Capsule 4 : How are coronavirus proteins encoded by its genetic instructions?

27 April 2020 Julian Venables

Capsule 4

How are coronavirus proteins encoded by its genetic instructions?

by Julian Venables and Philippe Fort

The genetic instructions to make and maintain our cells reside on two similar types of long molecule, DNA and RNA. Coronavirus injects an RNA that encodes the instructions to make more viruses. How does this genetic information encode these instructions?

Consider a simple code, the Morse code, which was invented nearly two centuries ago. Morse was used in the Second World War to send messages over the radio waves. The classic distress signal was SOS, for ‘save our souls’. The Morse alphabet has just two letters, dots and dashes. On the radio, a dot is a short sound ‘dit’ and a dash is a longer sound ‘dah’. In Morse then there are combinations of dits and dahs for each of the 26 letters of the alphabet. SOS is thus coded for by three dots, three dashes and three dots again. This is easily understood on the radio as dit-dit-dit, dah-dah-dah, dit-dit-dit!

Obviously the cell uses another code than Morse to make proteins, including the Spike protein. Spike protein is actually very long as it is made of 1273 building blocks. These blocks are molecules called amino acids. There are 20 different building blocks in the protein alphabet. To make a protein, each block is added, one by one, like the beads of a necklace, by a machine in the cell called the ‘ribosome’ that decodes the genetic information.

The carrier of genetic information is the same throughout the living world and it consists of four RNA letters: A, C, G and U. To decode the information, the ribosome reads the genetic letters three-at-a-time and with each triplet it knows which brick it has to add to the protein under construction. For example the first 5 letters of the Spike protein are MFVFL and these are coded by 15 genetic letters AUG UUU GUU UUU CUU. The first triplet, AUG, codes for the first amino acid, M. The second triplet, UUU, encodes the second amino acid, F, and so on. Note that the second and fourth building block of the Spike protein are both F, and that they are, indeed, encoded by the same triplet, UUU.

Our cells contain 30,000 different proteins that make up their substance. The Spike protein is one of just ten proteins encoded by the Coronavirus. This little clutch of ten proteins suffices to take advantage of everything that is already in the cell, to make enough viral proteins and nucleic acids to make thousands of viruses, which will end up rupturing the cell and going on to infect others. We will study the viral genome in more depth in the next capsule.

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