NEET 2025 biology notes: Viruses, Viroids, Prions and Lichens

                               

Introduction

In Whittaker's five kingdom classification, certain acellular organisms like viruses, viroids, prions, and lichens are not mentioned. This section provides a brief overview of these entities.

Viruses

  • Characteristics:
    • Non-cellular organisms with inert crystalline structure outside living cells.
    • Infect host cells, take over their machinery, and replicate themselves.
    • Can be considered both living (inside host cells) and non-living (outside host cells).
  • History and Discovery:
    • Dmitri Ivanowsky (1892): Identified microbes causing mosaic disease in tobacco.
    • M.W. Beijerinek (1898): Named the infectious agent "virus" and described it as Contagium vivum fluidum (infectious living fluid).
    • W.M. Stanley (1935): Showed that viruses could be crystallized and composed mainly of proteins.
  • Structure:
    •  Contain either RNA or DNA, never both.
    • Protein coat (capsid) made of capsomeres, arranged in helical or polyhedral forms.
  • Host Specificity:

o   Plant Viruses: Usually have single-stranded RNA.

o   Animal Viruses: Can have single or double-stranded RNA or double-stranded DNA.

o   Bacteriophages: Typically have double-stranded DNA.

  • Diseases Caused:
    • In humans: Mumps, smallpox, herpes, influenza, AIDS.
    • In plants: Mosaic formation, leaf rolling, yellowing, dwarfing, and stunted growth.

Viroids

  • Discovery:
    • T.O. Diener (1971): Identified viroids as infectious agents smaller than viruses causing potato spindle tuber disease.
  • Characteristics:
    • Consist solely of free RNA.
    • Lack a protein coat.
    • RNA is of low molecular weight.

Prions

  • Discovery:
    • Identified as infectious agents causing certain neurological diseases.
  • Characteristics:
    • Consist of abnormally folded proteins.
    • Similar in size to viruses.
  • Diseases Caused:
    • Bovine Spongiform Encephalopathy (BSE): Mad cow disease in cattle.
    • Creutzfeldt-Jakob Disease (CJD): Variant in humans.


Lichens

  • Nature:
    • Symbiotic associations between algae (phycobiont) and fungi (mycobiont).
    • Algae are autotrophic; fungi are heterotrophic.
  • Function:
    • Algae provide food through photosynthesis.
    • Fungi provide shelter, mineral nutrients, and water.
  • Environmental Indicator:
    • Lichens are sensitive to pollution and do not grow in polluted areas.

Mind Map: Viruses, Viroids, Prions, and Lichens

  1. Viruses
    • Non-cellular, inert outside host
    • Discovered by Dmitri Ivanowsky, M.W. Beijerinek, W.M. Stanley
    • Structure: RNA or DNA, protein coat (capsid)
    • Diseases: Mumps, smallpox, herpes, influenza, AIDS (humans); mosaic, leaf rolling (plants)
  2. Viroids
    • Discovered by T.O. Diener
    • Free RNA, no protein coat
    • Cause potato spindle tuber disease
  3. Prions
    • Abnormally folded proteins
    • Cause neurological diseases
    • Diseases: BSE (mad cow disease), CJD in humans
  4. Lichens
    • Symbiotic association of algae (phycobiont) and fungi (mycobiont)
    • Algae: autotrophic, Fungi: heterotrophic
    • Sensitive to pollution




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