Download Viruses, Viroids, and Prions

11/15/2016
PowerPoint® Lecture
Presentations prepared by
Bradley W. Christian,
McLennan Community
College
CHAPTER
13
Viruses,
Viroids, and
Prions
© 2016 Pearson Education, Ltd.
Figure 7.6 Evaluation of disinfectants by the disk-diffusion method.
Zone of inhibition
Chlorine
Hexachlorophene
O-phenylphenol
Quat
Staphylococcus aureus
(gram-positive)
Chlorine
O-phenylphenol
Hexachlorophene
Quat
Escherichia coli
(gram-negative)
Chlorine
Hexachlorophene
O-phenylphenol
Quat
Pseudomonas aeruginosa
(gram-negative)
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General Characteristics of Viruses
Learning Objective
13-1 Differentiate a virus from a bacterium.
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General Characteristics of Viruses
• Obligatory intracellular parasites
• Require living host cells to multiply
•
•
•
•
Contain DNA or RNA
Contain a protein coat
No ribosomes
No ATP-generating mechanism
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Table 13.1 Viruses and Bacteria Compared
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Host Range
• The spectrum of host cells a virus can infect
• Most viruses infect only specific types of cells
in one host
• Determined by specific host attachment sites and
cellular factors
• Bacteriophages—viruses that infect bacteria
• Range from 20 nm to 1000 nm in length
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Figure 13.1 Virus sizes.
Bacteriophage M13
800 × 10 nm
970 nm
Bacteriophages f2, MS2
24 nm
Poliovirus
30 nm
Rhinovirus
30 nm
Ebola virus
Adenovirus
90 nm
Rabies virus
170 × 70 nm
Prion
200 × 20 nm
Bacteriophage T4
225 nm
Tobacco mosaic virus
250 × 18 nm
Viroid
300 × 10 nm
Vaccinia virus
300 nm Chlamydia bacterium
elementary body
E. coli bacterium
3000 × 1000 nm
Human red blood cell
10,000 nm in diameter
Plasma membrane
of red blood cell
10 nm thick
300 × 200 × 100 nm
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Viral Structure
• Virion—complete, fully developed viral particle
• Nucleic acid—DNA or RNA can be single- or doublestranded; linear or circular
• Capsid—protein coat made of capsomeres (subunits)
• Envelope—lipid, protein, and carbohydrate coating on
some viruses
• Spikes—projections from outer surface
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Figure 13.2 Morphology of a nonenveloped polyhedral virus.
Nucleic acid
Capsomere
Capsid
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Figure 13.3 Morphology of an enveloped helical virus.
Nucleic acid
Capsomere
Spikes
Envelope
Envelope is made of…?
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General Morphology
•
•
•
•
Helical viruses—hollow, cylindrical capsid
Polyhedral viruses—many-sided
Enveloped viruses
Complex viruses—complicated structures
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Figure 13.4 Morphology of a helical virus.
Nucleic acid
Capsomere
Capsid
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Figure 13.5 Morphology of complex viruses.
65 nm
Capsid (head)
DNA
Sheath
Tail fiber
Pin
Baseplate
A T-even bacteriophage
Orthopoxvirus
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Solunetti:
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Isolation, Cultivation, and Identification of
Viruses
Learning Objectives
13-5 Describe how bacteriophages are cultured.
13-6 Describe how animal viruses are cultured.
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Growing Bacteriophages in the Laboratory
• Viruses must be grown in living cells
• Bacteriophages are grown in bacteria
• Bacteriophages form plaques, which are clearings on a
lawn of bacteria on the surface of agar
• Each plaque corresponds to a single virus; can be
expressed as plaque-forming units (PFU)
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Figure 13.6 Viral plaques formed by bacteriophages.
Plaques
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Figure 13.8 Cell cultures.
Normal
cells
A tissue is treated with enzymes
to separate the cells.
Cells are suspended
in culture medium.
Transformed
cells
Normal cells or primary cells grow in a monolayer across
the glass or plastic container. Transformed cells or
continuous cell cultures do not grow in a monolayer.
