Corona test: which tests are available and how reliable they are.
It’s a back and forth: Who will even get a corona test? What does PCR mean? And why not just test all of them? There’s a good reason for that.
How does the PCR test work?
PCR stands for the English term polymerase chain reaction, in German polymerase chain reaction. It was developed in 1983 by Kary Mullis. Mullis received the Nobel Prize in Chemistry for this in 1993, because the development was a milestone in molecular biology research. Still, he was a weirdo – he denied, among other things, that HIV is responsible for AIDS and man-made climate change as well.
The DNA that you want to replicate is often referred to as the starting DNA. At the beginning of the process, it is placed in a reaction vessel together with the replication enzymes and building blocks. This mix of molecules always contains very similar substances.
On the one hand, the individual “DNA letters” adenine, guanine, thymine and cytosine. The DNA is made up of these building blocks. In addition, there is a so-called DNA polymerase, an enzyme that can put these building blocks together. Then there are the primers. They show the polymerase where to start assembling the DNA building blocks.
The DNA is put into a reaction vessel together with the DNA letters, the polymerase and the primers. For example a small tube. This is then put into a so-called thermal cycler. This is a device that can automatically change the temperature and both heats and cools the tube during the PCR.
The process of the PCR
This causes the two strands of DNA to separate from each other. DNA is a double helix made up of two complementary strands. Complementary means that the structure of the two strands is dependent on one another and one can always deduce the structure of the other from one of the two strands of DNA.
After denaturation, the DNA is therefore present as a single strand. It is now, so to speak, free to be re-paired.
If the primer cannot find an exactly matching stretch of DNA, it cannot attach. The primers are therefore gene-specific. In the case of the corona tests, they are matched to certain genes of the SARS-CoV-2 virus. Namely, on genes that only occur in this form in SARS-CoV-2.
This completes the first cycle of the PCR, consisting of denaturation, attachment and extension. In order to continue to replicate the DNA, the temperature is simply raised by the thermal cycler to 94 degrees Celsius and the process starts all over again. The amount of DNA grows more and more because a larger number of templates are available each time. Hence the term “chain reaction”.
There are slight variations in performing PCRs. The temperatures of the respective steps can vary slightly, for example, depending on the primers used. In cases, we also do accumulation and elongation in the same temperature step.
From viral RNA to DNA
Just like polymerase, reverse transcriptase also needs a primer to help it find a starting point. Starting from the primer, the reverse transcriptase then attaches the complementary DNA building blocks to the virus RNA. The resulting DNA strand contains the same genetic information as the virus genome.
After the DNA-RNA double strand has been separated by heating, the DNA strand is used as a template for the PCR. After that, the cycles run like normal PCR.
Here, too, there are slight variations in the process, similar to normal PCR.
Luminous genes as a distinguishing feature
With real-time PCR, so-called DNA probes are located in the reaction vessel in addition to the gene-specific primers. Like the primer, these probes are gene-specific and only bind to the coronavirus gene segment sought during the deposition phase in which the primer also binds. The probes have a fluorescent label that is inactive as long as the probe is intact.
However, the probe does not remain intact. If, after the deposition phase, the polymerase begins to elongate, the probe is in the way. It is destroyed by the polymerase during the extension. This releases the fluorescence and the sample begins to glow.