Viruses – BIO102

Definition of a Virus

  • Etymology: The word "virus" is derived from Latin meaning "poison".

  • Historical Background:

    • Discovered by Dimitri Ivanosky (1892) and the term was coined by Beijerinck (1897).

    • Wendell Stanley (1935) identified viruses as comprising nucleic acids, proteins, and lipids.

    • Definitions have evolved:

      • Bawden (1943): Obligatory parasitic organism <200mµ.

      • Loria (1953): Sub-microscopic entity that reproduces only in living cells.

      • Luria and Darnell: Viruses have nucleic acid genomes (DNA/RNA) that reproduce inside living cells.

      • Konin et al. (2006): Small infectious agents replicating inside living cells.

  • Virology: The study of viruses, a branch of microbiology.

  • Diversity: Millions of viruses exist, with only about 5,000 well-documented.

Origin of Viruses

  • Unclear evolutionary history; some may originate from:

    • Plasmids (moving DNA within cells)

    • Bacteria

  • Considered at the edge of life due to:

    • Containing genetic material, reproduction, evolution through natural selection, yet lacking essential life characteristics.

Morphology of Viruses

  • Virion Structure: Composed of:

    • Nucleoid/Genome: Contains nucleic acids

    • Capsid: Protein coat

    • Some have a Viral Envelope (lipid layer derived from host cells).

Genetic Material

  • Located within the capsid; can be:

    • DNA or RNA (not both)

    • Forms: single/double stranded, linear/circular.

  • Examples of Nucleic Acids:

    • Linear DNA: Tobacco Mosaic Virus

    • Circular RNA: Bacteriophages

    • Double-stranded RNA: Reovirus

    • Single-stranded DNA: Influenza Virus

Capsid

  • Composed of capsomeres (protein subunits).

  • Types of shapes:

    • Spherical

    • Helical

    • Polyhedral

    • Binal/Complex

  • Functions: protects genetic material, aids in viral entry into host, supports synthesis in host cells, and serves as a basis for morphological classification.

Viral Envelope

  • A lipid layer found in some viruses, enhancing infectivity.

  • Proteins and carbohydrates from host cells may be incorporated.

  • Naked Viruses vs. Enveloped Viruses:

    • Enveloped viruses possess a viral membrane; others do not.

Characteristics of Viruses

  1. Obligate, acellular entities.

  2. Contain only one nucleic acid type: DNA or RNA.

  3. Smallest organisms, visible only via electron microscopy.

  4. Infect a variety of hosts (plants, animals, bacteria).

  5. Highly abundant across ecosystems.

  6. Replicate within living host cells.

  7. Not metabolically active outside host cells.

  8. Undergo genetic mutations.

  9. Can crystallize and be precipitated.

  10. Existence of races/strains.

  11. Host-specific with narrow or broad ranges.

  12. Effective at low doses.

  13. Resistant to antibiotics and extreme conditions.

  14. Diverse in shape, size ranges from 20-700nm.

  15. Transmissible between organisms.

  16. Considered both living and non-living entities.

Classification of Viruses

  • Proposed by Lwoff et al. (1962) based on:

    1. Type of Nucleic Acid:

      • Double-stranded RNA/DNA

      • Single-stranded RNA/DNA

    2. Architecture of Capsid:

      • Helical, Polyhedral, Spherical, Binal

    3. Host Range: Infectivity spectrum across organisms.

    4. Size: Varied from 2.5nm to 300nm.

    5. Site of Synthesis: Nucleus or cytoplasm of host cells.

    6. Presence of Viral Envelope: Enveloped or naked viruses.

    7. Transmission Methods: Through vectors, blood, saliva, etc.

Taxonomy of Viruses

  • Classification involves hierarchical levels:

    • Family: "-viridae"

    • Subfamily: "-virinae"

    • Order: "-virales"

    • Genus: "-virus"

    • Species: Named in English.

  • Example: HIV classified under Retroviridae family.

Uses of Viruses

  1. Cause diseases in humans and plants (virulence can vary).

  2. Regulation of ecosystems.

  3. Gene transfer between species: boosting genetic diversity.

  4. Used in gene therapy and nanotechnology.

  5. Scientific research applications in molecular biology.

  6. Virotherapy for targeting cancer and infection.

  7. Development of vaccines using synthetic viruses.

  8. Potential for biological warfare.

  9. Biological control methods in agriculture, but with limitations.

References

  1. Agrios, G.N. (2004). Plant Pathology. Elsevier Academic Press.

  2. Dube, H.C. (1982). A Text Book of Fungi, Bacteria and Viruses.

  3. Various online and academic sources.