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Chapter 12 Viruses, Viroids, and Prions
1
Not all pathogens are cellular!
• Many infections of humans, animals, plants,
and even bacteria are caused by acellular
(noncellular) particles
• Acellular infectious particles include
– Viruses
– Viroids
– prions
2
Characteristics of Viruses
• Virus
• Much smaller than bacteria(usually measured
in nanometers)
• Acellular infectious agent having either DNA
or RNA
• They do not have both!
• Cause many infections in humans, animals,
plants, and bacteria
• Cause most of the diseases that affect the
industrialized world
– Examples: common cold, influenza, herpes, SARS,
Polio, HIV
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Figure 13.4 Sizes of selected virions
E. coli (bacterium)
(1000 nm  3000 nm)
Red blood cell
(10,000 nm in diameter)
Bacterial
ribosomes
(25 nm)
Poliovirus
(30 nm)
Bacteriophage MS2
(24 nm)
Smallpox virus
(200 nm  300 nm)
Bacteriophage T4
(50 nm  225 nm)
Tobacco mosaic virus
(15 nm  300 nm)
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Characteristics of Viruses
• Cannot carry out any metabolic pathway
outside of a cell
• Neither grow nor respond to the environment
• Cannot reproduce independently
– Recruit the cell’s (animal, plant, or bacterial cells)
metabolic pathways to reproduce
• No cytoplasmic membrane, cytosol (liquid
portion of cytoplasm), organelles
• Have extracellular and intracellular state
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Characteristics of Viruses
• Extracellular State (Naked or Enveloped)
– Protein coat (capsid) surrounding nucleic acid
(naked)
– Some have a phospholipid envelope which
surrounds the capsid (enveloped)
– Outermost layer provides protection and
recognition sites for host cells
• Intracellular State
– Capsid removed
– Virus exists inside the cell as nucleic acid (DNA or
RNA)
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Figure 13.1 Virions-overview
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Differentiating Viruses
• We can differentiate viruses from one
another based on their Genetic Material
• Show more variety in genomes than cells
– The genetic material a virus contains is the
primary way scientists categorize and classify
viruses
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Differentiating Viruses
• We can differentiate viruses from one another
based on their Genetic Material
– Viral genome may be DNA or RNA, but never both
• dsDNA, ssDNA, dsRNA, ssRNA
– ds= double stranded, ss= single stranded
• Linear and segmented or single and circular
– Influenza virus genome has 8 linear segments of ssRNA
• Much smaller than genomes of cells
– Cells always have double stranded DNA
• ssDNA and dsRNA are almost nonexistent in cells
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Differentiating Viruses
• Hosts of Viruses
– Most viruses infect only particular host’s cells
• Species specific
– Dog viruses don’t infect humans
– May be so specific they infect only particular kind
of cell in a particular host
• HIV attacks helper T lymphocytes in humans but does
not infect muscle or bone cells
– Generalists – infect many kinds of cells in many
different hosts
• Rabies
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Figure 13.3 Hosts of viral infections-overview
Tobacco mosaic virus
infected leaf on left
Bacteria (blue/gray)
under attack from a
bacteriophage (pink)
Human WBC cytoplasmic
membrane with HIV
particles (blue) attached
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Differentiating Viruses
• Host specificity
– Due to viral surface proteins which have a precise affinity
(attraction) for complementary proteins on the host cell
membranes
Figure 13.7 Enveloped virion-overview
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Animal Virus Replication
• Replication of Animal Viruses
– Same basic replication pathway as
bacteriophages
1.
2.
3.
4.
5.
Recognition and Attachment (capsid or envelope proteins
recognize host cell receptors)
Entry (fusion with cell membrane or endocytosis)
Synthesis (DNA virus in the nucleus, RNA virus in the cytoplasm)
Assembly
Release (budding, exocytosis, lysis)
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• Replication of Animal Viruses
– Enveloped viruses cause persistent infections
• Released from cell by budding
– Naked viruses are released by exocytosis or lysis
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Bacteriophage Replication
• Dependent on hosts’ organelles and enzymes to
produce new viral particles
• Lytic replication
– Replication cycle usually results in lysis and death of
host cell
• Basic stages of lytic replication cycle
1.
