Chapter 2: Biological Classification
NCERT Biology | NEET-UG Preparation | Chapter Summary
Learning objective: Understand the development of biological classification, the five-kingdom system, the major characteristics of different kingdoms, and the differences between viruses, viroids, prions and lichens.
Viroids and Prions
Viroids
Viroids are infectious agents made of small, naked RNA molecules. Unlike viruses, they do not have a protein coat.
- They are smaller and structurally simpler than viruses.
- They contain RNA without a surrounding capsid.
- The discovery of viroids is associated with T. O. Diener in 1971.
- Viroids are known to cause diseases in plants.
Prions
Prions are infectious agents composed of abnormally folded proteins. They do not contain a conventional DNA or RNA genome.
- They are associated with certain transmissible neurodegenerative diseases.
- They differ from viruses and viroids in their protein-based nature.
| Feature | Virus | Viroid | Prion |
|---|---|---|---|
| Basic nature | Genetic material and protein coat | Naked RNA | Infectious protein |
| Protein coat | Present | Absent | Not applicable |
| Genetic material | DNA or RNA | RNA | No conventional nucleic-acid genome |
