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CHAPTER
13
Viruses,
Viroids, and
Prions
© 2016 Pearson Education, Inc.
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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
Ebola virus
Poliovirus
30 nm
Rhinovirus
30 nm
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
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300 × 200 × 100 nm
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
Check Your Understanding
 How could the small size of viruses have helped
researchers detect viruses before the invention of
the electron microscope?
13-1
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Viral Structure
Learning Objective
13-2 Describe the chemical and physical structure
of both an enveloped and a nonenveloped
virus.
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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
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Capsid
Figure 13.3 Morphology of an enveloped helical virus.
Nucleic acid
Capsomere
Spikes
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Envelope
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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Check Your Understanding
 Diagram a nonenveloped polyhedral virus that has
spikes.
13-2
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Taxonomy of Viruses
Learning Objectives
13-3 Define viral species.
13-4 Give an example of a family, genus, and
common name for a virus.
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Taxonomy of Viruses
•
•
•
•
Genus names end in -virus
Family names end in -viridae
Order names end in -ales
Viral species: a group of viruses sharing the
same genetic information and ecological niche
(host)
• Descriptive common names are used for species
• Subspecies are designated by a number
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Check Your Understanding
 How does a virus species differ from a bacterial
species?
13-3
 Attach the proper endings to Papilloma- to show
the family and genus that includes HPV, the
cause of cervical cancer.
13-4
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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.
13-7 List three techniques used to identify viruses.
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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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Growing Animal Viruses in the Laboratory
• In living animals
• In embryonated eggs
• Virus injected into the egg
• Viral growth is signaled by changes or death of the
embryo
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Figure 13.7 Inoculation of an embryonated egg.
Shell
Amniotic
cavity
Chorioallantoic
membrane
Air
sac
Chorioallantoic
membrane
inoculation
Amniotic
inoculation
Yolk
sac
Allantoic
inoculation
Shell
membrane
Albumin
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Allantoic
cavity
Yolk sac
inoculation
Growing Animal Viruses in the Laboratory
• In cell cultures
• Tissues are treated with enzymes to separate cells
• Virally infected cells are detected via their deterioration,
known as the cytopathic effect (CPE)
• Continuous cell lines are used
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Figure 13.8 Cell cultures.
Normal
cells
A tissue is treated with enzymes
to separate the cells.
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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.
Figure 13.9 The cytopathic effect of viruses.
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Viral Identification
• Cytopathic effects
• Serological tests
• Western blotting—reaction of the virus with antibodies
• Nucleic acids
• RFLPs
• PCR
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Check Your Understanding
 What is the plaque method?
13-5
 Why are continuous cell lines of more practical
use than primary cell lines for culturing viruses?
13-6
 What tests could you use to identify influenza
virus in a patient?
13-7
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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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Viral Replication: Virulent Bacteriophages
PLAY
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Animation: Viral Replication: Virulent
Bacteriophages
Viral Replication: Temperate Bacteriophages
PLAY
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Animation: Viral Replication: Temperate
Bacteriophages
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
Lytic
cycle
Cell lyses, releasing
phage virions.
Lysogenic
cycle
Phage DNA circularizes and enters
lytic cycle or lysogenic cycle.
OR
New phage DNA and
proteins are synthesized
and assembled into virions.
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Lysogenic bacterium
reproduces normally.
Prophage
Phage DNA integrates within the
bacterial chromosome by recombination,
becoming a prophage.
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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Transduction: Generalized Transduction
PLAY
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Animation: Transduction: Generalized
Transduction
Transduction: Specialized Transduction
PLAY
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Animation: Transduction: Specialized
Transduction
Check Your Understanding
 How do bacteriophages get nucleotides and
amino acids if they don't have any metabolic
enzymes?
13-8
 Vibrio cholerae produces toxin and is capable of
causing cholera only when it is lysogenic. What
does this mean?
13-9
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Table 13.3 Bacteriophage and Animal Viral Multiplication Compared
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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.
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Fusion of viral envelope
and plasma membrane
Entry of herpesvirus by fusion.
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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Viral Replication: Overview
PLAY
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Animation: Viral Replication: Overview
Viral Replication: Animal Viruses
PLAY
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Animation: Viral Replication: Animal
Viruses
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.
A papovavirus is a typical
DNA-containing virus that
attacks animal cells.
DNA
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.
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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A portion of viral DNA is
transcribed, producing
mRNA that encodes
"early" viral proteins.
The Biosynthesis of DNA Viruses
• Adenoviridae
• Double-stranded DNA, nonenveloped
• Respiratory infections in humans
• Tumors in animals
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Figure 13.16a DNA-containing animal viruses.
