Avirus(Latin for “poison”) is an obligate intracellular parasite that can only
replicate inside a living host cell. Once inside the living host cell, a virus
becomes integrated in the metabolism of its host, making a virus difficult to con-
trol by chemical means. You cannot kill a virus with antibiotics. Drugs that
destroy the host’s ability to be used by a virus for replication tend to also be
highly toxic and have a negatively and sometimes deadly effect on the host cell.
Before a virus enters a cell, it is a free virus particle called a virion. A virion
cannot grow or carry out any biosynthetic or biochemical function because it is
metabolically inert. Viruses are not cells. They vary in size from 20 nanometers
(polio virus) to 300 nanometers (smallpox virus) and cannot be seen under a
light microscope.
In 1933, microbiologist Wendell Stanley of the Rockefeller Institute for Medical
Research showed that viruses could be regarded as chemical matter rather than
as living organisms.VIRAL STRUCTURE
The major components of a virus are:- Nucleic acid core. The nucleic acid corecan either be DNA or RNA that
makes up the genetic information (genome) of the virus. RNA genomes
only occur in viruses. - Capsid. Acapsidis the protein coat that encapsulates a virus and protects
the nucleic acid from the environment. It also plays a role in how some
viruses attach to a host cell. A capsid consists of one or more proteins that
are unique to the virus and determine the shape of the virus. - Envelope. An envelopeis a membrane bilayer that some viruses have out-
side their capsid. If a virus does not have an envelope, the virus is called a
naked virus. Examples of diseases caused by naked viruses are chickenpox,
shingles, mononucleosis, and herpes simplex. A naked virus is more resist-
ant to changes and is less likely to be affected by conditions that can dam-
age the envelope. Environmental factors that can damage the envelope are:- Increased temperature
- Freezing temperature
- pH below 6 or above 8
- Lipid solvents
- Some chemical disinfectants (chlorine, hydrogen peroxide, and phenol)
Naked viruses are more resistant to changes in temperature and pH. Examples
of diseases caused by naked viruses include poliomyelitis, warts, and the com-
mon cold.
(^186) CHAPTER 12 Viruses, Viroids, and Prions