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The primary function of a virus is to infect host cells and create more viruses. The virus does this by taking over the host cell's protein and genetic material-making processes, forcing it to produce the new viruses. Exactly how viruses function in this manner is best understood by examining general viral structure, classification, and reproductive strategies.
There are three basic structures for standard viral capsids: icosahedral, helical, and complex. Icosahedral capsids are 20-sided, made of triangular capsomere subunits. The points of the triangular subunits join at 12 vertices about the shape. Although exact structure varies from virus type to virus type, a common arrangement is five or six neighboring triangular subunits at each vertex. Some viruses show more than one capsomere arrangement within the capsid. An example of a virus having an icosahedral structure is adenovirus, the virus that can cause acute respiratory disease or viral pneumonia in humans.
The helical viruses have protein subunits that curve about a central axis running the length of the virus. The fanlike arrangement of protein forms a three-dimensional ribbon-shaped structure that covers the viral genome. Some of these capsid structures are stiff and rodlike, while other helical viruses are more flexible. The influenza virus is an example of a virus with a helical capsid structure.
The third type of virus capsid structure is called complex. Although the structure is regular from virus to virus of the same type, the symmetry is not patterned enough to be fully understood. For example, poxvirus, the virus that causes smallpox in humans, has a complex capsid structure of over 100 proteins. Virologists are still trying to determine the exact arrangement of these proteins.
The combination of the capsid and the viral genome is known as a nucleocapsid. Some nucleocapsids are infective in this form and are known as naked viruses. Others require a surrounding lipid membrane derived from the host cell to be infective. The membrane envelope can encompass one or more nucleocapsids and usually contains on its surface at least one viral protein in addition to the host cell components. Viruses of this type are called enveloped or coated viruses.
Viruses are classified according to structural characteristics such as whether the virus genome is made of DNA or RNA. Both of these nucleic acids can form ladder-like structures where each side of the ladder is known as a strand. Viruses are differentiated by whether the DNA or RNA is single or doubled-stranded. The type of capsid structure and whether the virus is naked or enveloped are also considered. A few viral classifications take into account differences in replication strategy.
Author Info: Michelle L. Johnson M.S., The Gale Group Inc., Gale, Detroit, Gale Encyclopedia of Nursing and Allied Health, 2002This feature is for informational purposes only and should not be used to replace the care and information received from your healthcare provider. Please consult a healthcare professional with any health concerns you may have.
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