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Chapter 13 Viruses, Viroids
& Prions
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Viruses - General Characteristics
obligate intracellular parasites (completely inert when not in a host cell)
very small (filterable)
contain a single type of nucleic acid (RNA or DNA)
contain a protein coat which surrounds the nucleic acid
(sometimes the coat is surrounded by a lipid envelope)
multiply inside living cells by using the cell’s own molecular machinery
cause the synthesis of special structures to facilitate the transfer of the
virus to other cells
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Host Range
most viruses can only infect a limited range of cell types
(& usually only one host species)
bacteriophages (or phages) are viruses which infect bacteria
host range is determined by the specific binding of the virus to
particular structures , called receptors, on the surface of the host cell
virus binds to the receptor via hydrogen bonding
Viral size – range 20nm – 975nm (refer to fig 13.1)
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Viral Structure
Virion - complete, fully developed, infectious viral particle
composed of nucleic acid which is surrounded by a protein coat
which protects it from the environment and is the vehicle of
transmission from one host cell to the next
Variations seen in:
type of nucleic acid
type of protein coat or envelope
morphology
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Nucleic Acids Variations
single-stranded DNA viruses
double-stranded DNA viruses
single-stranded RNA viruses (+ strand or – strand)
+ strand viruses are able to directly synthesize proteins from their genome
- strand viruses must transcribe a + strand to serve as mRNA before
synthesizing proteins
double-stranded RNA viruses
depending on the virus, the nucleic acid can be linear or circular
(some, like influenza virus, have multiple nucleic acid segments)
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Capsid and Envelope
capsid: the protein coat surrounding the nucleic acid of the virus
capsomeres: protein subunits of the capsid
envelope: the capsid of some viruses is surrounded by a lipid, protein,
and carbohydrate envelope
in some animal viruses, this envelope is derived from the host cell
membrane as viral particles are extruded from the cell
– think about this, how would this strategy benefit the virus?
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spikes composes of protein-carbohydrate complexes on the surface
of the envelope can be used in attachment to host cells, also used in
identification of viruses
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General Morphology
Various shapes seen in viruses include…
Polyhedral viruses
Helical viruses
Enveloped viruses
Complex viruses
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many-sided
icosahedron (20 triangular faces)
is the most common form of
‘many-sided’
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long rods (rigid or flexible) consisting
of a hollow, cylindrical capsid which has
a helical structure.
Nucleic acid is within the hollow capsid
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surrounded by an envelope,
are roughly spherical
can have helical or polyhedral capsid
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Complex viruses
have complicated structures,
often both helical and polyhedral portions
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Viral Taxonomy
Viruses are grouped based on:
 nucleic acid type
 strategy for replication
 morphology
see Table 13.2
for virus families
a viral species is a group of viruses sharing the same genetic
information and ecological niche
no species epithets (names) have been given
instead common names are used
such as human immunodeficiency virus (HIV)
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Cultivation of Viruses
All viruses can only multiply inside host cells
Different techniques are required to grow & maintain viruses.
Techniques used depend on the normal host of the virus
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Cultivation of Bacteriophages
Bacteriophages – viruses that prey upon bacteria
grown in either liquid cultures of bacteria or in bacterial cultures
on solid medium
in a bacterial lawn on a plate, viruses can be detected by the formation
of plaques, zones of clearing produced by the killing of bacteria
(fig. 13.6) pg 379 – good photo
like colony counting in bacteria, each plaque represents one viral
infection, or plaque-forming unit (pfu)
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Cultivation of Animal Viruses
may be cultivated in living animals, embryonated (fertilized) eggs, or in
cell culture
Observers see cytopathic effect a cell deterioration as a result of virus
activity
This effect can be observed and counted similar to counting
viral plaques in bacteriophage
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Transduction
the process of…..
transferring DNA from one bacterial cell to another by a bacteriophage
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Multiplication of Bacteriophages
Lytic Cycle - all infected cells are lysed and killed (fig. 13.10, p. 383)
Lysogenic Cycle - most infected cells are lysed and killed
(fig. 13.12, p. 373)
Some cells have a phage which inserts in the chromosome and
becomes dormant (a prophage), this is called lysogeny
Cell replicates normally; at some later time, phage can enter lytic cycle
This phenomenon makes transduction possible
specialized transduction transfers specific genes;
generalized transfers any gene
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Multiplication of Animal Viruses
similar steps as in bacteriophages, but some differences
(see Table 13.3, p. 386) also see differences in multiplication of nucleic
acid between DNA and RNA viruses (Table 13.4, p. 388)
General steps: (in order)
Attachment to cell
Penetration of cell membrane
Uncoating
Biosynthesis
Maturation & Release
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Viruses and Cancer
viruses first shown to cause some cancers in 1908 (chicken leukemia)
hard to establish link due to:
 cancer may develop long after the viral infection
 cancer are not contagious like viral diseases
Transformation (in the context of cancer): the process of altering a normal cell
with a virus to make it cancerous – generally by inserting DNA
(done by an oncovirus )
an oncogene is the area of the cells DNA which is altered
transformed cells no longer demonstrate contact inhibition (p 396)
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Latent and Slow Viral Infections
Latent infection: virus remains dormant in the host and does not
produce the disease for some length of time
example: herpes simplex virus (cold sores)
Slow Viral Infection: disease progresses slowly
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Prions
a prion is an infectious protein particle
Prion  proteinaceous infectious particle
the protein is an altered normal cellular protein, the altered form is
capable of catalyzing the alteration of other molecules of the same
protein
example: scrapie, mad cow disease, Creutzfeldt-Jakob disease
Thus far 9 animal diseases fall into this category – all are neurological
spongiform encephalopathys (large voids develop in brain)
“Spongy brains”
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Healthy Cell
mRNA translated
into normal proteins
Prion transforms normal
protein into prion
Prion (altered/infectious
protein) enters normal cell
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prion
Prion Diseases – Non-Human Animals
Scrapie: sheep
TME (transmissible mink encephalopathy): mink
CWD (chronic wasting disease): muledeer, elk
BSE (bovine spongiform encephalopathy): cows (“Mad Cow”)
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Prion Diseases - Human
CJD: Creutzfeld-Jacob Disease. Cerebral involvement so dementia is
more common and patient seldom survives a year (originally thought to
be sporadic, but now known to be familial as well).
GSS: Gerstmann-Straussler-Scheinker syndrome. characterised by cerebellar
ataxia and concomitant motor problems, dementia less common and disease
course lasts several years to death.
FFI: Fatal familial Insomnia
(characterized by severe selective atrophy of the thalamus )
Alpers Syndrome - Prion disease of infants
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Kuru highlands of New Guinea. Funeral ritual of respect - ingesting brain
tissue of dead relatives
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