Crown in a haystack

Multiply to see

The main difficulty in detection of pathogens is that they are very small. Neither a virus nor a bacterium not make out under the microscope just.

More precisely, you can see, but it is necessary to catch them in sight with a microscope and preferably also pre-painted. But as to browse “manually” the whole blood sample or saliva as a whole, requires incredibly time-consuming, only one way out — breed the enemy.

You don’t even have the virus to infect human cells, we can propagate it artificially, or rather — not the whole virus, and its genome. For this purpose, PCR, polymerase chain reaction. In fact, it is the same thing that makes the cell when it copies its DNA when dividing. Only during the PCR, we copy the desired DNA, not once, but thousands until it will be enough that our device was able to “see”.

In the case of coronavirus, the method looks like. The virus is a single strand of RNA (and protein shell, which builds the infected cell by reading the information from the RNA). To start to multiply, it is necessary to translate it into DNA. To do this, molecular biologists (as well as viruses in the cell) using reverse transcription: the enzyme revertase builds DNA on the RNA matrix, actually rewriting the information into another language.

Then starts the actual PCR. In a solution of viral DNA add primers — priming for completing, polymerase responsible enzyme and nucleotides — the building materials for DNA. Each cycle of chain reaction consists of three stages:

  1. The solution is heated, the DNA splits into separate strands.
  2. The solution was cooled, priming-primers sit on certain stretches of DNA.
  3. The solution was heated, polymerase starts to work — recognizes the primers and starting from them, completes the second strand of DNA.

Then the cycle is repeated with the only difference that since the last time the DNA was twice as each strand polymerase added two more pair. Thus, in each cycle we “propagated” the viral genome, and the number of DNA molecules in the solution increases exponentially.

Previously, the amount of DNA was determined at the end of the reaction, the sample is treated with a dye for DNA and dispersed in the gel to highlight the desired small fragments. Then the sensitivity of the method depended in fact from human eyes — a strip of DNA on the gel had to be thick enough to be able to distinguish.

Now the method is designed to be simpler: in the mix for PCR directly add fluorescent dye to DNA. So the long filament start to glow, and the device with each cycle measures the intensity of the glow. Because of this, today the technique is called “real time PCR” (real-time PCR). Now we do not depend on their own eyes and are only limited by the sensitivity of the instrument to glow.

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