Virion Structure and Composition

Extracellular virus consists of the viral genome surrounded by a protective coat of protein; the resulting nucleocapsid may be surrounded by an outer lipid envelope in which viral proteins are embedded. Viral proteins confer specificity as to the range and type of host cell that may be infected. The assembly of spherical particles follows the principles of cubic symmetry with individual asymmetrical proteins clustered into symmetrical structures grouped around the cubic axis of symmetry to form an icosahedron. X-ray diffraction studies have revealed how viral proteins expose ligands important for the recognition of host cell receptors and the properties of antigenic variants. The properties of viral proteins determine temperature sensitivity and resistance to environment and chemical factors.