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Viruses, Viroids, Prions And Lichens

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Classification is everywhere, in living, non-living, etc.  Classification of organisms gives us an easy understanding of the family and the origin of the organisms. The process of classification has been carried on since the ages of Aristotle.

First came the Two kingdom classification which included plants (Plantae) and animals (Animalia). But there were a few drawbacks to this classification like no proper classification between eukaryotes and prokaryotes, unicellular and multicellular, and photosynthetic and non-photosynthetic organisms.

Photosynthetic organisms- The organisms that can use photosynthesis to produce their own food. Nonphotosynthetic organisms those not prepare their food on their own.

After a long time and several changes in the classification technique, R.H. Whittaker proposed the Five Kingdom Classification. Whittaker kept in mind several properties of the organisms like mode of reproduction, mode of nutrition, etc. His Five kingdoms included the following,

  1. Monera: Bacteria.
  2. Protista: Forams.
  3. Fungi: Sac fungi. Basidiomycota, etc.
  4. Plantae: Tomato, mango, etc.
  5. Animalia: Dog, cat, etc.

But, Whittaker did not include the organisms like viruses, viroid, Prions, and lichens in the classification, due to some unique features.


Viruses are not practically living organisms, they are noncellular organisms and are in an inactive state while outside the host body and when they get inside the host body they replicate themselves. Of course, the process will kill the host. 

  • The most popular disease caused by the virus is the mosaic disease of tobacco. 
  • An experiment carried out by M.W. Beijerinck introduced a new pathogen called “virus” and “Contagium Vivum Fluidum” (infectious living fluid). 
  • W.M. Stanley demonstrated that viruses could be crystallized and that crystals are mostly made up of proteins. Outside of their unique host cell, they are dormant. Viruses are parasitic to other organisms. 

Structure of a Virus



  • Viruses, outside the host, can crystallize and this crystal layer is consist of protein.
  • The virus also contains genetic material (either RNA or DNA) which is infectious.
  • There is a protein coat over the virus called capsid made of small capsomere which is arranged in a polyhedral structure.
  • A viruses size ranges from 30 nm to 50 nm
  • The Head of the virus contains the genetic material which is either DNA or RNA.
  • Below mentioned viruses have a similar structure but have a different shape.

The most common disease caused by viruses is Mumps, measles, Rubella, etc.



There are different types of virus-like the two mentioned above. One is a bacteriophage that only infects bacteria and reproduces inside the bacteria and bursts open when matured and the young virus infects the other bacteria cells. The right one is the Human Immunodeficiency Virus better known for causing AIDS (acquired immune deficiency syndrome) which is a disease caused in humans and is incurable but its symptoms can be treated.


After the discovery of viruses, scientists found another pathogen that was smaller than a virus. It didn’t have a protein layer like a virus and caused potato spindle tuber disease.

It was discovered by T.O. Diener in 1971.

Structure of Viroid 

  • Viroid only consists of RNA which is not protected by any layers of protein like a virus.
  • They only infect plants (eg. Potato spindle tuber disease)
  • They are smaller in size than a virus
  • The structure of viroid is never similar
  • As same as viruses, these viroids infect the host cell to reproduce and slowly kill the host during the process.


These are abnormally folded proteins, which can transmit several diseases. The word prion is derived from “proteinaceous infectious particle”. They were discovered by American biologist Stanley B. Prusiner. 

Structure of Prions

  • They are similar in size to Viroid (Significantly smaller than viroid)
  • Prions generate aberrant protein clumps known as amyloids, which accumulate in infected tissue and are linked to tissue damage and cell death.
  • Prions are abundant in one’s body but the one that is dangerous are structurally different.

The most common disease caused by Prions is bovine spongiform encephalopathy (BSE), Cr–Jacob disease (CJD), etc.

Difference between Viroid  and prions



Nucleic acid is present in viroidNucleic acid is not present in prions
In size comparison, viroid is bigger than prionsIn size comparison, prions are smaller than viroid
In terms of composition, viroid are composed of a single strand of RNAIn terms of composition, prions are composed of only protein
These are highly infectious RNA moleculesThese are highly infectious Protein molecules


Lichens are symbiotic (Mutual) relationship between algae and fungi in which algae prepare food and fungi provides shelter. In this relationship, algae are known as photobionts, and fungi are known as mycobionts. Lichens have properties different from those of their component organisms.



Structure of lichen

  • Physically, the most of the observed part of lichens is the filaments of its fungal symbiote
  • The algae part is situated below this fungal filament and is separately situated
  • The thick filaments help in attaching them to the surrounding. 

Types of lichens

  1. Fruticose– Grows outside of a standing branch tree
  2. Foliose– This species has two defined upper and lower sides.
  3. Squamulose– A lichen that is composed of small, often overlapping “scales” called squamulose.
  4. Leprose– This lichen has the physical appearance of dust.
  5. Gelatinous– This lichen has the physical appearance of jelly
  6. Crustose– a crust that strongly appears.

Conceptual Questions

Question 1: State the members of the Five kingdom classification.


Monera, Protista, Fungi, Plantae and Animalia.

Question 2:Why are lichens referred to as Symbiotic?


The lichens are the organisms that include two different organisms algae and fungi. They help each other to survive and cannot be differentiated through physical appearance. Hence they are called Symbiotic organisms.

Question 3: Name the viruses that cause infection in humans?


Pox virus (causes Chickenpox), HIV (Human immunodeficiency virus), Coronavirus (Covid-19)

Question 4: What can Prions do to the human body?


Prions mainly cause brain damage which may result in memory impairment, personality changes, etc.

Question 5:What is a bacteriophage?


A virus that infects only bacteria and replicates is called a Bacteriophage.

Question 6: What are prokaryotes and eukaryotes?


Eukaryotic organisms are the ones that have a nucleus and other cell membranes. Prokaryotes don’t have any nucleus and other membranes.

Last Updated : 28 Nov, 2022
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