polymerase chain reaction
/ PCR, say P-C-R /
Suppose you have a single faint sentence written somewhere in a vast library, and you need millions of clean copies of just that one sentence to read it or work with it. PCR is a molecular photocopier that does exactly this for DNA: starting from a tiny amount of a target sequence, it makes billions of copies of that one stretch in a couple of hours, in a test tube, with no living cell involved.
PCR works by repeating a simple three-step cycle, usually 25 to 40 times. First the DNA is heated to about 95 degrees Celsius so the two strands come apart (denaturation). Then it is cooled so that two short, lab-made DNA pieces called primers stick to the two ends of the target region, one on each strand (annealing). Then a heat-tolerant DNA-copying enzyme, Taq polymerase, extends each primer along its template, building a fresh complementary strand (extension). Each cycle roughly doubles the number of target copies, so the amount grows exponentially: after n cycles you have on the order of 2 to the n copies. The two primers are what define the target — only the DNA between and including them gets amplified.
PCR, invented by Kary Mullis in 1983, transformed biology: it lets you detect a virus from a swab, type DNA at a crime scene, diagnose a genetic disease, or prepare a gene for cloning, all from vanishingly small samples. But its power is also its danger. PCR copies whatever matching template is present, including stray contaminating DNA. A single stray molecule of the wrong DNA, or a leftover product from a previous reaction, can be amplified into a strong false signal, which is why clean technique and proper controls are not optional in a PCR lab.
To detect a virus in a nasal swab, a lab picks two primers that flank a stretch unique to that virus's genome. If the virus is present, that stretch is amplified billions of times and becomes easy to detect; if it is absent, there is nothing for the primers to copy and the reaction stays dark.
PCR turns a single target molecule into a detectable mountain of copies.
PCR amplifies any template that matches the primers, not just the one you want — contamination, including the lab's own previous products, is the most common cause of false positives, so negative controls are mandatory.