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Chapter 13
Characterizing
and Classifying
Viruses,
Viroids, and
Prions
© 2012 Pearson Education Inc.
Lecture prepared by Mindy Miller-Kittrell
North Carolina State University
Characteristics of Viruses
• Virus
– Minuscule, acellular infectious agent having
either DNA or RNA
– Causes many infections of humans, animals,
plants, and bacteria
– Causes most of the diseases that plague the
industrialized world
© 2012 Pearson Education Inc.
Characteristics of Viruses
• Cannot carry out any metabolic pathway
• Neither grow nor respond to the
environment
• Cannot reproduce independently
• Recruit the cell’s metabolic pathways to
increase their numbers
• No cytoplasmic membrane, cytosol,
organelles (with one exception)
• Have extracellular and intracellular state
© 2012 Pearson Education Inc.
Characteristics of Viruses
• Extracellular State
– Called virion
– Protein coat (capsid) surrounding nucleic acid
– Nucleic acid and capsid also called
nucleocapsid
– Some have phospholipid envelope
– Outermost layer provides protection and
recognition sites for host cells
• Intracellular State
– Capsid removed
– Virus exists as nucleic acid
© 2012 Pearson Education Inc.
Figure 13.1 Virions-overview
Characteristics of Viruses
• Genetic Material of Viruses
– Show more variety in nature of their genomes
than do cells
– Primary way scientists categorize and classify
viruses
– May be DNA or RNA, but never both
– dsDNA, ssDNA, dsRNA, ssRNA
– Linear and segmented or single and circular
– Much smaller than genomes of cells
© 2012 Pearson Education Inc.
Figure 13.2 The relative sizes of genomes
Partial genome
of E. coli
Viral
genome
Characteristics of Viruses
• Hosts of Viruses
– Most viruses infect only particular host’s cells
– Affinity of viral surface proteins for proteins on
host cell
– May be so specific they infect only particular
kind of cell in a particular host
– Generalists – infect many kinds of cells in
many different hosts
© 2012 Pearson Education Inc.
Figure 13.3 Hosts of viral infections-overview
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)
Characteristics of Viruses
• Capsid Morphology
– Capsids
– Provide protection for viral nucleic acid
– Means of attachment to host’s cells
– Composed of proteinaceous subunits called
capsomeres
– Capsomere made of single or multiple types of
proteins
© 2012 Pearson Education Inc.
Characteristics of Viruses
• Viral Shapes
– Three basic shapes
– Helical
– Polyhedral
– Complex
© 2012 Pearson Education Inc.
Figure 13.5 The shapes of virions-overview
Figure 13.6 Bacteriophage T4-overview
Characteristics of Viruses
• The Viral Envelope
– Acquired from host cell during viral replication or
release
– Envelope is portion of membrane system of host
– Composed of phospholipid bilayer and proteins
– Some proteins are virally coded glycoproteins
(spikes)
– Envelope’s proteins and glycoproteins often
play role in host recognition
© 2012 Pearson Education Inc.
Figure 13.7 Enveloped virion-overview
Table 13.2 Families of Human Viruses
Viral Replication
• Dependent on hosts’ organelles and enzymes
to produce new virions
• Lytic replication
– Replication cycle usually results in death and lysis
of host cell
• Stages of lytic replication cycle
–
–
–
–
–
Attachment
Entry
Synthesis
Assembly
Release
© 2012 Pearson Education Inc.
Viral Replication
ANIMATION Viral Replication: Overview
© 2012 Pearson Education Inc.
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
Number of infective virions
in medium (log scale)
Figure 13.9 Pattern of virion abundance in lytic cycle
Burst size
Entry
Synthesis and assembly
Time (minutes)
Attachment
Burst time
Viral Replication
ANIMATION Viral Replication: Virulent Bacteriophages
© 2012 Pearson Education Inc.
Viral Replication
• Lysogeny
– Modified replication cycle
– Infected host cells grow and reproduce normally
for generations before they lyse
– Temperate phages
– Prophages – inactive phages
– Lysogenic conversion results when phages carry
genes that alter phenotype of a bacterium
© 2012 Pearson Education Inc.
Figure 13.10 Bacteriophage lambda
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
Viral Replication
ANIMATION Viral Replication: Temperate
Bacteriophages
© 2012 Pearson Education Inc.
Viral Replication
• Replication of Animal Viruses
– Same basic replication pathway as
bacteriophages
– Differences result from
– Presence of envelope around some viruses
– Eukaryotic nature of animal cells
– Lack of cell wall in animal cells
© 2012 Pearson Education Inc.
Viral Replication
• Replication of Animal Viruses
– Attachment of animal viruses
– Chemical attraction
– Animal viruses do not have tails or tail fibers
– Have glycoprotein spikes or other attachment
molecules that mediate attachment
© 2012 Pearson Education Inc.
Figure 13.12 Three mechanisms of entry of animal viruses-overview
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
– Must consider
– How mRNA is synthesized
– What serves as template for nucleic acid replication
© 2012 Pearson Education Inc.
