The protein coats of viruses ::
In many viruses the DNA, which codes for new viruses, is protected inside an approximately spherical protein coat, assembled from multiple copies of viral proteins, following the principles outlined here. The adenovirus, shown in the electron micrograph below, causes minor respiratory infections. The protein coat assembles itself from 240 copies of a protein (the hexon) distributed over the twenty triangular faces of an icosahedron.
The twelve pentagonal vertices are occupied by a slender fibre protein, the tip of which recognises a receptor on the host cell, inducing the cell to take up the virus. The machinery of cells translates the DNA into new virus particles with unpleasant consequences.
Electron micrographs of Adenoviruses(R.C. Valentine, 1965, J.Mol.Biol. 13, 13)
i) A crystal of the fibre protein
ii) Icosahedral core
iii) Core with attached fibres
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