Viruses are parasites with a noncellular structure composed mostly of a protein coat around a nucleic acid.
Viruses usually are too small to be seen with the light microscope (100-2,000 Angstrom units) and must be
studied by electron microscopes. In one stage of their life cycle, in which they are free and infectious,
virus particles do not carry out the functions of living cells, such as respiration and growth; in the other
stage, however, viruses enter living plant, animal, or bacterial cells and make use of the host cell's chemical
energy and its protein and nucleic acid-synthesizing ability to replicate themselves. The existence of
submicroscopic infectious agents was suspected by the end of the 19th century. In 1892 the Russian botanist
Dimitri Iwanowski showed that the sap from tobacco plants infected with mosaic disease, contained an agent
that could infect other tobacco plants. In 1900 a similarly filterable agent was reported for foot and mouth
disease of cattle. In 1935 the American virologist W. M. Stanley crystallized tobacco mosaic virus; for that
work Stanley shared the 1946 Nobel Prize in Chemistry with J. H. Northrup and J. B. Summer. Later studies of
virus crystals established that the crystals were composed of individual virus particles, or virions. By the
early 21st cent. The understanding of viruses had grown to the point where scientists synthesized (2002) a
strain of poliovirus using their knowledge of that virus's genetic code and chemical components required.
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