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.
Viruses
Viruses are acellular infectious agents that can reproduce only inside living host cells. They are not placed within the five kingdoms of cellular organisms.
Structure
- A virus contains genetic material, either DNA or RNA, enclosed in a protein coat called a capsid.
- Some viruses possess an additional outer envelope.
- Viruses lack independent cellular machinery for reproduction and metabolism.
Important features
- They are obligate intracellular parasites.
- Outside a suitable host cell, they do not reproduce.
- Different viruses infect animals, plants, bacteria and other organisms.
- Bacteriophages are viruses that infect bacteria.
Examples and historical notes
- W. M. Stanley crystallised tobacco mosaic virus in 1935.
- Tobacco mosaic virus (TMV) contains RNA as its genetic material.
- Bacteriophages infect bacterial cells.
NEET focus: Know the distinction between the viral genome, capsid and envelope. Viruses are acellular and depend on host cells for reproduction.