Capsomere
Mastadenovirus
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The Biosynthesis of DNA Viruses
• Poxviridae
• Double-stranded DNA, enveloped
• Cause skin lesions
• Vaccinia and smallpox viruses (Orthopoxvirus)
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The Biosynthesis of DNA Viruses
• Herpesviridae
• Double-stranded DNA, enveloped
•
•
•
•
•
•
HHV-1 and HHV-2—Simplexvirus; cause cold sores
HHV-3—Varicellovirus; causes chickenpox
HHV-4—Lymphocryptovirus; causes mononucleosis
HHV-5—Cytomegalovirus
HHV-6 and HHV-7—Roseolovirus
HHV-8—Rhadinovirus; causes Kaposi's sarcoma
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Figure 13.16b DNA-containing animal viruses.
Capsomeres
Simplexvirus
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The Biosynthesis of DNA Viruses
• Papovaviridae
• Double-stranded DNA, nonenveloped
• Papillomavirus
• Causes warts
• Can transform cells and cause cancer
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The Biosynthesis of DNA Viruses
• Hepadnaviridae
• Double-stranded DNA, enveloped
• Hepatitis B virus
• Use reverse transcriptase to make DNA from RNA
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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
– strand is transcribed
from + viral genome.
KEY
Capsid
protein
+ or sense
strand of
viral genome
Uncoating releases
viral RNA and proteins.
Viral
genome
(RNA)
– or antisense
strand of
viral genome
ss = single-stranded
ds = double-stranded
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+ strand
mRNA is transcribed
from the – strand.
ssRNA;
+ or sense strand;
Picornaviridae
Viral
protein
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
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Cytoplasm
Host cell
Entry
and uncoating
Maturation
and release
ss = single-stranded
Nucleus
– strands are
incorporated
into capsid
Additional – strands are
transcribed from mRNA.
Uncoating releases
viral RNA and proteins.
Viral
genome
(RNA)
ssRNA; – or
antisense strand;
Rhabdoviridae
Viral
protein
Figure 13.17c Pathways of multiplication used by various RNA-containing viruses.
Attachment
Capsid
RNA
Cytoplasm
Host cell
Entry
and uncoating
Maturation
and release
Translation and synthesis
of viral proteins
KEY
Nucleus
RNA replication by viral RNA–
dependent RNA polymerase
RNA polymerase initiates production of
– strands. The mRNA and – strands form the
dsRNA that is incorporated as new viral genome.
+ or sense
strand of
viral genome
mRNA is produced inside the
capsid and released into the
cytoplasm of the host.
Uncoating releases
viral RNA and proteins.
Viral
genome
(RNA)
Viral
protein
– or antisense
strand of
viral genome
ss = single-stranded
ds = double-stranded
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Capsid proteins and RNAdependent RNA polymerase
dsRNA; + or sense
strand with – or antisense
strand; Reoviridae
The Biosynthesis of RNA Viruses
• Picornaviridae
• Single-stranded RNA, + strand, nonenveloped
• Enterovirus
• Poliovirus and coxsackievirus
• Rhinovirus
• Common cold
• Hepatitis A virus
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The Biosynthesis of RNA Viruses
• Togaviridae
• Single-stranded RNA, + strand, enveloped
• Alphavirus
• Transmitted by arthropods; includes chikungunya
• Rubivirus
• Rubella
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The Biosynthesis of RNA Viruses
• Rhabdoviridae
• Single-stranded RNA, − strand, one RNA strand
• Lyssavirus
• Rabies
• Numerous animal diseases
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The Biosynthesis of RNA Viruses
• Reoviridae
• Double-stranded RNA, nonenveloped
• Reovirus (respiratory enteric orphan)
• Rotavirus (mild respiratory infections and gastroenteritis)
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Biosynthesis of RNA Viruses That Use DNA
• Single-stranded RNA, produce DNA
• Use reverse transcriptase to produce DNA from the
viral genome
• Viral DNA integrates into the host chromosome as a
provirus
• Retroviridae
• Lentivirus (HIV)
• Oncoviruses
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Figure 13.19 Multiplication and inheritance processes of the Retroviridae.
Reverse
transcriptase
Capsid
Envelope
Virus
Two identical +
strands of RNA
Host cell
Retrovirus enters by fusion
between attachment spikes
and the host cell receptors.
Mature retrovirus
leaves the host cell,
acquiring an
envelope and
attachment spikes
as it buds out.
DNA of one
of the host cell's
chromosomes
Viral
enzymes
Uncoating releases the two
viral RNA strands and the
viral enzymes reverse
transcriptase, integrase,
and protease.
Viral DNA
Viral RNA
Reverse transcriptase
copies viral RNA to
produce doublestranded DNA.
Viral proteins are processed
by viral protease; some of the
viral proteins are moved
to the host plasma membrane.
