A. Virus particles are called virions.
1. Virions are composed of either RNA or DNA that is encased in a protein coat called a capsid.
2. They are either naked or enveloped, depending on whether the capsid is surrounded by a lipoprotein envelope.
3. Virions replicate only in living cells and therefore are obligate intracellular parasites.
4. They cannot be observed with a light microscope.
B. Viral genome.
1. Viral genome may be single stranded or double stranded, linear or circular, and segmented or nonsegmented.
2. Viral-specific enzymes, other proteins within the virion, or both may be associated with the genome.
C. Viral capsid >> capsomers >> (helical, icosahedral (a 20-sided polygon), or complex)
Viral nucleocapsid = capsid and enclosed viral genome = naked viruse
D. Viral envelope >> surrounds the nucleocapsid of enveloped viruses and is composed of viral-specific glycoproteins and host-cell-derived lipids and lipoproteins.