The virion

Abstract

Two basic patterns of virus structure can be recognized: (a) protein subunits arranged in a spherical shell with cubic symmetry (a crystallographic term indicating the relationship of subunits to each other) and (b) protein subunits arranged with helical symmetry. In both cases the protein subunits surround the nucleic acid molecule to form a structure known as a nucleocapsid. Helical nucleocapsids of animal viruses are always enveloped in a membrane which contains virus proteins and host cell lipids; many plant viruses and some bacteriophages exist as naked helical nucleocapsids. Only one group of viruses with cubic symmetry, the herpesviruses, is enveloped. In two groups of enveloped viruses (the togaviruses and the RNA tumour viruses) the arrangement of the protein subunits of the nucleocapsid is not known. In many groups of viruses with cubic symmetry, the spherical shell is not in direct contact with the nucleic acid but encloses another protein structure containing the viral genome; this is known as the virus core.