Viruses, Viroids, Prions & Lichens
🌡️ Why Aren’t They in the 5 Kingdoms?
Whittaker’s five-kingdom classification (Monera, Protista, Fungi, Plantae, Animalia) doesn’t include lichens, viruses, viroids, or prions because they’re unique acellular (not made of cells) or symbiotic organisms.
🦠 Viruses: The Tiny Invaders
- ❓ Living or Non-living?
- Outside host cells: Inert crystals (non-living)
- Inside host cells: Take over cell machinery to replicate (behave like living things)
- 🔬 Structure:
- Genetic material: Only RNA or DNA (never both!)
- Protein coat: Capsid (made of capsomeres)
- Shapes: Helical (e.g., TMV) or polyhedral (e.g., bacteriophage)
- ⚡ Key Facts:
- Discovered by Dmitri Ivanowsky (1892) in tobacco plants.
- Obligate parasites – can’t survive without a host.
- Diseases: Mumps, smallpox, AIDS (humans); mosaic formation, leaf curling (plants).
🌀 Viroids: Simpler Than Viruses!
- 📌 What they are:
- Infectious RNA only (no protein coat!).
- Smaller than viruses.
- 🥔 Discovered by: T.O. Diener (1971) in potatoes causing spindle tuber disease.
😵 Prions: Misfolded Proteins
- 🧠 What they are:
- Infectious proteins (no DNA/RNA!).
- Cause neurological diseases.
- 🐄 Diseases:
- BSE (Mad cow disease) in cattle.
- Cr–Jacob disease (CJD) in humans.
🍄 Lichens: Teamwork Champions!
- 🤝 What they are:
- Symbiotic association between algae + fungi.
- Algae (phycobiomt): Makes food 🥗 via photosynthesis.
- Fungi (mycobiomt): Provides shelter 🏠 + water/minerals.
- ⚠️ Superpower:
- Pollution indicators – they vanish in polluted areas!
📚 Biological Classification Recap
| Kingdom | Key Features |
|---|---|
| Monera | Bacteria (cosmopolitan, metabolic diversity) |
| Protista | Single-celled eukaryotes (e.g., algae, protozoans) |
| Fungi | Mostly saprophytic (e.g., mushrooms, molds) |
| Plantae | Eukaryotic, chlorophyll-containing (alternation of generations!) |
| Animalia | Multicellular, heterotrophic, no cell walls (holozoic nutrition) |
💡 NEET Must-Knows
- Viruses: Structure (capsid + RNA/DNA), obligate parasitism, diseases (e.g., AIDS, TMV).
- Viroids vs. Prions: Viroids = infectious RNA; Prions = infectious proteins (BSE/CJD).
- Lichens: Symbiosis (algae + fungi) + pollution indicators.
- Five Kingdoms: Criteria (cell structure, nutrition, reproduction, phylogeny).
- Plantae/Animalia: Alternation of generations (plants) & holozoic nutrition (animals).
