Chapter 13: Viruses, Viroids, and Prions

Chapter 13: Viruses, Viroids, and Prions

What is a “Virus?”

• Definition:

  • Virus is an obligate intracellular parasite.

  • Origin of the term "virus" comes from the Latin word for “poison.”

• Characteristics:

  • Cannot reproduce independently; must be inside a host cell.

  • Lack enzymes for metabolism and ribosomes for protein synthesis.

  • Exhibit specific host ranges (only infect certain hosts).

Are Viruses Living Things?

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• Short Answer: No

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• Comparison with Bacteria:
  

  Characteristic	Bacteria	Viruses
  Intracellular Parasite	No	Yes
  Plasma Membrane	Yes	No
  Binary Fission	Yes	No
  Pass Through Bacteriological Filters	No	Yes
  Possess DNA and RNA	Yes	No
  ATP-Generating Metabolism	Yes	No
  Sensitive to Antibiotics	Yes	No
  Sensitive to Interferon	No	Yes
  Historical Facts About Viruses

  • Discovery:

    • First virus discovered around 1900 as a “filterable agent” in Tobacco Mosaic Disease.

    • Visible in late 1930's via electron microscopes.

  Structure of Viruses

  • Composition:

    • Made up of:

    • Protein Capsid (protein shell)

    • Nucleic Acid Core (either DNA or RNA, not both)

    • Some viruses have a lipid envelope.

  Virus Examples

  • Size Comparison:

    • Adenovirus: 90 nm

    • Bacteriophage T4: 225 nm

    • COVID-19 (SARS-CoV2): 200 nm × 20 nm

    • Human Red Blood Cell: 10,000 nm in diameter

  Viral Taxonomy

  • Classification Criteria:

    1. Nucleic Acid Content: Only DNA or RNA, may be single or double-stranded.

    2. Capsid Shape: Helical, Polyhedral, Complex.

    3. Number of Capsomeres: Protein subunits forming the capsid.

    4. Presence/Absence of Envelope: Originates from host cell membrane.

    5. Serology: Antibody testing to classify viruses.

    6. Host Specificity: Specific to host species.

    7. Replication Cycle: Lytic or Lysogenic.

  Lytic vs. Lysogenic Cycle

  • Lytic Cycle:

    1. Attachment to host cell.

    2. Penetration and injection of DNA.

    3. Viral component synthesis.

    4. Assembly of new virions.

    5. Host cell lysis and release of new virions.

  • Lysogenic Cycle:

    • Phage DNA integrates into the host genome as a prophage and can later enter a lytic cycle.

  Basic Steps of Viral Replication

  1. Adsorption: Virus attaches to the host cell.

  2. Penetration: Virus or its genetic material enters the host cell.

  3. Uncoating: Viral capsid removed, releasing nucleic acid.

  4. Viral Component Biosynthesis: Host cell synthesizes viral components.

  5. Assembly and Maturation: New virions are assembled.

  6. Release: New virions escape the host cell, often through budding.

  Viruses and Cancer

  • Certain viruses carry oncogenes, leading to cancerous characteristics in infected cells.

  • Examples:

    • Epstein-Barr Virus = Burkitt’s Lymphoma.

    • Hepatitis B Virus = Liver Cancer.

    • Human Papillomavirus = Cervical Cancer.

  Viroids and Prions

  • Viroids:

    • Consist of naked circular RNA, infect plants, disrupt metabolism by interacting with host enzymes.

  • Prions:

    • Infectious proteins causing degenerative brain diseases (e.g., Mad Cow Disease).

    • Misfolded proteins that convert normal proteins into more prions, leading to diseases known as Transmissible Spongiform Encephalopathies.

  Culturing Viruses

  • Viruses need living cells to grow.

  • Bacteriophages create plaques in bacterial cultures.

  • Animal viruses can be grown in living organisms or embryonated eggs.

  Virus Identification Techniques

  • Cytopathic Effects: Changes in host cells.

  • Serological Tests: Detect antibodies against viruses.

  • Nucleic Acid Testing: Includes PCR and RFLP methodologies.