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.