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Viral Multiplication
Learning Objectives
13-8 Describe the lytic cycle of T-even
bacteriophages.
13-9 Describe the lysogenic cycle of bacteriophage
lambda.
13-10 Compare and contrast the multiplication cycle
of DNA- and RNA-containing animal viruses.
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Viral Multiplication
• For a virus to multiply:
• It must invade a host cell
• It must take over the host's metabolic machinery
• One-step growth curve
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Figure 13.10 A viral one-step growth curve.
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Multiplication of Bacteriophages
• Lytic cycle
• Phage causes lysis and death of the host cell
• Lysogenic cycle
• Phage DNA is incorporated in the host DNA
• Phage conversion
• Specialized transduction
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T-Even Bacteriophages: The Lytic Cycle
• Attachment: phage attaches by the tail fibers to
the host cell
• Penetration: phage lysozyme opens the cell wall;
tail sheath contracts to force the tail core and DNA
into the cell
• Biosynthesis: production of phage DNA and
proteins
• Maturation: assembly of phage particles
• Release: phage lysozyme breaks the cell wall
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Figure 13.11 The lytic cycle of a T-even bacteriophage.
Bacterial Bacterial
cell wall chromosome
Capsid
DNA
Capsid (head)
Sheath
Attachment: Phage attaches
to host cell.
Tail fiber
Baseplate
Tail
Pin
Cell wall
Plasma membrane
Penetration: Phage penetrates
host cell and injects its DNA.
Sheath contracted
Biosynthesis: Phage DNA directs
synthesis of viral components by the
host cell.
Tail core
Tail
DNA
Maturation: Viral components are
assembled into virions.
Capsid
Release: Host cell lyses, and new
virions are released.
Tail fibers
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Bacteriophage Lambda (λ): The Lysogenic
Cycle
• Lysogeny: phage remains latent
• Phage DNA incorporates into host cell DNA
• Inserted phage DNA is known as a prophage
• When the host cell replicates its chromosome, it also
replicates prophage DNA
• Results in phage conversion—the host cell exhibits
new properties
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Figure 13.12 The lysogenic cycle of bacteriophage λ in E. coli.
Phage DNA
(double-stranded)
Occasionally, the prophage may
excise from the bacterial chromosome
by another recombination event,
initiating a lytic cycle.
Phage attaches
to host cell and
injects DNA.
Bacterial
chromosome
Many cell
divisions
Lysogenic
cycle
Lytic
cycle
Cell lyses, releasing
phage virions.
Phage DNA circularizes and enters
lytic cycle or lysogenic cycle.
OR
New phage DNA and
proteins are synthesized
and assembled into virions.
Lysogenic bacterium
reproduces normally.
Prophage
Phage DNA integrates within the
bacterial chromosome by recombination,
becoming a prophage.
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Bacteriophage Lambda (λ): The Lysogenic
Cycle
• Specialized transduction
• Specific bacterial genes transferred to another
bacterium via a phage
• Changes genetic properties of the bacteria
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Figure 13.13 Specialized transduction.
Prophage
gal gene
Bacterial
DNA
Prophage exists in
galactose-using host
(containing the gal gene).
Galactosepositive
donor cell
gal gene
Phage genome excises,
carrying with it the adjacent gal
gene from the host.
gal gene
Phage matures and cell lyses,
releasing phage carrying gal
gene.
Phage infects a cell that cannot
utilize galactose (lacking gal
gene).
Galactosenegative
recipient cell
Along with the prophage, the
bacterial gal gene becomes
integrated into the new host's
DNA.
Lysogenic cell can now
metabolize galactose.
Galactose-positive
recombinant cell
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Multiplication of Animal Viruses
Attachment: viruses attach to the cell membrane
Entry by receptor-mediated endocytosis or fusion
Uncoating by viral or host enzymes
Biosynthesis: production of nucleic acid and
proteins
• Maturation: nucleic acid and capsid proteins
assemble
• Release by budding (enveloped viruses) or
rupture
•
•
•
•
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Figure 13.14 The entry of viruses into host cells.
