Polymerase Chain Reaction (PCR)

Scientist watching mice in laboratory
Research - Scientist watching mice in laboratory. Adam Gault/OJO Images/Getty Images

What is PCR Analysis?

Polymerase chain reaction (PCR) analysis is a laboratory technique designed to identify small amounts of DNA in a sample by a process known as amplification. During PCR amplification, the DNA of interest is copied repeatedly until there is enough of it for analysis and detection. For example, PCR can be used to identify small amounts of DNA from the organisms that cause gonorrhea or chlamydia that are present in a urine sample.

How does PCR Work?

The first step in PCR is to heat the sample so that the double-stranded DNA separates into two single strands - this is called denaturation. Then primers, short samples of DNA that match up to the ends of the DNA sequence of interest, are combined with the sample DNA. After this, a DNA polymerase is used to start DNA replication at the primer location. Finally the DNA is heated to separate the strands once more, and the whole PCR process begins again.

The amount of the DNA segment of interest present in the sample increases exponentially with each PCR cycle: one copy becomes two, then becomes four, then becomes eight, etc; so generally, only 20 to 40 cycles are needed to determine if the DNA in question is present (and, if it is, to provide a sufficient sample for analysis).

All the steps of a polymerase chain reaction -- denaturing the DNA, applying the primers, and elongating the DNA -- happen at different temperatures, so after the initial mixture is put together, the steps can be controlled through a process known as thermocycling, in which the temperature is held at the necessary levels for just long enough for each step to take place.

Thus, PCR is an efficient way of amplifying the amount of target DNA in a single test tube with little need for human intervention.

Polymerase chain reaction represented a revolution in biological technique when it was first developed in the early 1980s, and PCR's creator, Kary Mullis, won the Nobel Prize for Chemistry for his work in 1993.

Why is PCR Relevant to STD Testing?

Polymerase chain reaction, and related techniques like ligase chain reaction, are proving to be of growing importance for STD testing. This is because these techniques can directly identify small amounts of viral DNA or RNA in samples. Identifying the genetic code of a pathogen doesn't require the pathogen to be alive -- like bacterial culture or viral culture. It also doesn't require the infection to have occurred a long enough time ago for people to have developed a detectable antibody reaction (such as is detected by ELISA.) This means that PCR techniques can sometimes detect diseases earlier than other tests, and without as much need to be concerned about keeping samples alive or testing at exactly the right time.

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