Viruses, Viroids, and Prions

Viruses, Viroids, and Prions

Introduction to Microorganisms

  • Definition of microorganisms:

    • Include prokaryotes, such as Bacteria and Archaea.

    • Additional categories: Protozoa and Fungi.

    • Viruses, viroids, and prions are not classified as traditional organisms but are studied by microbiologists.

Viruses

Characteristics of Viruses

  • Small in size, significantly less than prokaryotic cells.

  • Acellular, lacking any cellular components.

  • Do not exhibit most characteristics of life, including:

    • Metabolic processes

    • Growth and development

    • Response to the environment

  • Require a host cell for reproduction.

  • Exist in two states:

    • Intracellular (virus)

    • Extracellular (virion)

  • Specificity: Certain viruses only infect specific organisms or cells.

    • There are viruses found that infect all forms of life.

Bacteriophages:

  • Definition: Viruses that specifically infect bacteria.

  • Visual representation included in the study notes.

Recent Study on Bacteriophages

  • A study published on 21 November 2017 highlighted:

    • Humans may absorb up to 30 billion bacteriophages daily, contributing to health by attacking bacteria.

Phage Therapy

  • Definition: Utilizes bacteriophages to treat bacterial infections.

    • Occasionally authorized for compassionate use when other therapies are unavailable.

Viral Structure

  1. Envelope

    • Host-derived membrane containing viral glycoproteins.

    • Not all viruses possess this structure; examples include enveloped and non-enveloped viruses (e.g., poliovirus).

  2. Capsid

    • Protein shell encasing the viral genome.

    • Formed from protein subunits called capsomeres.

    • Nucleocapsid: the combination of the capsid and nucleic acid.

  3. Other Structural Proteins

    • Includes matrix proteins and enzymes such as:

      • RNA-dependent RNA polymerase (RDRP): catalyzes the synthesis of RNA strands.

      • Reverse transcriptase (RDDP): synthesizes DNA from an RNA template, primarily in retroviruses.

  4. Nucleic Acid

    • The viral genome may be either DNA or RNA and can be linear or circular.

    • Typical size and structure:

      • Small genomes with as few as 3 genes, compared to bacteria with 1000-3000.

Viral Replication:

Bacteriophage Replication

  • Two cycles:

    1. Lytic Cycle: Virus infects and takes over the host bacterium, producing new phages and ultimately causing cell lysis (bursting) to release them.

    2. Lysogenic Cycle: Viral genome integrates into the host’s chromosome as a prophage, where it remains dormant and replicates with the host cell until triggered to enter the lytic cycle.

Animal Viral Replication

  • Similar basic steps as bacteriophages:

    1. Attachment: Virus binds to specific receptors/ligands on host cells.

    2. Entry and Uncoating: Virus enters host cell and sheds its capsid.

    3. Replication/Synthesis: Synthesis of new nucleic acids and proteins based on the type of virus.

    4. Assembly: Packaged into new viral particles.

    5. Release: Via cell lysis or budding (especially in enveloped viruses).

Latency in Animal Viruses

  • Some viruses can remain dormant within host cells for extended periods (e.g., years).

  • Reactivation can occur under certain conditions, as seen in chickenpox leading to shingles.

Viruses and Cancer

  • Approximately 20-25% of human cancers are virus-induced through mechanisms that may activate oncogenes or deactivate tumor suppressor genes.

Viroids

Characteristics of Viroids

  • Simplistic structures, consisting solely of circular RNA without a capsid.

  • Infectious agents primarily affecting plants.

  • Mechanism: Make their way into the host nucleus and use the host's RNA polymerase for replication.

Discovery of Viroids

  • Theodore Otto Diener in 1971 discovered viroids while investigating plant diseases, specifically the potato spindle tuber disease.

Notable Viroids

  • Cadang-Cadang: A lethal viroid affecting coconut palms, causing significant loss over decades.

Prions

Characteristics of Prions

  • Defined as proteinaceous infectious particles.

  • Composed of a single protein called PrP (prion protein).

  • Prions lack nucleic acids and induce abnormal folding in normal proteins, leading to neurodegenerative diseases.

Prion Diseases

  • Highlighted diseases caused by prions include:

    • Chronic Wasting Disease (CWD) in deer

    • Bovine Spongiform Encephalopathy (BSE), also known as Mad Cow Disease

    • Scrapie in sheep

Mechanism of Prion Infection

  • Prions cause misfolding in normal protein variants, leading to cellular death and neurodegeneration.

Effects of Prions on Brain Tissue

  • Lead to the formation of amyloids, resulting in tissue damage and cell death.

  • Structural stability of prions makes them resistant to denaturation.

Conclusion

  • Viruses, viroids, and prions represent a continuum of infectious agents varying in complexity, pathogenicity, and mechanisms of replication. Their implications for human health, agriculture, and the understanding of diseases underscore the importance of ongoing research in these areas.