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Ch 13. Viruses and Prions. Student Learning Outcomes. Differentiate a virus from a bacterium. Explain the difference between enveloped and nonenveloped viruses. Define viral species. Describe how bacteriophages and animal viruses are cultured.

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Ch 13

Viruses and Prions

student learning outcomes
Student Learning Outcomes

Differentiate a virus from a bacterium.

Explain the difference between enveloped and nonenveloped viruses.

Define viral species.

Describe how bacteriophages and animal viruses are cultured.

Compare and contrast the lytic and lysogenic cycles of bacteriophages.

Define oncogene and transformed cell.

Discuss the relationship between viruses and cancer.

Explain latent viral infections and give an example.

Discuss how a proteins can be infectious.

foundations of virology
Foundations of Virology

Non-living agents that infect all life forms (phages vs. animal viruses)

Viral cultivation differs from bacterial cultivation

 1,500 known viruses (estimates:  400,000 exist)

Advent of EM allowed for visualization of viruses

general characteristics of viruses
General Characteristics of Viruses
  • Obligatory intracellular parasites
  • Filterable
  • Virus = Latin for poison
  • Contain DNA or RNA
  • Contain a protein coat = capsidmade up of capsomeres. Various shapes
  • Some are enclosed by an envelope (naked vs. enveloped)
  • Some viruses have spikes (COH/protein)
  • Most viruses are tissue specific
  • Host range is determined by specific host attachment sites and cellular factors
host range and specificity
Host Range and Specificity

Virus / host cell interaction usually very specific (narrow host range) – due to?

Tissue tropism

taxonomy of viruses
Taxonomy of Viruses
  • No evidence for common viral ancestor.
  • Classification based on type of NA, strategy for replication, and morphology.
    • Familynames end in –viridae
    • Genus and species 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.
taxonomy of viruses1


Human immunodeficiency virus HIV-1, HIV-2

Taxonomy of Viruses
  • Herpesviridae
  • Herpesvirus
  • Human herpes virus HHV-1, HHV-2, HHV-3
isolation cultivation and identification of viruses
Isolation, Cultivation, and Identification of Viruses

Fig 13.6

  • Viruses must be grown in living cells

Bacteriophages form plaques on a lawn of bacteria

Animal viruses may be grown in cell culture, embryonated eggs, or living animals

Fig 13.8

virus identification
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
  • Novel methods such as Biophotonics
viral replication
Viral Replication
  • Obligate intracellular parasites using host cell machinery
  • Very limited number of genes encode proteins for
    • Capsid formation
    • Viral nucleic acid replication
    • Movement of virus into and out of cell
  • Kill or live in harmony within the host cell – Outside the cell, viruses are inert
bacteriophage the lytic cycle
Bacteriophage:The Lytic Cycle
  • Attachmentto cell surface receptors (chance encounter – no active movement)
  • Penetration– only genome enters
  • Biosynthesis–Production of phage DNA and proteins
  • Maturation– assembly to form intact phage
  • Releasedue to phage induced lysozyme production

See Fig 13.11

results of multiplication of bacteriophages
Results of Multiplication of Bacteriophages
  • Lytic cycle
    • Lytic or virulent phage
    • Phage causes lysis and death of host cell
  • Lysogenic cycle
    • Lysogenic or temperate phage
    • Phage DNA incorporated in host DNA  Prophage
    • Phage conversion
    • Specialized transduction

ANIMATION Viral Replication: Virulent Bacteriophages

ANIMATION Viral Replication: Temperate Bacteriophages

some animal viruses exit the host cells via budding
Some animal viruses exit the host cells via budding.

HSV envelopment and release

Compare to Fig. 13.20

multiplication of dna viruses
Multiplication of DNA Viruses

Foundation Fig 13.15

ANIMATION Viral Replication: Animal Viruses


Cancer  uncontrolled mitotic divisions

Benign vs. malignant tumors


3 important characteristics of cancer cells:

  • Rapid cell division
  • Loss of anchoring junctions and contact inhibition  metastasis
  • Dedifferentiation of cells
viruses and cancer
Viruses and Cancer
  • The genetic material of oncogenic viruses becomes integrated into the host cell’s DNA (provirus).
  • Conversion of proto-oncogenes to oncogenes
  • Activated oncogenes transform normal cells into cancerous cells
  • Transformed cells have increased growth, loss of contact inhibition, tumor-specific transplantantigens, and T antigens
  • Oncogenic Viruses are responsible for 10 % of human cancers
oncogenic dna viruses and rna viruses
OncogenicDNA Viruses andRNA Viruses
  • Papilloma virus (HPV)  cervical cancer
  • Epstein-Barr virus (EBV) Burkitt’s lymphoma
  • HV8  Kaposi’s sarcoma
  • Hepatitis B virus (HBV)  liver cancer
  • Hepatitis C virus (HCV)  liver cancer
  • human T-cell leukemia virus (HTLV-1)
latent and persistent viral infections
Latent and Persistent Viral Infections

Latent:Virus remains in asymptomatic host cell for long periods

Persistent:Disease processes occurs over a long period; generally is fatal

Fig 13.21

  • Small proteinaceous infectiousparticles (very resistant to inactivation)
  • Inherited and transmissible by ingestion, transplant, and surgical instruments
  • Causes spongiform encephalopathies:
    • Sheep scrapie,
    • Creutzfeldt-Jakob disease,
    • Gerstmann-Sträussler-Scheinker syndrome,
    • fatal familial insomnia,
    • mad cow disease
  • PrPC: Normal cellular prion protein, on cell surface. Involved in cell death.
  • PrPSc: Scrapie protein; accumulates in brain cells, forming plaques.
spongiform encephalopaties
Spongiform Encephalopaties

Caused by altered protein:

  • Mutation in normal PrPc gene (sporadic CJD), or
  • contact with the abnormal PrPSc protein

ANIMATION Prion: Overview

ANIMATION Prion: Characteristics

ANIMATION Prion: Diseases

The End