Provirus
Viral
proteins
Identical strands
of RNA
RNA
Transcription of the provirus
may also occur, producing
RNA for new retrovirus
genomes and RNA that
encodes the retrovirus
capsid, enzymes, and
envelope proteins.
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The new viral DNA
is transported into
the host cell's nucleus,
where it's integrated into
a host cell chromosome
as a provirus by viral
integrase. The provirus
may be replicated when
the host cell replicates.
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
 Describe the principal events of attachment, entry,
uncoating, biosynthesis, maturation, and release
of an enveloped DNA-containing virus.
13-10
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Viruses and Cancer
Learning Objectives
13-11 Define oncogene and transformed cell.
13-12 Discuss the relationship between DNA- and
RNA-containing viruses and cancer.
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Viruses and Cancer
• Several types of cancer are caused by viruses
• May develop long after a viral infection
• Cancers caused by viruses are not contagious
• Sarcoma: cancer of connective tissue
• Adenocarcinomas: cancers of glandular
epithelial tissue
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The Transformation of Normal Cells into Tumor
Cells
• Oncogenes transform normal cells into cancerous
cells
• Oncogenic viruses become integrated into the
host cell's DNA and induce tumors
• A transformed cell harbors a tumor-specific
transplant antigen (TSTA) on the surface and a
T antigen in the nucleus
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DNA Oncogenic Viruses
• Adenoviridae
• Herpesviridae
• Epstein-Barr virus
• Poxviridae
• Papovaviridae
• Human papillomavirus
• Hepadnaviridae
• Hepatitis B virus
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RNA Oncogenic Viruses
• Retroviridae
• Viral RNA is transcribed to DNA (using reverse
transcriptase), which can integrate into host DNA
• HTLV-1 and HTLV-2 cause adult T cell leukemia and
lymphoma
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Check Your Understanding
 What is a provirus?
13-11
 How can an RNA virus cause cancer if it doesn't
have DNA to insert into a cell's genome?
13-12
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Latent Viral Infections and Persistent Viral
Infections
Learning Objectives
13-13 Provide an example of a latent viral infection.
13-14 Differentiate persistent viral infections from
latent viral infections.
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Latent Viral Infections and Persistent Viral
Infections
• Latent virus remains in asymptomatic host cell for
long periods
• May reactivate due to changes in immunity
• Cold sores, shingles
• A persistent viral infection occurs gradually over
a long period; is generally fatal
• Subacute sclerosing panencephalitis (measles virus)
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Figure 13.21 Latent and persistent viral infections.
Acute infection
Latent infection
Persistent infection
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Table 13.5 Examples of Latent and Persistent Viral Infections in Humans
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Check Your Understanding
 Is shingles a persistent or latent infection?
13-13, 13-14
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Prions
Learning Objective
13-15 Discuss how a protein can be infectious.
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Prions
• Proteinaceous infectious particles
• Inherited and transmissible by ingestion,
transplant, and surgical instruments
• Spongiform encephalopathies
•
•
•
•
•
"Mad cow disease"
Creutzfeldt-Jakob disease (CJD)
Gerstmann-Sträussler-Scheinker syndrome
Fatal familial insomnia
Sheep scrapie
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Prions
• PrPC: normal cellular prion protein, on the cell
surface
• PrPSc: scrapie protein; accumulates in brain cells,
forming plaques
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Prions: Overview
PLAY
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Animation: Prions: Overview
Prions: Characteristics
PLAY
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Animation: Prions: Characteristics
Prions: Diseases
PLAY
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Animation: Prions: Diseases
Figure 13.22 How a protein can be infectious.
PrPSc
PrPc
PrPc produced by cells
is secreted to the cell
surface.
PrPSc may be acquired or
produced by an
altered PrPc gene.
PrPSc reacts with PrPc
on the cell surface.
PrPSc converts the PrPc
to PrPSc.
Lysosome
Endosome
PrPSc
The new
more PrPc.
converts
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PrPSc
The new
is taken
in, possibly by receptormediated endocytosis.
PrPSc accumulates in
endosomes.
PrPSc continues to accumulate
as the endosome contents are
transferred to lysosomes.
The result is cell death.
Plant Viruses and Viroids
Learning Objectives
13-16 Differentiate virus, viroid, and prion.
13-17 Describe the lytic cycle for a plant virus.
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Plant Viruses and Viroids
• Plant viruses: enter through wounds or via
insects
• Plant cells are generally protected from disease by an
impermeable cell wall
• Viroids: short pieces of naked RNA
• Cause potato spindle tuber disease
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Figure 13.23 Linear and circular potato spindle tuber viroid (PSTV).
PSTV
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Table 13.6 Classification of Some Major Plant Viruses
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Check Your Understanding
 Contrast viroids and prions, and for each name a
disease it causes.
13-15, 13-16
 How do plant viruses enter host cells?
13-17
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