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In the 1990s scientists discovered how to harness DNA polymerase. It is an enzyme that attaches to a single strand of DNA and then generates the matching sequence of DNA. This formed a technique called PCR - polymerase chain reaction. The essence of PCR is making a lot of copies of any DNA that matches the priming sequence.

DNA is made of base pairs of the following nucleotides Adenine, Guanine, Cytosine, Thymine. In double stranded DNA an A is paired with a T and a G is paired with a C. The single stranded sequences encode all the amino acids to make proteins using 3 nucleotide messages. Matching DNA will tend to bind to opposing strands.

Here is a link to a coding chart of DNA to amino acids.
https://www.researchgate.net/figure...quence-of-any-mRNA-Tortora-GJ_fig10_285387474

The DNA polymerase needs to start from a double stranded location, so labs mix in "primers". These are known sequences of DNA that can bind to test DNA if the test DNA is a match.

The way the test kits work in general:

1) Get a swab
2) isolate all the genetic material
3) isolate out RNA
4) reverse transcribe the RNA into DNA with a reverse transcriptase
5) add the primers that match whatever dna you are looking to detect. The nucleotides are often labeled with fluorescent materials that can be detected under UV light
6) start the PCR reaction
7) detect to see if DNA matching the primers was amplified

If the virus is present then primers that match the virus will bind to the virus DNA and allow amplification to happen. This will create consistent DNA strands of a predictable size. One method of detection is electrophoresis in a gel. Electrical voltage is applied to a gel, DNA has a charge and migrates through the gel and can be separated by size. This ensures the amplified DNA is the size you would expect for a particular virus.

Another is to have fluorescent DNA templates that can match and bind to viral strands. the electrophoresis method can be done without purchasing a premade kit. The template method (in the instructions below) would require a lab to purchase templates. You couldnt use templates directly against the original sample because there is not enough of the genetic material present to detect.

The test steps themselves probably only take a few hours total or less. The PCR is under an hour, template binding is probably just 15 minutes. Here is a link to an example protocol
https://www.cdc.gov/coronavirus/2019-ncov/lab/rt-pcr-detection-instructions.html#rRT-PCR-assays

As the virus mutates it is possible for the regions that match the primers to mutate and primers will no longer match. Scientists typically try to find conserved sequences to prime. The challenge with this is that conserved sequences tend to be the same across all strains. Conserved areas generally encode for proteins that are critical for the survival of the virus. Rapidly mutating sequences tend to be areas on the outside of the virus that allow the virus to change so the immune system can no longer recognize it. So you need to find an area that is different between strains, but not so rapidly mutating that your primers are useless.
 
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