Polymerase Chain Reaction (PCR) Testing

A Knowledge Base Article from Medika Life

Sometimes called “molecular photocopying,” the polymerase chain reaction (PCR) is a fast and inexpensive technique used to “amplify” – copy – small segments of DNA. Because significant amounts of a sample of DNA are necessary for molecular and genetic analyses, studies of isolated pieces of DNA are nearly impossible without PCR amplification.

Often heralded as one of the most important scientific advances in molecular biology, PCR revolutionized the study of DNA to such an extent that its creator, Kary B. Mullis, was awarded the Nobel Prize for Chemistry in 1993.

What is PCR used for?

Once amplified, the DNA produced by PCR can be used in many different laboratory procedures. For example, most mapping techniques in the Human Genome Project (HGP) relied on PCR.

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PCR is also valuable in a number of laboratory and clinical techniques, including DNA fingerprinting, detection of bacteria or viruses (particularly AIDS), and diagnosis of genetic disorders. This technique has featured prominently in global testing for the novel Corona Virus (SARS-nCoV2).

Image courtesy of National Human Genome Research Institute

How does PCR work?

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.

PCR’s role in Covid19 testing

The info-graphic below will help you visualize the testing procedures for Covd19. This article will focus only on the PCR testing and not on the identification of antibodies with Serological Testing. PCR will assess if you are infected, whereas the antibodies test checks to see if you were infected.

mage courtesy of Compoundchem.com Creative Commons License. Click to enlarge

The Diagnostic or PCR Test

This test uses a sample of mucus typically taken from a person’s nose or throat. The test may also work on saliva — that’s under investigation. It looks for the genetic material of the coronavirus. The test uses PCR (polymerase chain reaction), which greatly amplifies the viral genetic material if it is present. That material is detectable when a person is actively infected.

Generally speaking, in terms of producing a reliable result, these are the best tests. However, a few days may pass before the virus starts replicating in the throat and nose, so the test isn’t guaranteed to identify someone who has recently been infected. Swabs can also sometimes fail to pick up signs of active infection.

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