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The genetic materials and enzymes of a virus are enclosed by a surface structure called capsid. Some viruses also contain an envelope surrounding the capsid. The shape of a viral capsid is either helical or icosahedral. The latter is illustrated in the following two figures.
Figure 1-E-1. Icosahedron. (a) An icosahedron has 20 identical equilateral triangular faces. (b) In most icosahedral capsids, each triangular face is made up of three identical subunits. Hence, a capsid contains 60 subunits. The five subunits surrounding each vertex are arranged in a five-fold symmetry. (c) A large icosahedral capsid consists of more than 60 subunits. Some of triangular faces are made up of four subunits.
Figure 1-E-2. Structure of a subunit in the capsid of foot-and-mouth-disease virus. The capsid of foot-and-mouth-disease virus is an icosahedron, comprising 60 subunits. Each subunit is made up of four proteins: VP1, VP2, VP3 and VP4. From the symmetry property of an icosahedron, the structure of the whole capsid can be obtained.
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