General Concepts: Virus Shapes

Early study with tobacco mosaic virus (TMV) strongly suggested that viruses were composed of repeating subunits of protein which was later supported by crystallization of TMV. A major advancement in determining the morphology of virus was the development of negative stain electron microscopy . Another modification of classical electron microscopy is cryo-electron microscopy where the virus containing samples were rapidly frozen and examined at a very low temperature; this allows us to preserve the native structure of the viruses.

A virion is a complete virus particle that is surrounded by the capsid protein and encapsidates the viral genome (DNA or RNA). Sometime structure without nucleic acid can be visible under the electron microscope those structures are called as empty capsids . In some of the viruses like paramyxoviruses the nucleic acid is surrounded by the capsid proteins and the composite structures are referred as nucleocapsid . Some of the viruses contain the lipid envelope which surrounds the nucleocapsids. The envelopes are derived from the host cell membrane during the budding process. As the envelopes are derived from host cell membrane they contain many of the surface proteins present in the host cells.

There are two kinds of symmetry found among the viruses: icosahedral and helical . In theory the icosahedral symmetry may sometime referred as spherical based on the external morphology. Icosahedral symmetry has 12 vertices , 30 edges , and 20 faces . They also have two, three, or five fold symmetry based on the rotation through axes passing through their edges, faces, and vertices respectively (Figure 3.1). The viruses of this kind look spherical in shape. In helical symmetry the genomic RNA forms a spiral within the core of the nucleocapsids (Figure 3.2). The viruses of this kind look rodlike or filamentous . The viruses which contain large DNA genomes are more complex in structure, for example- poxviruses and herpesviruses.