polymerase chain reaction
Suppose you have one faint photocopy of a single page lost inside a thick book, and you want a stack of crisp copies of just that page. PCR does the molecular equivalent: starting from even a tiny amount of DNA, it makes millions to billions of copies of one chosen stretch, and nothing else, in a test tube.
It works by repeating a three-step cycle. The DNA is heated to separate its two strands; the mixture is cooled so short primers stick to the spots flanking the target; and a heat-stable DNA polymerase extends from each primer to build a new complementary strand. Each cycle roughly doubles the number of target molecules, so after thirty or so cycles a single starting molecule has become a vast, detectable amount. The whole process runs automatically in a machine called a thermal cycler.
PCR transformed biology and beyond because it is fast, sensitive, and specific. It powers DNA fingerprinting in forensics, the detection of viruses and bacteria in clinical tests, the diagnosis of genetic conditions, and countless research tasks. Its great power is also its main caution: because it amplifies so efficiently, a trace of stray contaminating DNA can be copied right alongside the real target.
From a single drop of blood at a crime scene, PCR can amplify specific DNA regions enough to produce a genetic profile, even when only a few cells' worth of DNA is present.
Tiny input, huge output — PCR's sensitivity is what makes forensic profiling possible.
The breakthrough that made PCR practical was a heat-stable polymerase, originally from a hot-spring bacterium, that survives the repeated heating steps that would destroy ordinary enzymes.