Demystifying the Polymerase Chain Reaction (PCR)

Polymerase chain reaction (PCR) is the process used by all those investigating DNA (deoxyribonucleic acid), from any source, to produce a sufficiently large sample for their investigation, if they only have a sample otherwise to small for their tests. This multiplication of the sample DNA is referred to as amplification.

PCR diagramFor example, forensic scientists may take a sample of DNA from a potential suspect by wiping a squib inside their mouth.

Although this in itself is insufficient for testing purposes, they will use PCR to multiply this sample many times, giving themselves a sufficiently large sample without inconveniencing the DNA donor. Without this process, they would be scraping the inside of the mouth bare or taking half the possible suspect’s blood to enable them to perform their tests.

The term is divided into two parts, “polymerase” and “chain reaction.” The word “polymerase” describes a particular class of enzyme; the “-use” suffix simply indicates that it is an enzyme while “polymer” indicates that it acts to form a chain of organic molecules.

The polymerase used in standard PCR is a DNA-polymerase; it acts to create polymers of deoxyribonucleic acid. All living organisms have DNA-polymerases; they are necessary to enable cells to divide. The polymerase used in PCR is called TAQ polymerase because it comes from an extremophile prokaryote called Thermus aquaticus. It is a type of bacteria classified under the taxonomic classification system as a member of the domain Archaea. Archaea and Eubacteria are separated into two different domains because there are very large differences between them genetically, although both are prokaryotes. The third domain in the taxonomic system is eukaryotes; these have a more complex cell structure; all plants and animals, including humans, are eukaryotes.

The “chain reaction” portion of the term signifies that the chemical reaction is ongoing as long as the process continues. Most people are familiar with the concept of “chain reaction” about nuclear reactions, where the fission of one atom triggers the fission of those around it. The meaning is essentially the same. Each cycle of the PCR process doubles the amount of DNA so that in quite a short time you can have an abundance of identical DNA from what may be a very small sample initially.

The process used is quite simple and elegant, which is why it has become such a standard tool for all those involved in a genetic examination.