The beautiful structure to the left is a reconstruction of the protein shell of the cowpea mosaic virus. This is a virus that infects the cowpea, a type of bean cultivated in warmer climes than the UK*. Viruses are the simplest form of life. They simply consist of a protein shell that contains and protects their genetic material, which is inside this shell. This virus’s coat is made of 120 molecules, of just two different types of protein – shown in green and yellow here. Inside are two molecules of RNA that cary the virus’s genes, plus a couple more proteins.
The remarkable thing is that beautiful structures like virus shells can form by themselves and completely spontaneously, like a Lego car assembling itself. This process is called self-assembly, and lots of types of molecules self-assemble into various structures, but I think virus shells are by far the most elegant.
And there is a reason for this elegance. You can see that the shell is very symmetrical, in fact it possesses what is called icosahedral symmetry. If it looks kind of familiar this is probably because a football has the same symmetry. The reason why the cowpea mosaic virus and many many other viruses have this symmetry is easy to understand if we look at the picture of the football to the right. Note that it is made from two types of panel, shown as black pentagons and white hexagons in the image.
Basically, if you do some math you can show that with just two types of objects: a hexagon and a pentagon, you can build a complete closed shell. I guess whoever invented the football figured this out sometime a hundred or a few hundred years ago. Evolution hit on the same idea billions of years ago.
Viruses have only tiny amounts of genetic material, which is only enough to code for a few proteins. They can’t have large amounts of RNA or DNA** because that physically would not fit inside the protein shell. The shell is tiny, only around 30 nanometres across.
So due to space constraints in the shell, it needs to be built from the fewest number of components possible, as each component needs a gene for it, and so each component adds to the genetic material that has to be squeezed inside the coat. Two is a small number of components so that is good, but then if you make a shell from two components you are basically forced to make a shell with beautiful icosahedral symmetry. The cowpea mosaic virus is forced to go for the solution with beautiful symmetry.
*There are huge numbers of viruses with similar shells. The cowpea one is a standard one that virologists study a lot as it is easy to produce.
** Our genes are coded for in DNA, as are the genes of some viruses, but some viruses encode their genes in RNA instead. The cowpea mosaci virus is one, HIV is another.