Chapter 13 Characterizing Viruses,
Viroids, and Prions
1
Not all pathogens are cellular!
• Many infections of humans, animals, plants,
and even bacteria are caused by _________
____________________
• Acellular infectious particles include
– _____________
– _____________
– _____________
2
General characteristics of viruses,
viroids, and prions
•
•
•
•
Simple compared to cells
Lack cell membranes
Composed of 1 or a few organic molecules
Lack most of the characteristics of life
described in chapters 3, 5, 6, and 7
3
Shared Characteristics of Viruses
• Virus
– Minuscule (usually measured in nanometers),
acellular infectious agent having either ____
or ______
• _______________________
– Cause many infections in humans, animals,
plants, and bacteria
– Cause most of the diseases that plague the
industrialized world
– Examples: common cold, influenza, herpes,
SARS, Polio, HIV
4
Shared Characteristics of Viruses
• __________carry out any metabolic pathway
outside of a cell
• Neither _________________to the environment
• Cannot _____________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
5
Shared Characteristics of Viruses
• Extracellular State
– Protein coat (________) surrounding nucleic acid
– Some have a phospholipid ____________which
surrounds the capsid
– Outermost layer provides ____________and
_____________sites for host cells
• Intracellular State
– Capsid removed
– Virus exists as nucleic acid
6
Figure 13.1 Virions-overview
7
Differentiating Viruses
• We can differentiate viruses from one
another based on their _________________
• Show more variety in genomes than cells
– The genetic material a virus contains is the
primary way scientists categorize and classify
viruses
8
Differentiating Viruses
• We can differentiate viruses from one another
based on their Genetic Material
– Viral genome may be DNA or RNA, but never both
• _________________________________
– 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 __________have ___________________DNA
• ssDNA and dsRNA are almost nonexistent in cells
9
Differentiating Viruses
• Hosts of Viruses
– Most viruses infect only particular host’s cells
• _________________
– 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
– _______________– infect many kinds of cells in
many different hosts
• Rabies
10
Differentiating Viruses
• Host specificity
– Due to viral surface ____________which have a precise
__________ (attraction) for complementary proteins on
the host cell membranes
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
• Hosts of Viruses
– ____________________can be infected by a virus
• Archaeal, bacterial, plant, protozoan, fungal, animal
– Most research focuses on animal and bacterial
viruses
• A virus that infects a bacterium is called a
________________or phage
• Bacteriophages out number all bacteria, archaea, and
eukaryotes put together!
13
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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Differentiating Viruses
• Capsid Morphology
– Capsids
• Provide ________________for viral nucleic acid
• Means of _________________to host’s cells
• Composed of proteinaceous subunits called
_______________
– Capsomere made of single or multiple types of proteins
15
Differentiating Viruses
• Viral Shapes
– Three basic shapes
• ___________(capsomeres bond in a spiral fashion)
• ___________(close to spherical, geodesic dome)
• ___________(many different shapes, bullet
shaped rabies virus)
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Figure 13.5 The shapes of virions-overview
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Figure 13.6 Bacteriophage T4-overview
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Differentiating Viruses
• The ________________
– Acquired from host cell during viral replication or
release
• Envelope is portion of _____________system of _______
– Composed of ____________bilayer and _________
• Some proteins are virally coded glycoproteins (spikes)
– Envelope’s proteins and glycoproteins often play
role in host recognition
19
Figure 13.7 Enveloped virion-overview
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Viral Replication
• Dependent on hosts’ organelles and enzymes to
produce new viral particles
• ___________________
– Replication cycle usually results in ______and ______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 13.8 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
• __________________
– Modified replication cycle
– Infected host cells _______________________for
generations before they lyse
– Inactive bacteriophage is called a _____________
– ______________occurs and the prophage is
excised from the host chromosome
• Induction can occur through DNA damaging chemicals,
UV light, X rays
• After induction the _______________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
Viral Replication
• Replication of Animal Viruses
– Same basic replication pathway as
bacteriophages
1.
