Virus structure and classification

A major branch of virology is virus classification. Viruses can be classified according to the host cell they infect: animal viruses, plant viruses, fungal viruses, and bacteriophages (viruses infecting bacteria, which include the most complex viruses). Another classification uses the geometrical shape of their capsid (often a helix or an icosahedron) or the virus's structure (e.g. presence or absence a lipid envelope). Viruses range in size from about 30 nm to about 450 nm, which means that most of them cannot be seen with light microscopes. The shape and structure of viruses can be studied with electron microscopy, with NMR spectroscopy, and most importantly with X-ray crystallography.

The most useful and most widely used classification system distinguishes viruses according to the type of nucleic acid they use as genetic material and the viral replication method they employ to coax host cells into producing more viruses:

DNA viruses (divided into double-stranded DNA viruses and the much less common single-stranded DNA viruses),

RNA viruses (divided into positive-sense single-stranded RNA viruses, negative-sense single-stranded RNA viruses and the much less common double-stranded RNA viruses),

reverse transcribing viruses (double-stranded reverse-transcribing DNA viruses and single-stranded reverse-transcribing RNA viruses including retroviruses).

In addition virologists also study subviral particles, infectious entities even smaller than viruses: viroids (naked circular RNA molecules infecting plants), satellites (nucleic acid molecules with or without a capsid that require a helper virus for infection and reproduction), and prions (proteins that can exist in a pathological conformation that induces other prion molecules to assume that same conformation).

The latest report by the International Committee on Taxonomy of Viruses (2005) lists 5450 viruses, organized in over 2,000 species, 287 genera, 73 families and 3 orders.

The taxa in virology are not necessarily monophyletic. In fact, the evolutionary relationships of the various virus groups remain unclear, and three hypotheses regarding their origin exist:

Viruses arose from non-living matter, separately from and in parallel to other life forms, possibly in the form of self-reproducing RNA ribozymes similar to viroids.

Viruses arose from earlier, more competent cellular life forms that became parasites to host cells and subsequently lost most of their functionality; examples of such tiny parasitic prokaryotes are Mycoplasma and Nanoarchaea.

Viruses arose as parts of the genome of cells, most likely transposons or plasmids, that acquired the ability to "break free" from the host cell and infect other cells.

It is of course possible that different alternatives apply to different virus groups.

Of particular interest here is mimivirus, a giant virus that infects amoebae and carries much of the molecular machinery traditionally associated with bacteria. Is it a simplified version of a parasitic prokaryote, or did it originate as a simpler virus that acquired genes from its host?

The evolution of viruses, which often occurs in concert with the evolution of their hosts, is studied in the field of viral evolution. While viruses reproduce and evolve, they don't engage in metabolism and depend on a host cell for reproduction. The often-debated question of whether they are alive or not is a matter of definition that does not affect the biological reality of viruses.

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