Fig. 2.2

Symmetry forms of viral capsids. (a) Helical symmetry; the symmetry planes run parallel to the longitudinal or transverse axis of the particle (e.g. tobacco mosaic virus capsid, nucleocapsid of paraviruses or orthomyxoviruses). (b) Cubic-spherical symmetry; icosahedron with rotational symmetry whose centres of the symmetry axes are at the vertices of the icosahedron (fivefold symmetry axis) in the middle of the triangle (threefold symmetry axis) and along the edges (twofold symmetry axis). Picornaviruses, parvoviruses and adenoviruses are examples of viruses with such capsid forms