2.
3.
4.
5.
6.
Recognition and Attachment
Entry
Chromosome degraded
Synthesis
Assembly
Release
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Viral Replication
• Replication of Animal Viruses
– Attachment of animal viruses
• Chemical attraction between glycoproteins or
proteins on the virus exactly fitting
_______________________on animal cells
– Animal viruses do not have tails or tail fibers
like bacteriophages do
• Have glycoprotein spikes or other attachment
molecules that mediate attachment
• http://highered.mcgrawhill.com/classware/ala.do?alaid=ala_1697939
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Figure 13.12 Three mechanisms of entry of animal viruses-overview
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Viral Replication
• Replication of Animal Viruses
– Synthesis of animal viruses
• Requires different strategy depending on its nucleic
acid
– DNA viruses often enter the nucleus
– RNA viruses often replicate in the cytoplasm
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Viral Replication
• Replication of Animal Viruses
– Assembly and release of animal viruses
• Most DNA viruses assemble in nucleus
• Most RNA viruses develop solely in cytoplasm
– Number of viruses produced depends on type of
virus and size and initial health of host cell
– Enveloped viruses cause ____________________
• Released from cell by _______________
– Naked viruses are released by __________or _____
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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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Viral Replication
• Replication of Animal Viruses
– Latency of animal viruses (chicken pox and herpes
virus)
– Called __________viruses or proviruses
• When animal viruses remain dormant in host cells
– May be prolonged for years with no viral activity
– Different than ___________
• Some latent viruses do not become incorporated into host
chromosome
• _______________________________________________________
– Incorporation of provirus into host DNA is _________
• HIV
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The Role of Viruses in Cancer
• Viruses cause ___________ of human cancers
– Some 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
– Some interfere with tumor repression
– Specific viruses are known to cause human cancers
• Kaposi’s sarcoma (HIV)
• Cervical cancer (HPV)
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Culturing Viruses in the Laboratory
• Viruses must be _____________________________
________________________________________
• Culturing Viruses in Mature Organisms
– In bacteria
– In plants and animals
• Culturing Viruses in Embryonated
Chicken Eggs
– Inexpensive, among the largest of cells, free of
contaminating microbes, and contain a nourishing yolk
• Culturing Viruses in Cell (Tissue) Culture
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Figure 13.17 Viral plaques in a lawn of bacterial growth on the surface of an agar plate
Bacterial lawn
Viral plaques
35
Are Viruses Alive?
• Infectious agents with both living and non-living characteristics
– Living characteristics:
•
•
– Nonliving characteristics:
•
•
•
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Not all viruses are bad!
• A previously unknown virus has been found
which attackes tiny marine alga that multiplies
and forms algal blooms
– Algal blooms can often be seen from space!
• Deplete water of oxygen
• Harming fish and other marine life
• A bacteriophage has been discovered which
transfers genes for photosynthetic machinery
into cyanobacteria increasing the rate of
photosynthesis
– Up to 10 million of these viruses have been found in a single milliliter
of seawater!
– Much of the oxygen we breath may be attributed to the action of this
virus on cyanobacteria
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Other Parasitic Particles: Viroids and
Prions
• Characteristics of ____________
– Extremely small, _______________________
__________________________
– Similar to RNA viruses, but ________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
40
Characteristics of Prions
• Proteinaceous infectious agents
•
• Cause spongiform encephalopathies:
– Mad cow
– Scrapie
– Kuru
– Creutzfeld-Jakob syndrome
• _____________to proteases, UV light, heat,
disinfectants
41
Prions
• Characteristics of Prions
– Prion diseases
• Fatal neurological degeneration, fibril deposits in brain,
and loss of brain matter
• Large vacuoles form in brain
– Characteristic spongy appearance
• Spongiform encephalopathies
– Prions only destroyed by incineration or
autoclaving in 1 N NaOH
42
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
43
Figure 13.22 The two stable, three-dimensional forms of prion protein (PrP)-overview
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