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.