Variations in the structure of viruses.

All viruses contain nucleic acid (DNA or RNA) surrounded by a protein capsid. They may or may not have an outer envelope surrounding the capsid. (a) Tobacco mosaic virus (TMV) has a capsid made of 2,130 identical protein subunits, helically arranged around a strand of RNA. (b) Adenoviruses have polyhedral capsids containing fibre proteins with a knob. (c) Many animal viruses, including the influenza virus, have an envelope composed of a lipid bilayer and spike glycoproteins. The lipid bilayer is obtained from the host cell when the virus buds from the plasma membrane. (d) Some bacteriophages, such as T4, have capsids with accessory structures that facilitate invasion of a bacterial cell.