• Geoff Russell

Covid-19 testing explainer (I)

Updated: Sep 9

The virus which causes the Covid-19 disease is called SARS-CoV-2. The CoV stands for "corona virus", obviously. There are many different corona viruses, but the one that most people may have heard of is SARS. The SARS virus causes the SARS disease ... which is nice and simple, but for obscure reasons, the name of the current corona virus causing so much trouble is different from the disease it causes. That's actually quite normal. Many different kinds of virus can cause a common cold and they have different names. Perhaps another virus will come along which will cause a disease very similar to Covid-19 and it'll be good to have given them different names!

A SARS-Cov-2 virus particle is a bundle of genetic material inside a little envelope. Our genetic material is DNA, but the SARS-Cov-2 genetic material is RNA, which is slightly different. In either case, you can think of this stuff as just letters like those you are reading. Normal letters come in 26 kinds; a-z. But DNA or RNA letters only come in 4 kinds; For DNA the letters are A,G,C, and T and for RNA they are A,G,C and U.

The SARS-Cov-2 genetic material is about 29,000 letters long. So it looks like this:

1 attaaaggtt tataccttcc caggtaacaa accaaccaac tttcgatctc ttgtagatct 61 gttctctaaa cgaactttaa aatctgtgtg gctgtcactc ggctgcatgc ttagtgcact 121 cacgcagtat aattaataac taattactgt cgttgacagg acacgagtaa ctcgtctatc 181 ttctgcaggc tgcttacggt ttcgtccgtg ttgcagccga tcatcagcac atctaggttt 241 cgtccgggtg tgaccgaaag ... etc

The above is a real sequence from a genetic database. While the actual virus has RNA, the sequence is stored in DNA form ... meaning it has a 't' instead of the 'u' it would have if it were RNA.

So if you have a sample from the throat of a person with suspected Covid-19 then you need to extract RNA from that sample and check that it's sequence of letters matches the sequence above. Alternatively you can look for the antibodies your immune system makes to fight the disease.

In summary, testing is done in one of two ways: 1) looking for the RNA of the virus or 2) looking for the proteins (antibodies are proteins) that show that your body is fighting the virus.

In Part II, we'll look at the practical difficulties of the first method ... testing RNA.

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