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Ifirst heard about coronaviruses in 1999. Their special cunning is in the huge length and complexity of their RNA genome. RNA is much less stable than DNA, so RNA viruses tend to be short. We measure them approximately in kilobases (kb) of information. Polio is a mere 7 kb, influenza stacks up at 14, and Ebola weighs in at 19. Severe Acute Respiratory Syndrome Coronavirus 2 (Sars-CoV-2), the causative agent of Covid-19, is 30 kb. That’s quite normal for a coronavirus, but close to the chemical limits of information storage for RNA – about as long as a strand of RNA can be without collapsing. The viruses therefore need some clever tricks to survive. I remember being fascinated by the RNA ‘pseudoknot’ and ‘slippery sequence’, which allow the viral genome to be read in two different ways simultaneously; the virus can regulate expression of different genes according to the way they are read.
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