What is PCR?

PCR Stands for Polymerase Chain Reaction

Scientist pipetting sample into multi well tray for analytical testing in laboratory
Getty Images/Andrew Brookes

The definition of PCR, which stands for polymerase chain reaction, is a biochemical technique performed in laboratories that is used to copy small segments of DNA (which is why it is It is also sometimes referred to as "molecular photocopying"). There are many medical, legal and research uses for PCR, including virus and bacteria identification, forensic matching and diagnosing disease (such as genetic disorders).

For hepatitis and HIV patients, one important use of PCR is determining the viral load, which is the amount of virus in the blood. Another use is finding the genotype of viruses, such as hepatitis B or hepatitis C. In addition, most mapping techniques in the Human Genome Project relied on PCR.

Basically, PCR is a way to make many copies of specific parts of DNA (DNA is the complex molecule in our cells that carries our genetic code). This simple concept (amplification, in the language of PCR) is so important that Kary B. Mullis, the person who in the 1980s developed the process, won a Nobel Prize (later, in 1993) for his work.

Before PCR, obtaining enough DNA for analysis was a time-consuming (days to weeks) and complicated process. Now, PCR can have the same result in hours. Having a relatively large amount of DNA to work with has made testing faster, cheaper and more accurate.

The National Human Genome Research Institute describes the PCR process in more detail:

To amplify a segment of DNA using PCR, the sample is first heated so the DNA denatures, or separates into two pieces of single-stranded DNA. Next, an enzyme called "Taq polymerase" synthesizes - builds - two new strands of DNA, using the original strands as templates. This process results in the duplication of the original DNA, with each of the new molecules containing one old and one new strand of DNA. Then each of these strands can be used to create two new copies, and so on, and so on. The cycle of denaturing and synthesizing new DNA is repeated as many as 30 or 40 times, leading to more than one billion exact copies of the original DNA segment.

The entire cycling process of PCR is automated and can be completed in just a few hours. It is directed by a machine called a thermocycler, which is programmed to alter the temperature of the reaction every few minutes to allow DNA denaturing and synthesis.


National Center for Biotechnology Information (NCBI), National Institutes of Health. "Polymerase Chain Reaction." http://www.ncbi.nlm.nih.gov/probe/docs/techpcr/

National Human Genome Research Institute. "PCF Fact Sheet." https://www.genome.gov/10000207