Viruses, Viroids, and Prions

1
Viruses, Viroids, and Prions

• Bits and Pieces that cause disease

2
Viruses

• Viruses contain DNA or RNA and a protein coat
• Some are enclosed by an envelope
• Some viruses have spikes
• Most viruses infect only specific types of cells
  in one host
• Host range is determined by specific host
  attachment sites and cellular factors

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Viruses
Figure 13.1
4
Helical Viruses
Figure 13.4a, b
5
Polyhedral Viruses
Figure 13.2a, b
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Complex Viruses
Figure 13.5a
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Viral Taxonomy

• Family names end in -viridae
• Genus names end in -virus
• Viral species A group of viruses sharing the
  same genetic information and ecological niche
  (host). Common names are used for species
• Subspecies are designated by a number

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Viral Taxonomy

• Herpesviridae
• Herpesvirus
• Human herpes virus 1
• HHV 2
• HHV 3
• Retroviridae
• Lentivirus
• Human Immunodeficiency Virus 1
• HIV 2

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Growing Viruses

• Viruses must be grown in living cells.
• Bacteriophages form plaques on a lawn of
  bacteria. ?

Figure 13.6
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Growing Viruses

• Animal viruses may be grown in living animals or
  in embryonated eggs.

Figure 13.7
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Growing Viruses

• Animal and plants viruses may be grown in cell
  culture.
• Continuous cell lines may be maintained
  indefinitely.

Figure 13.8
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Virus Identification

• Cytopathic effects
• Serological tests
• Detect antibodies against viruses in a patient
• Use antibodies to identify viruses in
  neutralization tests, viral hemagglutination, and
  Western blot
• Nucleic acids
• RFLPs
• PCR

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Virus Identification
Figure 13.9
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Bacterial cell wall
Bacterial chromosome
Capsid
DNA
Capsid
Sheath
Tail fiber
Tail
1
AttachmentPhage attaches to host cell.
Base plate
Pin
Cell wall
Plasma membrane
2
PenetrationPhage pnetrates host cell and
injects its DNA.
Sheath contracted
Tail core
3
Merozoites released into bloodsteam from liver
may infect new red blood cells
Figure 13.10.1
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Tail
DNA
4
MaturationViral components are assembled into
virions.
Capsid
5
ReleaseHost cell lyses and new virions are
released.
Tail fibers
Figure 13.10.2
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Multiplication of Bacteriophages (Lytic Cycle)

• Attachment Phage attaches by tail fibers to
  host cell
• Penetration Phage lysozyme opens cell wall,
  tail sheath contracts to force tail core and
  DNA into cell
• Biosynthesis Production of phage DNA and
  proteins
• Maturation Assembly of phage particles
• Release Phage lysozyme breaks cell wall

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One-step Growth Curve
Figure 13.11
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The Lysogenic Cycle
Figure 13.12
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Lytic vs Lysogenic

• Lytic cycl3
• Phage causes lysis and death of host cell
• Lysogenic cycle
• Prophage DNA incorporated in host DNA

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Specialized Transduction
Prophage
gal gene
1
Prophage exists in galactose-using host
(containing the gal gene).
Bacterial DNA
gal gene
2
Phage genome excises, carrying with it the
adjacent gal gene from the host.
Galactose-positive donor cell
3
Phage matures and cell lyses, releasing phage
carrying gal gene.
gal gene
4
Phage infects a cell that cannot utilize
galactose (lacking gal gene).
Galactose-negative recipient cell
5
Along with the prophage, the bacterial gal gene
becomes integrated into the new hosts DNA.
6
Lysogenic cell can now metabolize galactose.
Galactose-positive recombinant cell
Figure 13.13
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Multiplication of Animal viruses

• Attachment Viruses attaches to cell
  membrane
• Penetration By 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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Attachment, Penetration, and Uncoating
Figure 13.14
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Release of an enveloped virus by budding
Figure 13.20
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Multiplication of DNA Virus
1
Papovavirus
Virion attaches to host cell
7
Virions are released
Host cell
DNA
Capsid
2
Virion penetrates cell and its DNA is uncoated
DNA
Cytoplasm
6
Virions mature
Capsid proteins
mRNA
5
Late translation capsid proteins are synthesized
3
Early transcription and translation enzymes are
synthesized
4
Late transcription DNA is replicated
Figure 13.15
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Pathways of Multiplication for RNA-Containing
Viruses
Figure 13.17
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Multiplication of a Retrovirus
Reverse transcriptase
DNA
Capsid
Virus
Two identical stands of RNA
1
Retrovirus penetrates host cell.
Host cell
DNA of one of the host cells chromosomes
5
Mature retrovirus leaves host cell, acquiring an
envelope as it buds out.
Reverse transcriptase
2
Virion penetrates cell and its DNA is uncoated
Viral RNA
Identical strands of RNA
4
Transcription of the provirus may also occur,
producing RNA for new retrovirus genomes and RNA
that codes for the retrovirus capsid and envelope
proteins.
Viral proteins
RNA
3
The new viral DNA is tranported into the host
cells nucleus and integrated as a provirus. The
provirus may divide indefinitely with the host
cell DNA.
Provirus
Figure 13.19
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Cancer

• Activated oncogenes transform normal cells into
  cancerous cells.
• Transformed cells have increased growth, loss of
  contact inhibition, tumor specific transplant and
  T antigens.
• The genetic material of oncogenic viruses becomes
  integrated into the host cell's DNA.

