All viruses are obligate intracellular parasites; they inhabit a no-man's-land between the living and the non-living worlds, and possess characteristics of both. They are now known to differ radically from the simplest true organisms, bacteria, in a number of respects:
• they cannot be observed using a light microscope
• they have no internal cellular structure
• they contain either DNA or RNA, but not both*
• they are incapable of replication unless occupying an appropriate living host cell
• they are incapable of metabolism
• individuals show no increase in size.
When inside a host cell, viruses show some of the features of a living organism, such as the ability to replicate themselves, but outside the cell they are just inert chemical
Some viruses have DNA and RNA at different phases of their growth cycle. See p. 253
Table 10.1 Some milestones in the history of virology
1892 Tobacco Mosaic Disease (TMD) shown to be caused by a filterable agent.
1898 Proposal that TMD is due to a novel type of infectious agent. Demonstration of first viral disease in animals (foot and mouth).
1901 Demonstration of first human viral disease (yellow fever).
1915/1917 Discovery of bacterial viruses (bacteriophages).
1918 Spanish influenza pandemic
1935 TMV is first virus to be crystallised.
1937 Separation of TMV into protein and nucleic acid fractions.
1939 Viruses visible under electron microscope
1955 Spontaneous reassembly of TMV from protein and
1971 Discovery of viroids.
1980 Sequencing of first complete viral genome (CaMV)
1982 Sequencing of first RNA genome (TMV) Recombinant Hepatitis B vaccine Discovery of prions
1983 Discovery of HIV, thought to be causative agent of
1990 Retrovirus used as vector in first human gene therapy trial.
2001 BSE outbreak in UK
2003 Outbreak of new human viral disease (SARS) in SE
Iwanowsky Beijerinck Loeffler & Frosch Reed
Twort, d'Herelle Stanley
Bawden & Pirie
& Ruska Fraenkel-Conrat
& Williams Diener Frank
Montaigner and Gallo Anderson
TMV, Tobacco mosaic virus; CaMV, Cauliflower mosaic caulimovirus.
structures, thus fuelling the debate as to whether they can be considered to be life forms. A particular virus has a limited host range, that is, it is only able to infect certain cell types. Nobody is sure how viruses evolved; Box 10.1 describes some current ideas.
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