Figure 13.13 Synthesis of proteins and genomes in animal RNA viruses-overview
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 persistent infections
– Naked viruses are released by exocytosis or lysis
© 2012 Pearson Education Inc.
Figure 13.14 The process of budding in enveloped viruses
Enveloped
virion
Budding of
enveloped virus
Viral
glycoproteins
Viral capsid
Cytoplasmic
membrane
of host
Number of infective virions in medium
Figure 13.15 Pattern of virion abundance in persistent infections
Entry
Synthesis and Release of
virions
assembly
Time
Attachment
Viral Replication
ANIMATION Viral Replication: Animal Viruses
© 2012 Pearson Education Inc.
Viral Replication
• Replication of Animal Viruses
– Latency of animal viruses
– When animal viruses remain dormant in host cells
– May be prolonged for years with no viral activity
– Some latent viruses do not become incorporated
into host chromosome
– Incorporation of provirus into host DNA is
permanent
© 2012 Pearson Education Inc.
The Role of Viruses in Cancer
• Animal’s genes dictate that some cells can
no longer divide or are prevented from
unlimited division
• Genes for cell division “turned off” or genes
inhibiting division “turned on”
• Neoplasia
– Uncontrolled cell division in multicellular animal
– Mass of neoplastic cells is tumor
• Benign vs. malignant tumors
– Metastasis
– Cancers
© 2012 Pearson Education Inc.
Figure 13.16 The oncogene theory of the induction of cancer in humans
Normal state:
DNA
Protooncogene
Represses
Gene for repressor
mRNA
Repressor
Result: No cancer
First “hit”:
Virus inserts promoter
DNA
Oncogene
Represses
Gene for repressor
mRNA
Repressor
Result: Still no cancer
Second “hit”:
Virus inserts into represssor gene
DNA
Oncogene
mRNA
Protein
No repressor
protein because
gene is segmented
Causes cell division Result: Cancer
The Role of Viruses in Cancer
• Environmental factors that contribute to
the activation of oncogenes
–
–
–
–
Ultraviolet light
Radiation
Carcinogens
Viruses
© 2012 Pearson Education Inc.
The Role of Viruses in Cancer
• Viruses cause 20–25% of human cancers
– Some carry copies of oncogenes as part of their
genomes
– Some promote oncogenes already present in host
– Some interfere with tumor repression
– Specific viruses are known to cause ~15% of
human cancers
– Burkitt’s lymphoma
– Hodgkin’s disease
– Kaposi’s sarcoma
– Cervical cancer
© 2012 Pearson Education Inc.
Culturing Viruses in the Laboratory
• 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
© 2012 Pearson Education Inc.
Figure 13.17 Viral plaques in a lawn of bacterial growth on the surface of an agar plate
Bacterial lawn
Viral plaques
Figure 13.18 Inoculation sites for the culture of viruses in embryonated chicken eggs
Air sac
Injection into
chorioallantoic
membrane
Injection into
chorioallantois
Injection into
embryo
Injection into
amnion
Injection into
yolk sac
Culturing Viruses in the Laboratory
• Culturing Viruses in Cell (Tissue) Culture
– Consists of cells isolated from an organism
and grown on a medium or in a broth
– Two types of cell cultures
– Diploid cell cultures
– Continuous cell cultures
© 2012 Pearson Education Inc.
Figure 13.19 An example of cell culture
Are Viruses Alive?
• Some consider them complex pathogenic
chemicals
• Others consider them the least complex
living entities
– Use sophisticated methods to invade cells
– Have the ability to take control of their host cell
– Are able to replicate themselves
© 2012 Pearson Education Inc.
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
– May appear linear due to H bonding
© 2012 Pearson Education Inc.
Figure 13.20 The RNA strand of the small potato spindle tuber viriod (PSTV)
Genome of bacteriophage T7
PSTV
Figure 13.21 One effect of viroids on plants
Other Parasitic Particles: Viroids and 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
© 2012 Pearson Education Inc.
Other Parasitic Particles: Viroids and Prions
ANIMATION Prions: Overview
© 2012 Pearson Education Inc.
Figure 13.22 The two stable, three-dimensional forms of prion protein (PrP)-overview
Other Parasitic Particles: Viroids and Prions
• Characteristics of Prions
– Normally, nearby proteins and polysaccharides
force PrP into cellular shape
– PrP mutations result in formation of prion Pr
© 2012 Pearson Education Inc.
Other Parasitic Particles: Viroids and Prions
ANIMATION Prions: Characteristics
© 2012 Pearson Education Inc.
Other Parasitic Particles: Viroids and 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
© 2012 Pearson Education Inc.
Figure 13.23 The brain of a sheep with the prion disease called scrapie
Vacuole
Other Parasitic Particles: Viroids and Prions
ANIMATION Prions: Disease
© 2012 Pearson Education Inc.
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