, which pulls the two strands of the double helix apart. Then it cools to around 50 to 65 degrees, letting primers, short custom-made pieces of single-stranded DNA, latch onto the ends of the target region. Getting that temperature right matters: too cold and primers stick in the wrong places, too hot and they fail to stick at all. Finally an enzyme called DNA polymerase builds new matching strands from loose building blocks, commonly at about 72 degrees. Every new strand then serves as a template in the next round, which is why growth is exponential until ingredients run low and output levels off.
The hidden hero is a heat-proof enzyme. Ordinary polymerase falls apart at the high temperatures of the separation step, so early users had to add fresh enzyme by hand every cycle, a slow and expensive chore. Taq polymerase, taken from a bacterium called Thermus aquaticus that thrives in heat, survives the cycling. Another enzyme, Pfu, proofreads its work more accurately but runs slower, and labs sometimes blend a little of it with Taq.
A machine called a thermal cycler automates the temperature swings, often using a Peltier device that heats or cools simply by reversing its electric current. PCR now underpins genetic testing, the study of ancient DNA, detection of infectious agents, parentage testing and forensic DNA profiling.
Source: Polymerase chain reaction
