Summary of Viruses, Viroids, and Prions

Overview of Viruses

  • Definition: Viruses are noncellular particles that require a host to multiply.

  • History: First studied in relation to rabies by Louis Pasteur and named by Dmitri Ivanovski.

Properties of Viruses

  • Characteristics:

    • Obligate intracellular parasites.

    • Ultramicroscopic size (20 nm to ~450 nm).

    • Not considered living organisms.

  • Structure: Comprises a protein shell (capsid) and a nucleic acid core (DNA or RNA).

  • Form Variations: Naked (no envelope) or enveloped (with lipid outer layer).

Viral Classification

  • Nomenclature: Family names end in -viridae, genera in -virus.

  • Taxonomy: 7 orders, 104 families, 505 genera recognized by the International Committee on the Taxonomy of Viruses.

Viral Replication Cycle

  1. Adsorption: Virus attaches to host cell receptors.

  2. Penetration: Virus enters the cell.

  3. Uncoating: Viral nucleic acid is released.

  4. Synthesis: Production of viral components.

  5. Assembly: New virus particles form.

  6. Release: Liberation via lysis or budding.

Virus Types and Human Diseases

  • DNA Viruses: e.g., Herpesvirus (HSV), Papillomavirus (HPV).

  • RNA Viruses: e.g., Retroviruses (HIV), Orthomyxoviruses (Influenza).

Persistent Infections and Oncogenic Viruses

  • Persistent Infections: Infections that last long-term without immediate cell death.

  • Oncogenic Viruses: Can induce cancer, e.g., HPV (cervical cancer), EBV (Burkitt's lymphoma).

Non-Viral Agents

  • Prions: Infectious proteins causing neurodegenerative diseases.

  • Viroids: Simple RNA molecules affecting plants.

  • Satellite Viruses: Require helper viruses for replication.