Viruses, Viroids, Prions & Lichens | Additional Study Material for NEET PDF Download

Introduction

• In the five-kingdom classification of Whittaker, there is no mention of some acellular organisms like viruses and viroids and lichens.
• All of us who have suffered the ill effects of the common cold or ‘flu’ know what effects viruses can have on us, even if we do not associate it with our condition.
• Viruses did not find a place in classification since they are not truly ‘living’ if we understand living as those organisms that have a cell structure.
• The 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.

Tobacco Mosaic Virus (T.M.V.)

Tobacco Mosaic Virus

Tobacco mosaic virus (TMV) is a positive-sense single-stranded RNA virus species in the genus Tobamovirus that infects a wide range of plants, especially tobacco and other members of the family Solanaceae.

Bacteriophage

Structure of Bacteriophage

• The name virus that means venom or poisonous fluid was given by Pasteur. 
• D.J. Ivanowsky (1892) recognized certain microbes as a causal organism of the mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria-proof filters.
• M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid Contagium vivum fluidum (infectious living fluid).
• W.M. Stanley (1935) showed that viruses could be crystallized and crystals consist largely of proteins. They are inert outside their specific host cell. Viruses are obligate parasites. 
• In addition to proteins viruses also contain genetic material, that could be either RNA or DNA. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic material is infectious. 
• In general, 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 the 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, and influenza. AIDS in humans is also caused by a virus. 
• In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing, and stunted growth. 

Viroids

• In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. 
  Viroids
• It was found to be a free RNA, it lacked the protein coat that is found in viruses, hence the name viroid.
• The RNA of the viroid was of low molecular weight.

Prions

• It is usually a harmless protein, found in the brain. It is responsible for several neurodegenerative diseases among humans and animals.
• Moreover, the prion’s course of action is unlike any bacteria or virus. The reason being, it is just protein lacking any genetic material. In case a misfolded protein enters a human body, it affects the correctly-folded protein to affect adversely.
• Furthermore, in the early 1980s, Stanley B. Prusiner and his colleagues first identified the prions. Additionally, the term prion came from “proteinaceous infectious particle”. 
• This misfolded protein enters a human or animal brain via infection. However, it can also arise from a mutation of the gene that encodes protein.
• However, scientists have not fully understood the mechanism till date.

➤ What is a Prion Infection?

• Prion infection or transmissible spongiform encephalopathies (TSE) is a family of uncommon progressive neurodegenerative disorder. It affects the brain of both humans and animals. Additionally, prions are the causative agents of TSEs. Notably, this abnormal folding of protein can result in brain damage and this disease is usually fatal

Lichens

• Lichens are symbiotic associations i.e. mutually useful associations, between algae and fungi. 
  Lichens
• The algal component is known as phycobiont and the fungal component as mycobiont, which is autotrophic and heterotrophic, respectively. 
• Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner. 
• So close is their association that if one saw a lichen in nature would never imagine that they had two different organisms within them. 
• Lichens are very good pollution indicators: They do not grow in polluted areas.