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Oncogenic Viruses

• Oncogenic RNA viruses
• Retroviridae
• Viral RNA is transcribed to DNA which can
  integrate into host DNA
• HTLV 1
• HTLV 2

• Oncogenic DNA Viruses
• Adenoviridae
• Heresviridae
• Poxviridae
• Papovaviridae
• Hepadnaviridae

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Latent vs Persistent

• Latent Viral Infections
• Virus remains in asymptomatic host cell for long
  periods
• Cold sores, shingles
• Persistent Viral Infections
• Disease processes occurs over a long period,
  generally fatal
• Subacute sclerosing panencephalitis (measles
  virus)

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Prions

• Infectious proteins
• Inherited and transmissible by ingestion,
  transplant, surgical instruments
• Spongiform encephalopathies Sheep scrapie,
  Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Sc
  heinker syndrome, fatal familial insomnia, mad
  cow disease
• PrPC, normal cellular prion protein, on cell
  surface
• PrPSc, scrapie protein, accumulate in brain cells
  forming plaques

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Prions
PrPSc
PrPc
2
3
4
1
Lysosome
Endosome
5
6
7
8
Figure 13.21
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• Plant Viruses
• Plant viruses enter through wounds or via insects
• Viroids
• Viroids are infectious RNA potato spindle tuber
  disease

Figure 13.22
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Some Plant Viruses
Table 13.6
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Virus Families

• Single-stranded DNA, nonenveloped viruses
• Parvoviridae
• Human parvovirus
• Fifth disease
• Anemia in immunocompromised patients

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Double-stranded DNA, nonenveloped viruses

• Mastadenovirus
• Respiratory infections in humans
• Tumors in animals

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Double-stranded DNA, nonenveloped viruses

• Papillomavirus (human wart virus)
• Polyomavirus
• Cause tumors, some cause cancer

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Double-stranded DNA, nonenveloped viruses

• Orthopoxvirus (vaccinia and smallpox viruses)
• Molluscipoxvirus
• Smallpox, molluscum contagiosum, cowpox

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Double-stranded DNA, nonenveloped viruses

• Simplexvirus (HHV1 and HHV 2)
• Varicellavirus (HHV 3)
• Lymphocryptovirus (HHV 4)
• Cytomegalovirus (HHV 5)
• Roseolovirus (HHV 6)
• HHV 7
• Kaposi's sarcoma (HHV 8)
• Some herpesviruses can remain latent in host cells

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Double-stranded DNA, nonenveloped viruses

• Hepadnavirus (Hepatitis B virus)
• Use reverse transcriptase to produce DNA from mRNA

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Single-stranded RNA, strand, nonenveloped

• Enterovirus
• Enteroviruses include poliovirus and
  coxsackievirus
• Rhinovirus
• Hepatitis A virus

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Single-stranded RNA, strand, nonenveloped

• Hepatitis E virus
• Norovirus (Norwalk agent) causes gastroenteritis

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Single-stranded RNA, strand, nonenveloped

• Alphavirus
• Alphaviruses are transmitted by arthropods
  include EEE, WEE
• Rubivirus (rubella virus)

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Single-stranded RNA, strand, nonenveloped

• Arboviruses can replicate in arthropods include
  yellow fever, dengue, SLE, and West Nile viruses
• Hepatitis C virus

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Single-stranded RNA, strand, nonenveloped

• Coronavirus
• Upper respiratory infections

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Single-stranded RNA, strand, one RNA strand

• Vesiculovirus
• Lyssavirus (rabies virus)
• Cause numerous animal diseases

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Single-stranded RNA, strand, one RNA strand

• Filovirus
• Enveloped, helical viruses
• Ebola and Marburg viruses

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Single-stranded RNA, strand, one RNA strand

• Paramyxovirus
• Morbillivirus
• Paramyxovirus causes parainfluenza, mumps and
  Newcastle disease

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Single-stranded RNA, strand, one RNA strand

• Hepatitis D virus
• Depends on coinfection with Hepadnavirus

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Single-stranded RNA, strand, multiple RNA
strands

• Influenzavirus (Influenza viruses A and B)
• Influenza C virus
• Envelope spikes can agglutinate RBCs

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Single-stranded RNA, strand, multiple RNA
strands

• Bunyavirus (CE virus)
• Hantavirus

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Single-stranded RNA, strand, multiple RNA
strands

• Arenavirus
• Helical capsids contain RNA-containing granules
• Lymphocytic choriomeningitis
• VEE and Lassa Fever

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Single-stranded RNA, two RNA strands, produce DNA

• Lentivirus (HIV)
• Oncogenic viruses
• Use reverse transcriptase to produce DNA from
  viral genome
• Includes all RNA tumor viruses

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Double-stranded RNA, nonenveloped

• Reovirus (Respiratory Enteric Orphan)
• Rotavirus
• Mild respiratory infections and gastroenteritis
• Colorado tick fever