Chapter+7+Viruses+and+Prions

Chapter 7: Viruses and Prions

Introduction to Viruses

  • Abundance: Viruses infect all types of cells (bacteria, algae, fungi, protozoa, plants, animals).

  • Active and inactive states: Viral activity is categorized as active (capable of infection) or inactive (unable to infect) instead of alive or dead.

  • Obligate intracellular parasites: Require host cells for reproduction.

  • Virion: The complete, infective form of a virus outside of a host cell.

Impact of Viruses on Evolution

  • Influence genetic makeup: Viruses shape the evolution of cells, tissues, and organisms.

  • Human genome: 40% to 80% may consist of remnants from ancient viral infections.

  • Part of microbiome: Viruses are components of the normal microbiome.

Viral Size Range

  • Smaller than bacteria: Examples range from Parvoviruses (~0.02 μm) to Pandoravirus (similar in size to some bacteria).

  • Visualization: Requires electron microscopy for observation.

Virus Structure

  • Basic components: Includes genetic material (DNA or RNA), a protective capsid, and sometimes an outer lipid envelope.

  • Capsid: A protein coat surrounding the genetic material.

  • Envelope: Derived from the host cell membrane for protection and infection assistance.

  • Glycoproteins (spikes): Essential for attachment to host cells.

Capsid Types

  • Shapes of viral capsids: (a) Helical, (b) Polyhedral, (c) Complex.

Viral Components

  • Capsid: The protein shell encasing the viral genome, made of capsomeres.

  • Nucleocapsid: The combination of the viral genome and capsid.

  • Envelope: Lipid membrane from the host cell, found in some viruses.

  • Spikes: Protruding structures for host cell attachment.

Complex Capsids

  • Found in bacteriophages that infect bacteria.

  • Non-symmetrical shapes and multi-protein types.

  • Example: T4 bacteriophage has a complex capsid with a tail.

Helical Capsids

  • Composed of rod-shaped capsomeres in a continuous helical structure.

  • Naked helical viruses: Rigid nucleocapsid example - Tobacco mosaic virus.

  • Enveloped helical viruses: Flexible nucleocapsid (examples: Influenza, measles, rabies virus).

Icosahedral Capsids

  • Three-dimensional, 20-sided structures with symmetrical capsomeres.

  • Naked viruses (e.g., adenoviruses) lack an envelope; enveloped viruses (e.g., hepatitis B) possess one.

Viral Genome

  • Can be DNA or RNA; not both.

  • Viral genes: Limited to those essential for host invasion and activity redirection (e.g., Hepatitis B has four genes).

Viral Classification by Genome

  • Classified by nucleic acid type and replication strategy (Baltimore classification):

    • Class I: dsDNA (e.g., Herpes simplex virus).

    • Class II: ssDNA (e.g., Parvovirus B19).

    • Class III: dsRNA (e.g., Rotavirus).

    • Class IV: (+)ssRNA (e.g., Poliovirus).

    • Class V: (-)ssRNA (e.g., Influenza).

    • Class VI: ssRNA-RT (e.g., HIV).

    • Class VII: dsDNA-RT (e.g., Hepatitis B).

Viral Life Cycle

  1. Adsorption: Virus attaches to host cell receptors.

  2. Penetration: Virus enters via fusion or endocytosis.

  3. Uncoating: Removal of viral capsid releasing genome.

  4. Synthesis: Host machinery produces viral components.

  5. Assembly: New viral particles are formed.

  6. Release: New viruses exit via budding (enveloped) or lysis (naked).

Adsorption

  • Virus encounters and attaches to the host cell's receptor sites.

Host Range and Infection

  • Host range: Compatibility of viral proteins with host cell receptors.

    • Restricted: Hepatitis B (liver cells only).

    • Moderate: Poliovirus (intestinal and nerve cells).

    • Broad: Rabies virus (various mammalian cells).

Penetration and Uncoating for Animal Viruses

  • Endocytosis: Virus is engulfed by the cell.

  • Uncoating: Enzymes dissolve the envelope/capsid releasing the virus into the cytoplasm.

Synthesis: Genome Replication and Protein Production

  • DNA viruses replicate in the nucleus; RNA viruses in cytoplasm.

  • Retroviruses (e.g., HIV) reverse transcribe RNA to DNA and integrate into the host genome.

Release of Mature Viruses

  • Enveloped viruses: Released by budding; do not destroy the host cell.

Replication of RNA Viruses

  • (+)ssRNA can be directly read by ribosomes; (-)ssRNA requires synthesis of (+)ssRNA for protein production.

Features in the Multiplication Cycle of RNA and DNA Animal Viruses

  • Specific mechanisms for RNA versus DNA replication cycles.

Viruses and Human Health

  • Common and serious diseases caused by viruses: colds, influenza, chickenpox, herpes, etc.

  • High mortality viruses: Ebola, rabies.

  • Chronic conditions linked to viruses: HIV/AIDS, hepatitis.

Challenges in Treating Viral Infections

  • Rapid mutation rates complicate effective long-term treatment development.

  • Limited antiviral drug options often targeting specific life cycle stages.

  • Vaccines are the best prevention but not available for all viral infections.

Designing Antiviral Therapies

  • Targets for antiviral drug development:

    • Entry inhibitors: Block virus entry.

    • Reverse transcriptase inhibitors: Used in HIV treatment.

    • Protease inhibitors: Block viral protein processing.

    • Neuraminidase inhibitors: Prevent release of new particles.

    • Interferons: Antiviral properties from host immune system.

Summary of Viral Properties

  • Non-living agents needing host cells to replicate.

  • Simple structure, diverse and significant roles in health and ecology.

Recap and Key Takeaways

  • Understanding viruses is crucial for developing treatments and preventing diseases.

  • Prions, while not viruses, pose similar health threats with distinct mechanisms.