Viruses, Viroids, AND Lichens

In the five kingdom classification of Whittaker, there is no mention of some acellular organisms like viruses and viroids, and lichens.

Viruses:

• Viruses are @@non-cellular@@ organisms that are characterized by having ==an inert crystalline structure== outside the living cell.

  • Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.

• The name virus (venom or poisonous fluid) was given by @@Pasteur. D.J. Ivanowsky.@@

• The Tobacco Mosaic Virus was found to be smaller than bacteria because they passed through bacteria-proof filters.

  • ^^M.W. Beijerinek^^ demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and is called the fluid ^^Contagium vivum fluidum.^^

• @@W.M. Stanley@@ @@showed that@@ @@viruses could be crystallized@@ @@and crystals consist largely of@@ @@proteins@@.

  • They are inert outside their specific host cell.
  • They are @@obligate parasites.@@
  • Viruses also contain genetic material, that could be ==either RNA or DNA== but never both.
  • Viruses are nucleoproteins and the genetic material is infectious.
  • Viruses that ==infect plants have single-stranded RNA== and viruses that @@infect animals have either single or double-stranded RNA or double-stranded DNA.@@
  • Bacterial viruses or bacteriophages (viruses that infect bacteria) are usually double-stranded DNA viruses.

  

• The ==protein coat called capsid== made of small subunits called @@capsomeres protects the nucleic acid.@@

  • These capsomeres are arranged in helical or polyhedral geometric forms.

• Viruses cause diseases like mumps, smallpox, herpes, AIDS, and influenza.

• In plants, the symptoms can be:

  • @@The mosaic formation@@
  • @@leaf rolling and curling@@
  • @@yellowing and vein clearing@@
  • @@dwarfing@@
  • @@stunted growth.@@

Viroids:

• ^^T.O. Diener^^ discovered a new infectious agent that was smaller than viruses and %%caused potato spindle tuber disease.%%
  • It was found to be a ^^free RNA.^^
  • It %%lacked the protein coat%% that is found in viruses.
  • The ^^RNA of the viroid was of low molecular weight^^^^.^^

Lichens:

• They are symbiotic associations i.e. mutually useful associations, @@between algae and fungi.@@
• The algal component is known as @@phycobiont@@ and the fungal component is @@mycobiont@@@@,@@ which is autotrophic and heterotrophic, respectively.
  • @@Algae prepare food@@ for fungi and @@fungi provide shelter@@ and absorb mineral nutrients and water for their partner.
  • Lichens are very good pollution indicators – they do not grow in polluted areas.

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