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.
- 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
- 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)
- Viroids
- Discovered by T.O. Diener
- Free RNA, no protein coat
- Cause potato spindle tuber disease
- Prions
- Abnormally folded proteins
- Cause neurological diseases
- Diseases: BSE (mad cow disease), CJD in
humans
- Lichens
- Symbiotic association of algae (phycobiont)
and fungi (mycobiont)
- Algae: autotrophic, Fungi: heterotrophic
- Sensitive to pollution
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