Host plasma membrane
proteins at site of
receptor-mediated
endocytosis
Entry of pig retrovirus by
receptor-mediated endocytosis.
Fusion of viral envelope
and plasma membrane
Entry of herpesvirus by fusion.
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Figure 13.20 Budding of an enveloped virus.
Viral capsid
Host cell plasma membrane
Viral protein
Bud
Bud
Envelope
Release by budding
Lentivirus
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The Biosynthesis of DNA Viruses
• DNA viruses replicate their DNA in the nucleus of
the host using viral enzymes
• Synthesize capsid in the cytoplasm using host cell
enzymes
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Figure 13.15 Replication of a DNA-Containing Animal Virus.
Papovavirus
RELEASE
Virions are
released.
ATTACHMENT
Virion attaches
to host cell.
DNA
A papovavirus is a typical
DNA-containing virus that
attacks animal cells.
Host cell
Capsid
MATURATION
Virions
mature.
Nucleus
ENTRY and
UNCOATING
Virion enters cell, and
its DNA is uncoated.
Cytoplasm
Capsid proteins
Viral DNA
BIOSYNTHESIS
Viral DNA is replicated,
and some viral proteins
are made.
Capsid
proteins
mRNA
Late translation;
capsid proteins are
synthesized.
A portion of viral DNA is
transcribed, producing
mRNA that encodes
"early" viral proteins.
KEY CONCEPTS
Viral replication in animals generally follows these steps: attachment, entry,
uncoating, biosynthesis of nucleic acids and proteins, maturation, and release.
Knowledge of viral replication phases is important for drug development strategies,
and for understanding disease pathology.
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The Biosynthesis of RNA Viruses
• Virus multiplies in the host cell's cytoplasm using
RNA-dependent RNA polymerase
• ssRNA; + (sense) strand
• Viral RNA serves as mRNA for protein synthesis
• ssRNA; – (antisense) strand
• Viral RNA is transcribed to a + strand to serve as mRNA
for protein synthesis
• dsRNA—double-stranded RNA
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Figure 13.17a Pathways of multiplication used by various RNA-containing viruses.
Attachment
Capsid
RNA
Nucleus
Host cell
Cytoplasm
Entry
and uncoating
Maturation
and release
Translation and synthesis
of viral proteins
RNA replication by viral RNA–
dependent RNA polymerase
Uncoating releases
viral RNA and proteins.
– strand is transcribed
from + viral genome.
KEY
Viral
protein
Viral
genome
(RNA)
Capsid
protein
+ or sense
strand of
viral genome
– or antisense
strand of
viral genome
+ strand
ss = single-stranded
ssRNA;
+ or sense strand;
Picornaviridae
mRNA is transcribed
from the – strand.
ds = double-stranded
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Figure 13.17b Pathways of multiplication used by various RNA-containing viruses.
Attachment
Capsid
RNA
Translation and synthesis
of viral proteins
Capsid
protein
KEY
RNA replication by viral RNA–
dependent RNA polymerase
The + strand (mRNA) must first be
transcribed from the – viral genome
before proteins can be synthesized.
+ or sense
strand of
viral genome
– or antisense
strand of
viral genome
ds = double-stranded
Cytoplasm
Entry
and uncoating
Maturation
and release
ss = single-stranded
Nucleus
Host cell
– strands are
incorporated
into capsid
Additional – strands are
transcribed from mRNA.
Uncoating releases
viral RNA and proteins.
Viral
genome
(RNA)
Viral
protein
ssRNA; – or
antisense strand;
Rhabdoviridae
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Table 13.2 Families of Viruses That Affect Humans (1 of 4)
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Table 13.2 Families of Viruses That Affect Humans (2 of 4)
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Table 13.2 Families of Viruses That Affect Humans (3 of 4)
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Table 13.2 Families of Viruses That Affect Humans (4 of 4)
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Check Your Understanding
ü Describe the principal events of attachment, entry,
uncoating, biosynthesis, maturation, and release
of an enveloped DNA-containing virus.
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