Virus
A virus is a parasite that must infect a living cell to reproduce. Although viruses share several features with living organisms, such as the presence of genetic material (DNA or RNA), they are not considered to be alive. Unlike cells, which contain all the structures needed for growth and reproduction, viruses are composed of only an outer coat (capsid), the genome, and, in some cases, a few enzymes. Together these make up the virion, or virus particle. Many illnesses in humans, including AIDS, influenza, Ebola fever, the common cold, and certain cancers, are caused by viruses. Viruses also exist that infect animals, plants, bacteria, and fungi.
Physical Description and Classification
Viruses are distinguished from free-living microbes, such as bacteria and fungi, by their small size and relatively simple structures. Diminutive viruses such as parvovirus may have a diameter of only 25 nanometers (nm, 10-9 meters). Poxviruses, the largest known viruses, are about 300 nanometers across, just at the detection limits of the light microscope. Typical bacteria have diameters of 1,000 nanometers or more. Information on the structure of viruses has been obtained with several techniques, including electron
| Nucleic Acid | Polarity | Family | Examples | Host | Diseases/pathologies |
| ss DNA | + | Parvoviridae | parvovirus B19 | humans | erythema infectiosum (fifth disease) |
| ds DNA | +/- | Myoviridae | Bacteriophage T4 | E.
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Copyrights
Virus from Macmillan Science Library: Genetics. Copyright © 2001-2006 by Macmillan Reference USA, an imprint of the Gale Group. All rights reserved.
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