Chapter 13_Viruses, viroids and prions

Viral Architecture and Taxonomy

Virion Structure

Virion: A complete viral particle that is capable of infecting a host.

  • Genetic Material: Can be DNA or RNA; characteristics include:

    • Linear or circular structures.

    • Single-stranded or double-stranded configurations.

    • The genetic material carries the information necessary for virus replication and can also contain genes that help the virus evade the host immune system.

  • Capsid: A protective protein coat made of individual protein subunits called capsomeres.

    • Functions to protect the viral genetic material from degradation and assists in the attachment to host cells.

  • Spikes: Present in animal viruses; these glycoproteins protrude from the viral surface and are crucial for attachment to host cells by binding specific receptors.

    • They can also play a role in the fusion of the viral envelope with the host cell membrane.

  • Nucleocapsid: Combination of the capsid and the encapsulated nucleic acids.

    • Some viruses have a nucleocapsid that directly enters the host cell, while others require the whole virion.

Types of Viruses

  • Non-enveloped (naked): Lack a lipid envelope, typically more resistant to environmental factors including detergents and heat.

    • Examples include Enteroviruses like Poliovirus.

  • Enveloped: Consist of a lipid bilayer derived from the host cell membrane, containing matrix proteins in between the envelope and capsid.

    • Examples include Influenza virus and HIV, which are more susceptible to desiccation and environmental stresses.

Shapes

  • Icosahedral: A geometric structure with 20 faces, e.g., Adenovirus, providing a stable configuration.

  • Helical: Spiral shape, e.g., Tobacco mosaic virus, where the capsid proteins are arranged in a helix around the nucleic acid.

  • Complex: Viruses with more intricate structures, such as the T4 bacteriophage which has a distinct head and tail morphology.

Taxonomy

  • International Committee on Taxonomy of Viruses (ICTV): Over 6,000 viruses classified.

  • Classification criteria: Based on nucleic acid type, host range, and method of transmission.

  • Families: Typically end in -viridae (e.g., Herpesviridae).

  • Genera: End in -virus, e.g., Enterovirus.

  • Species: Named after the disease they cause, e.g., Poliovirus.

  • Informal groupings: Based on shared infection routes or reservoirs, such as respiratory viruses or enteric viruses.

Mechanisms of Transmission

  • Enteric: Fecal-oral route (e.g., Enteroviruses), often linked to contaminated food and water.

  • Respiratory: Spread via respiratory droplets (e.g., Influenza), primarily through coughing and sneezing.

  • Sexually Transmitted: Transmitted through sexual contact (e.g., HIV), with specific receptors involved.

  • Zoonotic: Viruses that can transfer from animals to humans, such as Ebola virus, often requiring close contact with animal hosts or their fluids.

Phage Infections

Types of Phage Infections

  • Lytic (Virulent) Phages: These phages cause host cell lysis resulting in the release of new phage particles. The process involves five key steps:

    1. Attachment to the host cell surface.

    2. Injection of viral DNA into the host.

    3. Replication of viral components using the host's cellular machinery.

    4. Assembly of new virions within the host cell.

    5. Release of new virions through cell lysis, which can result in cell death.

  • Lysogenic (Temperate) Phages: Incorporate their genetic material into the host genome, allowing for a dormant phase before potential lysis.