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.