2.
3.
4.
5.
6.
Recognition and Attachment
Entry
Chromosome degraded
Synthesis
Assembly
Release
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Figure 13.12 Three mechanisms of entry of animal viruses-overview
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Figure 13.8 The lytic replication cycle in bacteriophages-overview
Attachment
Bacteriophage
genome
Entry
Tail sheath
Outer
membrane
Peptidoglycan
Cytoplasmic
membrane
Bacterial
chromosome
Entry
Attachment
Phage
DNA
Lytic replication
cycle of bacteriophage
Bacterial
chromosome
degraded
Release
Synthesis
Phage
proteins
Assembly
Assembly
Base
Tail
Sheath
DNA
Capsid
Mature head
Tail fibers
Mature virion
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Viral Replication
• Lysogeny
– Modified replication cycle
– Infected host cells grow and reproduce normally
for generations before they lyse
– Inactive bacteriophage is called a prophage
– Induction occurs and the prophage is excised from
the host chromosome
• Induction can occur through DNA damaging chemicals,
UV light, X rays
• After induction the lytic cycle will occur
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Figure 13.11 The lysogenic replication cycle in bacteriophages: phage lambda and E. coli
Attachment
Prophage
in chromosome
Entry
Lambda
phage
Lytic
cycle
Lysogeny
Synthesis
Release
Replication of
chromosome
and virus;
cell division
Assembly
Induction
Further replications and
cell divisions
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Transduction
Figure 13.14 The process of budding in enveloped viruses
Enveloped
virion
Budding of
enveloped virus
Viral
glycoproteins
Cytoplasmic
membrane
of host
Viral capsid
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The Role of Viruses in Cancer
• Viruses cause 20–25% of human cancers
– Some viruses carry copies of oncogenes as part of
their genomes
• Oncogenes are involved in cell division and are usually repressed
(not activated) and no cancer results
– Some promote oncogenes already present in host
– Specific viruses are known to cause human cancers
• Kaposi’s sarcoma (HIV)
• Cervical cancer (HPV)
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Are Viruses Alive?
• Infectious agents with both living and non-living
characteristics
– Living characteristics:
• Reproduce, but only in living host cells
• Can mutate
– Nonliving characteristics:
• Acellular: no cytoplasm or organelles
• Cannot carry out metabolism on their own
• Have DNA or RNA but not both
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Other Parasitic Particles: Viroids and
Prions
• Characteristics of Viroids
– Extremely small, circular pieces of RNA that
are infectious and pathogenic in plants
– Similar to RNA viruses, but lack capsid
– No known animal diseases are known to be
caused by viroids
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Figure 13.21 One effect of viroids on plants
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Characteristics of Prions
• Proteinaceous infectious agents
• Are ONLY protein
• Cause spongiform encephalopathies:
–
–
–
–
Mad cow
Scrapie
Kuru
Creutzfeld-Jakob syndrome
• Resistant to proteases, UV light, heat,
disinfectants
• Prions only destroyed by incineration or autoclaving in
NaOH
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Prions
• Characteristics of Prions
– Prion diseases
• Fatal neurological degeneration, and loss of brain matter
• Large vacuoles form in brain
– Characteristic spongy appearance
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Chronic wasting disease
Prions
• Characteristics of Prions
– Proteinaceous infectious agents
– Cellular PrP protein
• Made by all mammals
• Normal structure with -helices called cellular PrP
– Prion PrP
• Disease-causing form with -pleated sheets called prion
PrP
– Prion PrP changes shape of cellular PrP so it
becomes prion PrP
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Figure 13.22 The two stable, three-dimensional forms of prion protein (PrP)-overview
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