The characteristic shape of a virus particle is determined by its protein coat or capsid. In the non-enveloped viruses, the capsid represents the outermost layer, and plays a role in attaching the virus to the surface of a host cell. It also acts to protect the nucleic acid against harmful environmental factors such as UV light and desiccation, as well as the acid and degradative enzymes encountered in the gastrointestinal tract.
The capsid is made up of a number of subunits called capsomers (Figure 10.5), and may comprise a few different protein types or just one. The number of cap-somers is constant for a particular viral type. This repetitive subunit construction is necessitated by the small amount of protein-encoding RNA/DNA in the viral genome. The capsomers have the ability to interact with each other spontaneously to form the completed capsid by a process of self-assembly. This would be less easily achieved if there were large numbers of different protein types. Capsomers are arranged symmetrically, giving rise to two principal capsid shapes, icosahedral and helical (Figure 10.5). Both shapes can be found in either enveloped or non-enveloped viruses. Complex viruses, such as certain bacteriophages, contain elements of both helical and icosahedral symmetry.
A number of plant viruses, including the well-studied tobacco mosaic virus, have a rodlike structure when viewed under the electron microscope (Figure 10.5a). This is caused by a helical arrangement of capsomers, resulting in a tube or cylinder, with room in the centre for the nucleic acid element, which fits into a groove on the inside. The diameter of the helix is determined by the nature of the protein(s) making up the capsomers; its length depends on the size of the nucleic acid core.
An icosahedron is a regular three-dimensional shape with 20 triangular faces, and 12 points or corners (Figure 10.5). The overall effect is of a roughly spherical structure.
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