1. Viruses are simple, acellular entities consisting of one or more molecules of
either DNA or RNA enclosed in a coat of protein (and sometimes, in addition,
substances such as lipids and carbohydrates). They can reproduce only within
living cells and are obligately intracellular parasites.
2. Viruses are cultured by inoculating living hosts or cell cultures with a
virion preparation. Purification depends mainly on their large size relative
to cell components, high protein content, and great stability. The virus
concentration may be determined from the virion count or from the number
of infectious units.
3. All viruses have a nucleocapsid composed of a nucleic acid surrounded by
a protein capsid that may be icosahedral, helical, or complex in structure.
Capsids are constructed of protomers that self-assemble through
noncovalent bonds. A membranous envelope often lies outside the
nucleocapsid.
4. More variety is found in the genomes of viruses than in those of procaryotes
and eucaryotes; they may be either single-stranded or double-stranded DNA or
RNA. The nucleic acid strands can be linear, closed circle, or able to assume
either shape.
5. Viruses are classified on the basis of their nucleic acid’s characteristics,
capsid symmetry, the presence or absence of an envelope, their host, the
diseases caused by animal and plant viruses, and other properties.