PCR, or polymerase chain reaction, uses repeating cycles of heating and cooling to replicate strands of DNA from a sample. PCR can amplify and copy a single gene from a sample multiple times.
PCR requires four main components. The first is the DNA sample containing the section, or sections, for copying. Secondly, PCR requires a primer. Primers are short segments of DNA a scientist creates to match the DNA sample.
The next requirement of PCR is DNA polymerase, an enzyme that copies DNA. Human DNA polymerase denatures, or breaks down, at PCR temperatures, so researchers often use DNA polymerase from a heat-tolerant bacteria. Finally, PCR requires nucleotides: adenine, guanine, cytosine and thymine. These are the base pairs that provide the coding elements of DNA.
Researchers first heat the PCR mixture to a temperature that denatures the DNA double helix. A cooling step follows, which allows the primers to bind to the sample DNA. Another healing cycle follows during which DNA polymerase elongates the DNA strand. Repeating these steps multiple times creates many new copies of the original DNA sample in as few as three hours.
PCR is useful in the diagnosis of viral diseases and some forms of cancer. It is also a common tool in forensic science for replicating small samples from crime scenes.