What is a polymerase chain reaction

PCR is a laboratory diagnostic method aimed at identifying the causative agents of infectious diseases. The three-letter version is an abbreviation of the name “polymerase chain reaction”. Actually, this name reflects the essence of the method, but in order to understand it, you will have to thoroughly recall the school course in biology.

But first, a little history. The PCR method was developed in 1983 by Carey Mullis, for which he was awarded the Nobel Prize. Initially, the method was used mainly for scientific purposes, but then, having discerned its prospects and effectiveness, the method began to be promoted into practical medicine.

In the literature, such a figurative description of PCR is very common: it is a method by which scientists can find a needle in a haystack and then build a stack of these needles. In principle, a very accurate description. If we continue the comparison, the needle is a small section of the genetic material of the microorganism, and the haystack is the human body in which this microorganism has settled.

What the PCR analysis shows

The analysis allows you to detect the presence of genetic material of infectious pathogens. PCR in gynecology and urology is widely used to detect latent and difficult-to-diagnose infections.

How it works

DNA, a double-stranded deoxyribonucleic acid consisting of a sequence of four nucleotides, which are usually denoted by the letters A (adenine), G (guanine), T (thymidine) and C (cytosine), is responsible for genetic information in a living organism of any size. One of the basic rules of genetics is the rule of complementarity, that is, the nucleotides of neighboring DNA strands are connected only in a certain order: A with T, G with C, and nothing else.

Each living creature (bacteria, virus, fish, beast) has its own DNA, and to identify a specific organism, it is enough to have only a small section of this repository of genetic information. Some types of microorganisms, for example, the human immunodeficiency virus, store genetic information in another nucleic acid – RNA, but its fragments can also be found using PCR.

For each pathogen, a specific genetic detector has been created, a reference DNA fragment, which, according to the principle of complementarity, accurately detects “its” DNA fragment and triggers a reaction to create a huge number of its copies.

One PCR cycle takes about three minutes, the number of copies grows exponentially. Thus, in a few hours, the number of fragments increases several billion times. It is clear that now it is quite easy to determine which pathogen is in this particular infection.


Theory is, of course, great, but what do we get at the end? What practical benefit do people get when they go in search of harmful microorganisms, armed with PCR?

  • Of course, one of the main advantages is the versatility of the method. PCR allows detecting any DNA and RNA, even when other methods are powerless. The equipment used for PCR is standard, it does not depend on what exactly and where exactly we are looking for;
  • The next advantage is high specificity. In the material sent for research, a unique nucleotide sequence is determined that is characteristic only for a particular pathogen. Thus, we can say that the specificity of the method reaches 100%. In addition, this makes it possible to simultaneously, in the same material, search for several pathogens without any damage to the quality of the response;
  • The method has the highest sensitivity – we have already said that it is possible to find only one fragment of the pathogen’s genetic material;
  • The undoubted advantage of the method is its efficiency. Setting up a reaction takes several hours, thus, all diagnostics, from the moment of submitting the material for analysis to obtaining the result, will take only one day;
  • PCR helps to determine the pathogen. Thus, it became possible to accurately diagnose the disease even in the incubation period, when there are no clinical or laboratory signs of the disease.