All virions have at least one protein coat, the capsid or nucleocapsid that encases and protects the nuclei acid genome. The protein coat of all but the most complex animal viruses display helical or icosahedral symmetry. Apart from its protective role, the outer surface of the virus is responsible for recognition of the host cell . This involves the binding of virus-attachment protein to a cellular receptor molecule. Each subunit is made of a number of identical or nonidentical viral proteins.
Icosahedral symmetry is the most efficient way to arrange protein subunits in a closed shell.
Symmetry and asymmetry Figure 4. The site of virus assembly varies for different viruses. Incorporation of the viral genome requires its discrimination from a large population of cellular nucleic acid and its packaging in the interior of the virion. By direct contact of the genome with nucleuocapsid or capsid proteins. The structural units of the icosahedral capsids of many small RNA viruses contain proteins that interact with the viral nucleic acids at the inner surface of the capsid shell.