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.