Viral Structure, Classification, and Replication
18-3 Virion structures. Nonenveloped (naked) viruses consist of a genome surrounded by a protein shell, or capsid. Shown here is an icosahedral capsid, the most common type in nonenveloped viruses. Enveloped viruses have a membrane that surrounds the nucleocapsid, which can have an icosahedral, icosadeltahedral, or helical shape. The helical nucleocapsid, found only in most enveloped (−) RNA viruses, is formed by association of viral proteins, including RNA polymerase, with the genome.