Biology
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Viruses, Viroids and Prions

In this article, we will be discussing about various cellular organisms that did not find a place in the five-kingdom classification, i.e., viruses, viroids and prions.

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Posted by barisbilgic, 6/10/2021
Viruses, Viroids and Prions

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R. H. Whittaker, in his five-kingdom classification, did not mention cellular organisms like viruses, viroids, and prions. However, these organisms are important entities of the environment having significant effects on humans, plants, and other living organisms. Let us learn more about them.

What are Viruses?

Viruses mean venom or poisonous fluid. You must have heard about the common cold or ‘flu’ and somebody who has suffered from it knows what effects viruses can have on us.

Viruses are noncellular particles made up of genetic material (DNA) and protein having the power to invade living cells.

Also, you must be wondering why viruses did not find a place in the classification? The reason is, viruses are not
considered as truly ‘living’. That is, they are both living and non-living. Let us see how.

Are viruses living or non living?

Viruses are both living and non-living or neither living nor non-living. That is, they show some properties of the living organisms but fail to show other properties.

To explain, viruses have a cell structure with genetic material and the ability to replicate which shows their living nature. On the other hand, they can not maintain a constant internal state i.e. homeostasis, which is living organisms show. Thus, we can not really classify them as living or non-living.

Discovery of viruses

Dmitri Ivanowsky in 1892, found out certain microbes causing the mosaic disease of tobacco having a size smaller than bacterias.

Further, in 1898, M.W. Beijerinek studied filtered plant juices and demonstrated that the extract of the infected plants of tobacco causes infection in healthy plants. He coined the term “virus” for the new pathogen and called the fluid Contagium vivum fluidum meaning, infectious living fluid.

W.M. Stanley in 1935, showed that a virus could be crystallized. However, these crystals were largely made of proteins and that they were inert outside their specific host cell.

Characteristics of viruses

  1. Viruses are nucleoproteins, containing proteins and genetic material which can be RNA or DNA. This genetic material is infectious in nature.
  2. None of the virus can have both RNA and DNA.
  3. They have a protein coat which is called as the capsid made of small subunits called capsomeres. Furthermore, these capsomeres are arranged in helical or polyhedral geometric forms.
  4. They are capable of reproducing only when they are inside a host cell.
  5. Viruses infecting plants have single stranded RNA.
  6. Viruses infecting animals have either single or double stranded RNA or double stranded DNA.
  7. Bacteriophages are the viruses that infects the bacteria. They are usually double stranded DNA.
Chapter 2 Biological Classification - NCERT Solutions for Class 11-science  Biology CBSE - TopperLearning
Tobacco Mosaic Virus (TMV) showing RNA and Capsid. Source: NCERT.
NCERT Ebook for Biological Classification - Biological Classification -  Chapter 2 - NCERT Biology - XI
Bacteriophage. Source: NCERT.

Viruses affect us in many ways, for example, diseases like mumps, smallpox, herpes, and influenza are all caused by. They also cause some serious diseases like AIDS.

Furthermore, in plants, the symptoms of a viral infection can be mosaic formation, leaf rolling and curling, yellowing, vein clearing, dwarfing, and can also affect its growth.

What are Viroids?

Viroids mean “virus-like” and they are a short strand of circular RNA capable of self-replication. 

Unlike viruses, viroids do not have a protein coat (capsid) to protect their genetic information. It is a free RNA of low molecular weight.

Discovery of viroid

In 1971, Theodor Diener, a pathologist found an acellular particle to cause potato tuber spindle disease, which causes slower sprouting and various deformities in potato plants. Also, these could take control of the host machinery to replicate their RNA genome, similar to viruses. Thus, he named them “viorids”.

What are Prions?

“Prions are agents consisting of an abnormal protein that transmits certain infectious neurological diseases.” These agents are similar in size to viruses. They have no DNA or RNA. “PrP” is the main protein in human and mammalian prion diseases.

Furthermore, prions cause diseases like bovine spongiform encephalopathy (BSE) commonly called mad cow disease in cattle and its analogous variant in humans is Cr–Jacob disease (CJD).

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