Biological Classification - Lichens, Viruses, Viroids and Prions

Lichens are symbiotic associations, consisting of an algal component ($Phycobiont$) and a fungal component ($Mycobiont$). The alga prepares food while the fungus provides shelter and absorbs mineral nutrients and water.

Viruses are non-cellular organisms characterized by having an inert crystalline structure outside the living cell. They are obligate parasites.

The genetic material of a virus can be either $DNA$ or $RNA$, but never both. Generally, plant viruses have single-stranded $RNA$ ($ssRNA$), while animal viruses have either $ssRNA$, double-stranded $RNA$ ($dsRNA$), or double-stranded $DNA$ ($dsDNA$).

The protein coat of a virus is called the $capsid$, made of small subunits called $capsomeres$, which protects the nucleic acid.

Viroids were discovered by $T.O. Diener$ in $1971$. They are smaller than viruses, lack a protein coat, and contain free $RNA$ of low molecular weight.

Prions are infectious agents composed of abnormally folded proteins. They are similar in size to viruses and cause neurological diseases like Bovine Spongiform Encephalopathy ($BSE$) and Cr-Jacob disease ($CJD$).

Lichens are very good pollution indicators; they do not grow in areas polluted with sulfur dioxide ($SO_2$).