Polymerase chain reaction (PCR) is a molecular diagnostic tool that allows for in vitro amplification of DNA at a rapid pace. The steps of PCR are: denaturation, annealing (hybridization), and extension. DNA doubles during each PCR cycle, resulting in exponential accumulation of the targeted DNA fragment. This ability to rapidly amplify a specific nucleic acid sequence in a short period of time has revolutionized molecular diagnostics.
A major advantage of PCR is that only a small amount of initial intact genomic DNA is required. PCR can be effective with only a single strand of intact DNA. This is often critical for forensic analysis where only trace amounts of DNA are available. PCR has also been used to amplify and analyze ancient DNA for anthropologic and archeological purposes. Invented in 1983 by an American biochemist Kary Mullis, PCR is now commonly used in many medical and clinical research laboratories with a broad variety of clinical applications.