BIOL 2011L Lecture Notes - Lecture 5: Retrovirus, X-Ray Crystallography, Virus Classification

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26 Nov 2017
A major branch of virology is virus classification. Viruses can be classified according to the
host cell they infect: animal viruses, plant viruses, fungal viruses, and bacteriophages
(viruses infecting bacterium, which include the most complex viruses). Another
classification uses the geometrical shape of their capsid (often a helix or an icosahedron) or
the virus's structure (e.g. presence or absence of a lipid envelope). Viruses range in size
from about 30 nm to about 450 nm, which means that most of them cannot be seen with
light microscopes. The shape and structure of viruses has been studied by electron
microscopy, NMR spectroscopy, and X-ray crystallography.
The most useful and most widely used classification system distinguishes viruses according
to the type of nucleic acid they use as genetic material and the viral replication method they
employ to coax host cells into producing more viruses:
DNA viruses (divided into double-stranded DNA viruses and single-stranded DNA viruses),
RNA viruses (divided into positive-sense single-stranded RNA viruses, negative-sense single-
stranded RNA viruses and the much less common double-stranded RNA viruses),
reverse transcribing viruses (double-stranded reverse-transcribing DNA viruses and single-
stranded reverse-transcribing RNA viruses including retroviruses).
The latest report by the International Committee on Taxonomy of Viruses (2005) lists 5450
viruses, organized in over 2,000 species, 287 genera, 73 families and 3 orders.
Virologists also study subviral particles, infectious entities notably smaller and simpler than
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