PCR (Polymerase Chain Reaction) is a method to selectively copy ("amplify") a segment of DNA. By repeatedly duplicating the DNA, PCR can generate billions of copies from even a single starting molecule. Because only the desired segment of DNA is amplified, PCR isolates a "needle in a haystack", allowing minute quantities of pathogens to be detected and providing abundant DNA for other molecular biology techniques.
In PCR, a sample undergoes a series of heating and cooling cycles in the presence of DNA polymerase, an enzyme that replicates DNA. During every cycle, each existing copy of target DNA serves as a template for synthesizing a new copy. This chain reaction doubles the number of target DNA each cycle; an exponential amplification. PCR reactions typically involve 30 - 40 cycles which are automated by a PCR thermocycler.
PCR is widely used to diagnose infectious diseases and hereditary disorders. It is also used in veterinary diagnostics, food safety testing, water & beverage quality monitoring, and forensics. In research, PCR is used to monitor gene expression, perform gene cloning & manipulation, and prepare DNA for sequencing.