Index >> Virus Structure and Classification >> Virus Polyhedral (Icosahedral) Symmetry

Virus Polyhedral (Icosahedral) Symmetry

Crick and Watson have shown that the polyhedral capsids can have three possible types of symmetry, viz. tetrahedral, octahedral and icosahedral.

It has been shown that an icosahedron is the most efficient shape for the packing and bonding of the subunits of a near spherical virus Therefore viruses are icosahedral rather than tetrahedral of octahedral.

A large number of intermolecular bonds can be formed in this type of structure, and it therefore has low free energy. An icosahedron is a regular polyhedron with 20 faces formed by equilateral triangles, and 12 intersecting points or corners.

As mentioned previously, each capsid consists of many capsomeres. Each capsomere is composed of few monomers which form polygonal rings, each with a central space of up to 40 A.
The monomers are the structural units, and are made up of one or more polypeptide chains.

Number of capsomeres

According to the rules of crystallography, only a certain number of capsomeres can be present in an icosahedral capsid.

The minimum number of capsomeres can theoretically be 12, followed by 32,42,72,92,162,252,362,492,642 and 812. Of these capsomeres, 12 are pentameres occupying the 12 corners, while the rest are hexameres.

The actual number of capsomeres found in different viruses are: φX174, 12; turnip yellow mosaic virus and poliovirus, 32; polyoma virus and papilloma virus, 72; reoviruses, 92; herpesviruses, 162; adenoviruses, 252, and tipula iridescent virus,812.

φX174. The bacteriophage φXI74 contains 12 capsomeres. It has been suggested that each capsomere is actually a cluster of five units. Therefore the capsid is probably made up of 60 identical units.