Viruses and Viroids Overview

Viruses

• Definition: Viruses are microscopic infectious agents that require a living host cell to replicate.
• Structure: Composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid); some have an outer lipid envelope.
• Reproduction: Hijack host cellular machinery to reproduce; can follow lytic (destructive) or lysogenic (integrative) cycles.
• Classification: Classified based on:
  • Type of nucleic acid (DNA or RNA)
  • Presence of an envelope
  • Shape (icosahedral, helical, complex)
• Diseases: Cause numerous diseases in humans, animals, and plants (e.g., influenza, HIV, COVID-19).

Viroids

• Definition: Small, circular RNA molecules that infect plants, lacking a protein coat.
• Characteristics:
  • The smallest known infectious pathogens
  • Only consist of a short strand of RNA
  • Replicate within host cells using host's own machinery
• Associated Diseases: Cause various plant diseases (e.g., potato spindle tuber disease).

Prions

• Definition: Infectious agents composed solely of misfolded proteins; lack nucleic acids.
• Mechanism: Induce normal proteins in the brain to misfold, leading to brain damage and disease.
• Characteristics:
  • Resistant to heat and proteases
  • Cause neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease, BSE - "mad cow disease").
• Transmission: Can be transmitted through contaminated food or medical procedures.

Lichen

• Definition: Symbiotic association between fungi (mycobiont) and photosynthetic organisms (photobiont, usually algae or cyanobacteria).
• Characteristics:
  • Grow in diverse environments (rocks, trees, soil).
  • Serve as indicators of air quality and environmental health.
  • Can reproduce sexually (fungal spores) or asexually (fragmentation, soredia).
• Role in Ecosystems:
  • Contribute to soil formation and nutrient cycling.
  • Serve as food for various organisms and habitat for small fauna.

Viruses

• Microscopic infectious agents requiring a living host cell to reproduce.
• Composed of genetic material (DNA or RNA) encased in a protein coat (capsid), with some having an outer lipid envelope.
• Replicate by hijacking host cellular machinery, following lytic (destructive) or lysogenic (integrative) cycles.
• Classified based on their nucleic acid type (DNA or RNA), presence of an envelope, and shape (icosahedral, helical, complex).
• Viruses cause various diseases in humans, animals, and plants, such as influenza, HIV, and COVID-19.

Viroids

• Small, circular RNA molecules lacking a protein coat that infect plants.
• The smallest known infectious pathogens, consisting only of a short strand of RNA.
• Replicate within host cells using the host's own machinery.
• Cause various plant diseases, like potato spindle tuber disease.

Prions

• Infectious agents made solely of misfolded proteins, lacking nucleic acids.
• Induce normal proteins in the brain to misfold, leading to brain damage and disease.
• Resistant to heat and proteases.
• Cause neurodegenerative diseases such as Creutzfeldt-Jakob disease and Bovine Spongiform Encephalopathy (BSE, "mad cow disease").
• Transmitted via contaminated food or medical procedures.

Lichens

• Symbiotic association between fungi (mycobiont) and photosynthetic organisms (photobiont, typically algae or cyanobacteria).
• Grow in diverse environments, including rocks, trees, and soil.
• Serve as indicators of air quality and environmental health.
• Reproduce sexually through fungal spores or asexually through fragmentation and soredia.
• Contribute to soil formation and nutrient cycling within ecosystems.
• Provide food for various organisms and habitat for small fauna.