The polymerase chain reaction (PCR) is a technique in which DNA is replicated enzymatically in vitro. Like amplification of DNA within living cells, the technique allows a small amount of DNA to be amplified exponentially. It can be performed without restrictions on the form of DNA and can be extensively modified to perform a wide array of genetic manipulations.
PCR is used to amplify specific regions of a DNA strand. This can be a single gene, part of a gene, or a non-coding sequence. PCR typically amplifies only short DNA fragments, usually up to 10 Kb. Some methods can copy fragments up to 47 Kb in size, which is still much less than the chromosomal DNA of a eukaryotic cell - for example, a human cell contains about three billion base pairs.
PCR is commonly used in medical and biological research labs for a variety of tasks, such as:
- The detection of hereditary diseases;
- Identification by genetic profiling;
- Diagnosis of infectious diseases;
- The cloning of genes;
- Evolutionary studies;
- Paternity testing.
PCR was invented by Kary Mullis in 1983 whilst working in California for Cetus, one of the first biotechnology companies. His job was to make short chains of DNA for other scientists. Mullis has written that he thought of PCR while cruising along the Pacific Coast Highway 1 one night in his car. He was thinking of a new way to analyse mutations in DNA when he realised that he had instead invented a method of amplifying any DNA region. Mullis has said that before his trip was over, he was already savouring the prospects of a Nobel Prize. He shared the Nobel Prize in Chemistry with Michael Smith in 1993.