Polymerase chain reaction—also known by its acronym PCR—is a laboratory technique that amplifies or copies any piece of DNA very quickly without using cells; DNA amplification is a method in which a small piece of DNA is copied thousands of times using PCR. DNA amplification is used in cloning, to detect small amounts of DNA in a sample, and to distinguish different DNA samples, as in DNA fingerprinting (see below).

Simply put, the process is as follows: DNA is incubated in a test tube with a special kind of DNA polymerase, a supply of nucleotides, and short pieces of synthetic, single-strand DNA. With a special machine, PCR can make billions of copies of a particular segment of DNA in a few hours (each cycle of the PCR procedure takes only about five minutes). At the end of the cycle, the DNA segment—even one with hundreds of base pairs—will be doubled. PCR is much faster than the days it took to clone a piece of DNA by making a recombinant plasmid and letting it replicate within bacteria. PCR was developed in 1983 by the American biochemist Kary Mullis (1944–) at Cetus Corporation, a California biotechnology firm; in 1993 Mullis, along with British-born Canadian chemist Michael Smith (1932–2000), won the Nobel Prize in Chemistry for development of PCR.