Viruses, Viroids and Prions - Practice Questions & MCQ

Virus:

• A virus is a nuclear proteinaceous entity which requires a living host to replicate by utilising the replication machinery of its host
• Viruses were discovered by Ivanowski (1892)
• Beijerinck called viruses as ‘contagium vivum fluidum’ which refers to living infectious fluid.
• The term Virus was coined by Pasteur which refers to poison.
• Viruses are obligate parasites.
• Viruses have not given any place in the classification because they have acellular structure and inert crystal like when present outside the host. When present in the host they replicate and weaken the host and sometimes eventually kill the host.
• Tobacco Mosaic Virus (TMV), Human Immunodeficiency Virus (HIV) are RNA containing Viruses.
• Viruses are considered to be intermediate between living and non-living entities.
• VIruses resemble non-livings as they lack protoplast, unable to live independent of a living cell, energy storing system, respiration, growth, division etc are absent.
• Viruses resemble livings in being formed of organic macromolecules, absence of genetic material, ability to multiply, the occurrence of mutations, having a few enzymes responsible for multiplication.

 
Composition of virus
 
Viruses have two components i.e. nucleoid or genome and capsid. In some, an envelope is present.  
1. Nucleoid-  It is the genome of a virus. It consists of a single molecule of nucleic acid that can be linear or circular. Nucleic acid can be either DNA or RNA. DNA containing viruses are called deoxy viruses and RNA containing viruses are called riboviruses.
dsDNA viruses- T2, T4 bacteriophage, coliphage, Pox virus, Adenovirus, Cauliflower mosaic virus,
ssDNA- PHI into 174
dsRNA- Reovirus, a tumour virus
ssRNA- TMV, Retroviruses,
2. Capsid - Protein covering around the genetic material i.e. nucleoid of viruses. The capsid consists of subunits called capsomeres
3. Envelope- Loose membranous covering that occurs in some animal viruses. Viruses without envelopes are called naked. Envelope consists of proteins, lipids and carbohydrates. It has subunits called peplomer. E.g. HIV, Herpes virus
 

Some Examples of Viruses:

• Tobacco mosaic virus- RNA is ssRNA, infects members of family Solanaceae.
• Poxvirus- Nucleoid is dsDNA, two core enzymes i.e. RNA polymerase and phosphohydrolase, are present in them, the causative agent of smallpox.
• AIDS virus- Consists of ssRNA and outermost glycoprotein layer, causes Acquired Immunodeficiency Syndrome (AIDS).

 
Viroids: 

• These are the smallest self-replicating, infectious particles having RNA without a protein coat 
• These were discovered by Raymer and Diener as the causal organism of Spindle Tuber disease in Potato. Since protein coat was absent in them, they were called as Viroid.
• Viroids have circular single-stranded RNA with low molecular weight.
• Cadang-Cadang in Coconut is also caused by Viroid.

 
Prions:

• Prions are infectious protein particles.
• These were discovered by Stanley Prusiner while they were working on scrapie disease of sheep and mad cow disease.
• These cause Scrapie disease of Sheep and Mad cow disease of Cow and
• Prions also cause Kuru (laughing death) disease, Mad cow disease, Alzheimer's disease etc.