General Properties of Viruses

Viruses are a unique group of infectious agents whose distinctiveness resides in their simple, acellular organization and pattern of reproduction. A complete viruses particle or virion consists of one or more molecules of DNA or RNA enclosed in a coat of protein, and sometimes also in other layer. These additional layers may be very complex and contain carbohydrates, lipids, and additional proteins.

Viruses can exist in two phases: extracellular and intracellular.

Virions, the extracellular phase, posses few if any enzymes and cannot reproduce independent of living cells. In the intracellular phase, viruses exist primarily as replicating nucleic acids that induce host metabolism to synthesize virion components; eventually complete virus particles or virions are released.

In summary, viruses differ from living cells in at least three ways:

1. Their simple, acellular organization;

2. The presence of either DNA or RNA, but not both, in almost all virions ( human cytomegalovirus has a DNA genome and four mRNAs);

3. Their inability to reproduce independent of cells and carry out cell division as prokaryotes and eukaryotes do. Although some acteria such as Chlamydia and rickettsia are obligate intracellular parasites like viruses, they do not meet the first two criteria.