Icosahedral models of rotational symmetry

Icosahedral models (left to right) on fivefold, threefold, and

twofold axes of rotational symmetry (Copyright © 1996 The University of

Texas Medical Branch at Galveston)

Helical Symmetry

 Single stranded RNA viruses such as paramyxoviruses, orthomyxoviruses and

rhabdoviruses have helical symmetry

 The capsid is in form of a helix

 Helical viruses resemble long rods that may be rigid or flexible

 The flexuous helical nucleocapsid is always contained within a lipoprotein envelope

 The envelope is lined internally by a matrix protein (M-protein)

 The M-protein may be rigid as in the case of bullet shaped rhabdoviruses or readily

distorted as in orthomyxoviruses and paramyxoviruses

 The capsid of helical viruses is formed by the insertion of protein units between each

turn of the nucleic acid helix.

 The capsid protein helix thus coincides with that of the nucleic acid and the length of the

helix is determined by the length of the RNA molecule

 Helical capsid devoid of nucleic acid cannot be formed.

 In RNA viruses, each capsomere consists of a single polypeptide molecule

The helical structure of the rigid tobacco mosaic virus rod (Copyright © 1996 The

University of Texas Medical Branch at Galveston)

Complex Symmetry

 Large viruses with large genome have complicated symmetry

 For example, poxviruses have complex symmetry which is neither icosahedral nor helical

For more about symmetry. please see previous post.