Polymerase Chain Reaction (PCR) and STD Testing

Scientist watching mice in laboratory
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Polymerase chain reaction (PCR) analysis is a laboratory technique. The purpose of PCR testing is to find small amounts of DNA in a sample, using 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 of PCR is to create primers. These are short sequences of DNA that match up to the ends of the DNA sample you are trying to detect. They are the trick to finding, amplifying, and detecting a particular piece of DNA. That piece of DNA can be used to identify a pathogen. It can also be used to do things like detect genes for antibiotic resistance.

Once you have your primers, the next step in PCR is to heat the sample so that the double-stranded DNA separates into two single strands — this is called denaturation. Then the primers 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. With that, the whole PCR process begins again.

The amount of the DNA segment of interest present in the sample increases exponentially with each PCR cycle. In the first cycle one copy becomes two. Then two copies become four, then become eight, etc. This exponential growth means that, generally, only 20 to 40 cycles are needed to determine if the DNA in question is present. (If the DNA is present, 20-40 cycles are also enough 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. That means that after the initial mixture is put together, the steps can be controlled through a process known as thermocycling. Thermocycling means that 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 fact, it can be accomplished 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. PCR's creator, Kary Mullis, won the Nobel Prize in Chemistry for his work in 1993.

Why PCR Is 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 — unlike 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 (the way infections are detected by ELISA.) This means that PCR techniques can sometimes detect diseases earlier than other tests. Even better, STDs can be detected without as much need to be concerned about keeping samples alive or testing at exactly the right time.

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