Biological Classification

Viruses, Prions, Viroids, Lichens

Lichens (Mycophycomycetes):

  • These dual organisms have a permanent association of a fungus (mycobiont) and an alga (Phycobiont)
  • The fungus is represented by ascomycete (or) sometimes basidiomycete
  • Algal partner is generally Blue-green algae
  • Lichens are sulphur dioxide (pollutant) indicators.

Viruses, Viroids, Prions:

  • Pasteur coined the term virus
  • Ivanowski discovered viruses from a diseased tobacco plant
  • Beijerinck called the fluid isolated from the diseased plant  “Contagium Vivum fluidum”
  • Viruses are obligate parasites, which cannot grow in a non-living culture medium.
  • Inert virus taken out of the host cell is called virion, which can be crystallised and stored indefinitely.
  • Important components of a virus are (i) Nucleoid (DNA or RNA) (ii) Capsid (iii) Envelope (iv) Enzymes.
  • Virus attacking a bacterium is called Bacteriophage.
  • Phagic reproduction is of two types : (i) Lytic cycle & (ii) Lysogenic cycle.
  • In the lytic cycle, the viral genome does not integrate with host DNA. But in the lysogenic cycle, the viral genome gets integrated with host DNA to form prophage.
  • A viroid is an RNA particle, where there is no protein coat, which affects only plants
  • Prions are formed due to a mutation in PRNP gene, that are glycoproteins which act as infectious agents.

Part-1: View this video for the topic from 0:15 to 9:40

Part-2: View this video for the topic from 0:09 to 7:38

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