🐡Virus

 HISTORY

 

Virus : Latin word, which means "poisonous fluid" or "venom" or "secretion" (According to Pasture).

 

Ivanowsky separated a micro organism from the sap of infected plant and named "TMV". He reported that 

viruses are smaller than bacteria and they can pass through the bacterial proof filters. 

Beijerinck demonstrated that the extract of the infected plants of tobacco could cause infection in healthy 

plants and called the fluid as "Contagium vivum fluidum" (Infectious living fluid)

 

Stanley crystallized TMV first time and Nobel prize was awarded to him.

 

CHARACTERISTIC FEATURES OF VIRUSES

 

1. These are submicroscopic & non-cellular organisms generally smaller than 200 mµ/200nm.

 

2. They are obligate intracellular parasites.

 

3. They have either RNA or DNA. No virus contains both DNA and RNA.

 

4. They are inert out side their specific host cell in crystalline form.

 

5. They contain nucleic acid so they are capable of protein synthesis by the help of ribosomes of host cell.

 

MORPHOLOGY AND STRUCTURE OF VIRUSES 

Size of Viruses :

 

TMV - 300 mm × 20 mm or 300 nm × 20 nm

 

SHAPE 

Spherical - Influenza virus, HIV 

 Rod shaped – TMV l Tadpole like – Bacteriophage

 

Chemical composition : 

1. Nucleic acid : Either RNA or DNA 

 Generally in plant viruses, ssRNA is present but in Cauliflower mosaic virus dsDNA is present but Gemini 

viruses (curly top virus) have ssDNA.

 

Generally in animal viruses, double stranded DNA is present but in some animal viruses, RNA is present. Which 

is single or double stranded.

 

2. Protein coat : 

It is known as capsid and made up of small sub unit called capsomeres. 

Central core & Capsid are collectively known as nucleocapsid.

 

Symmetry of viruses : 

1. Helical symmetry: Capsomeres are arranged in helical manner in the capsid, e.g.. TMV and Mumps virus etc. 

2. Icosahedral : It is a type of polyhedral e.g., Herpes virus, Adeno virus, f × 174 bacteriophage

 

SOME IMPORTANT VIRUSES

 

1. TMV (Tobacco Mosaic Virus)  

It is the most thoroughly studied virus and was discovered 

by the Russian worker D. Ivanowsky (1892). 

It has helical symmetry. 

Having single stranded RNA 

In its capsid number of capsomeres are 2130. 

5% RNA and 95% protein are present in TMV.

 

2. Bacteriophage Virus

 

The Virus which infect the bacteria are called bacteriphage. 

 Bacteriophage was discovered by F.W. Twort and Named by Felix d' Herelle 

Bacteriophages are generally double stranded DNA virus.  

 

In QB - bacteriophage - ss RNA is present. 

l In f × 174 – bacteriophage - ss DNA is present.

 

Types of bacteriophages :

 

Broadly of 2 types - Depending on the interaction of 

phages with bacterial cell. 

1. Non-virulent phages or Temperate phage or Lysogenic 

phage. 

2. Virulent phages or Lytic phage. 

Transduction : 

When a bacteriophage transfers of genetic material from one bacterium (Donor cell) to another bacterium 

(receptor cell) it is called transduction. 

Discovered by - Zinder & Lederberg (1952) in Salmonella typhimurium.

 

Plant Diseases coused by viruses :- 

1. Tobacco mosaic disease. 2. Leaf curl of papaya. 

3. Yellow vein mosaic of lady finger. 4. Potato leaf roll 

5. Bunchy top of banana. 6. Tungro disease of rice. 

7. Tomato leaf curl. 

8. Chlorosis, necrosis, dwarfing and stunted growth are also caused by virus.

3. Cyanophage 

 

The virus which infect blue green algae are known as cyanophage. Double stranded DNA. e.g. LPP-1

 

4. Mycophages

 

Viruses, infecting fungi are called Mycophages. ds RNA present.

 

Viroids (Free infectious RNA 

l T.O. Diener (1971) discovered some new infectious agents, which are smaller than viruses, called viroids. 

l Viroids contain only very low molecular weight RNA (ss RNA) and not the protein coat. 

l Viroids cause Potato spindle tuber disease,

Prions or Slow viruses Smallest Proteinaceous infectious agent.