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Chapter 16
The Viruses: Introduction
and General
Characteristics
1
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Viruses
• acellular infectious agents
• virologists
– scientists that study viruses
• virology
– study of viruses
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Early Development of
Virology
• many epidemics of viral diseases
occurred before anyone understood the
nature of their causative agents.
• historical evidence suggests that
epidemics caused by measles and
smallpox viruses were among the causes
for the decline of the Roman Empire
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Early Attempts to Prevent
Viral Disease
• Lady Wortley Montagu (early 1700s)
– proponent of inoculation of children with
material from smallpox lesions
– observed this practice among Turkish
women
• Edward Jenner (1798)
– published case reports of successful attempts
to prevent smallpox by exposure to cowpox
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Discovery of Viruses
• Charles Chamberland (1884)
– developed porcelain bacterial filters
used later in discovery of viruses
• Dimitri Ivanowski (1892)
– demonstrated that causative agent of
tobacco mosaic disease passed through
bacterial filters
– thought agent was toxin
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Discovery of Viruses…
• Martinus Beijerinck (1898-1900)
– showed that causative agent of tobacco
mosaic disease was still infectious after
filtration
– referred to agent as filterable virus
• Loeffler and Frosch (1898-1900)
– showed that hoof-and-mouth disease in
cattle was caused by filterable virus
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Discovery of Viruses…
• Walter Reed (1900)
– showed that yellow fever in humans was
caused by filterable virus transmitted by
mosquitoes
• Ellerman and Bang (1908)
– showed that leukemia in chickens was
caused by a virus
• Peyton Rous (1911)
– showed that muscle tumors in chickens were
caused by a virus
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Discovery of Bacterial
Viruses
• Frederick Twort (1915)
– first to isolate viruses that infect bacteria
(bacteriophages or phages)
• Felix d’Herelle (1917)
– firmly established the existence of
bacteriophages
– devised method for enumerating them
– demonstrated that bacteriophages only
reproduce in live bacteria
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Discovery of Chemical
Nature of Viruses
• W. M. Stanley (1935)
– crystallized tobacco mosaic virus (TMV)
– showed that TMV was composed mostly of
protein
• F. C. Bawden and N. W. Pirie (1935)
– separated TMV particles into protein and
nucleic acid components
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General Properties of
Viruses
• virion
– complete virus particle
– consists of 1 molecule of DNA or RNA
enclosed in coat of protein
– may have additional layers
– cannot reproduce independent of living
cells nor carry out cell division as
procaryotes and eucaryotes do
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Generalized Structure of
Viruses
Figure 16.1
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The Structure of Viruses
• virion size range is ~10-400 nm in diameter
with most viruses too small to be seen with the
light microscope
• all virions contain a nucleocapsid which is
composed of nucleic acid (DNA or RNA) and a
protein coat (capsid)
– some viruses consist only of a nucleocapsid, others
have additional components
• envelopes
– virions having envelopes = enveloped viruses
– virions lacking envelopes = naked viruses
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The Size and Morphology of
Selected Viruses
Figure 16.2
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Capsids
• large macromolecular structures
which serve as protein coat of virus
• protect viral genetic material and
aids in its transfer between host cells
• made of protein subunits called
protomers
14
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Helical Capsids
• shaped like hollow tubes with
protein walls
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Tobacco Mosaic Virus Structure
Figure 16.3
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Influenza Virus – an Enveloped Virus
with a Helical Nucleocapsid
Figure 16.4
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Icosahedral Capsids
• an icosahedron is a regular polyhedron
with 20 equilateral faces and 12 vertices
• it is one of nature’s favorite shapes
• capsomers
– ring or knob-shaped units made of 5 or 6
protomers
– pentamers (pentons) – 5 subunit capsomers
– hexamers (hexons) – 6 subunit capsomers
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Figure 16.5
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Figure 16.6
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Figure 16.7
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Viruses with Capsids of
Complex Symmetry
• many viruses do not fit into the
category of having helical or
icosahedral capsids
• examples are the poxviruses and
large bacteriophages
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Figure 16.8
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Figure 16.9
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Viral Envelopes and Enzymes
• many viruses are bound by an outer,
flexible, membranous layer called
the envelope
– in eucaryotic viruses some envelope
proteins, which are viral encoded, may
project from the envelope surface as
spikes or peplomers.
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Figure 16.10
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Virion Enzymes
• it was first erroneously thought that
all virions lacked enzymes
• now known a variety of virions have
enzymes
– some are associated with the envelope
or capsid but most are within the
capsid
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Viral Genome Acids
• A virus may have single or double
stranded DNA or RNA
• the size of the nucleic acid also varies
from virus to virus
• genomes can be linear or circular
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Table 16.1
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Figure 16.11
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Generalized Illustration of
Virus Reproduction
Figure 16.12
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The Cultivation of Viruses
• requires inoculation of appropriate
living host
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Hosts for animal viruses
• suitable animals
• embryonated eggs
• tissue (cell) cultures
– monolayers of animal cells
– plaques
• localized area of cellular destruction and lysis
• cytopathic effects
– microscopic or macroscopic degenerative
changes or abnormalities in host cells and
tissues
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Figure 16.13
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Figure 16.14
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Figure 16.15
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Hosts for Bacterial and Archael
Viruses
• usually cultivated in broth or agar
cultures of suitable, young, actively
growing bacteria
• broth cultures lose turbidity as
viruses reproduce
• plaques observed on agar cultures
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Figure 16.16
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Hosts for Plant Viruses
• plant tissue cultures
• plant protoplast cultures
• suitable whole plants
– may cause localized necrotic lesions or
generalized symptoms of infection
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Figure 16.17
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Virus Purification
• four commonly used methods
– differential centrifugation and density
gradient centrifugation
– precipitation of viruses
– denaturation of contaminants
– enzymatic digestion of host cell
constituents
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Differential centrifugation
Figure 16.18
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Density gradient centrifugation
Figure 16.19 (a)
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separates based on size and
density
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Figure 16.19 (b)
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Differential precipitation
• commonly uses ammonium sulfate
or polyethylene glycol
• separates viruses from other
components in a virus preparation
mixture
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Denaturation and precipitation
of contaminants
• can be achieved with heat, pH, and
organic solvents
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Enzymatic degradation of
cellular constituents
• proteases and nucleases used to
remove cellular proteins and nucleic
acids
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Virus Assays
• used to determine quantity of viruses
in a sample
• two types of approaches
– direct
• count particles
– indirect
• measurement of an observable effect of
the virus
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Particle counts
virus particles
• direct counts
– made with
an electron
microscope
Figure 16.20
• indirect counts
– e.g., hemagglutination assay
• determines highest dilution of virus that causes
red blood cells to clump together
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Measuring concentration of
infectious units
• plaque assays
– dilutions of virus preparation made
and plated on lawn of host cells
– number of plaques counted
– results expressed as plaque-forming
units (PFU)
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Measuring concentration of
infectious units…
• infectious dose and lethal dose assays
– determine smallest amount of virus
needed to cause infection or death of
50% of exposed host cells or organisms
– results expressed as ID50 or LD50
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Determination
of LD50
Figure 16.21
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Table 16.2
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Principles of Virus
Taxonomy
• lack of information on origin and evolutionary
history makes viral classification difficult
• a uniform classification system was developed
in 1971 by the International Committee for
Taxonomy of Viruses (ICTV)
– in most current report described ~2,000 viruses
– classification based on numerous characteristics
• most important – nature of genome
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