viruses, history and origin of viruses, types of viruses, semester 3, botany notes,

  In nature, there are many ultramicroscopic particles known as viruses. A virus is a small particle composed of two types of substances, protein and nucleic acid; they are seen only with the help of electron microscope. Viruses have an important position in comparative study of living and nonliving, because they are entities sharing the characters of both they cause wide range of disease in plant and animals. The study of viruses has become so important and detailed that their study is treated in a new branch known as Virology.

  VIRUSES  

Definition: -

Various definition of viruses has been given, some of them are as follow: -

"A virus particle is in effect, a minute package of DNA or RNA surrounded by a protein coat or capsid." 

"A genetic element enclosed in a protein coat". 

"A core of nucleic acid either DNA or RNA surrounded by a protein coat shell".

 Green (1935) define viruses as "the smallest unit showing reproductive properties considered typical of life".

Bawden (1949) defined viruses as "obligate parasite too small to be seen".

According to Luria (1953) viruses are "submicroscopic entities capable of being introduced into a specific living cell and reproducing inside such cells only."

HISTORY: -

In 1886 Mayer described Tobacco Mosaic Disease but he could not find any casual agent. Credit for the discovery of viruses goes to the Russian botanist Iwanowski (1892) who found the causes of tobacco. He reported that the sap of infected plant filtered through bacteria proof filter was equally potent in causing the disease and conclude that the cause of disease was not bacterium but some other a smaller particle known as Virus.

Beijerinck (1898) called them "Contagium vivum fluidum " (i.e, living fluid infectant).

• In the following year the other filterable viruses" were discovered that were responsible for infection in plant animals and bacteria Bacteriophages (Virus infecting bacteria) were discovered by Twort (1915) and d. Herelle (1917). Shafferman and Moris (1963) discover Cyanophage that cause infection in blue green algae. In 1935 W.M. stanley crystallized TMV and showed that crystals retained their infectivity even when stored for indefinite period in a bottle. That the viruses could be crystallized showed that were not cells, but much simpler chemical entities. 
• Bawden and Pirie (1937) studies the chemical nature of TMV particles and reported that they are nucleoproteins.

CHARACTER OF VIRUSES: -

• Viruses are ultramicroscopic infections particles ranging from 20 to 35 nanometer in diameter.
• They're nucleoproteins.
• They contain DNA or RNA but not both which is either single standard or double standard.
• They DNA or RNA is surrounded by a protein coat that may be attached to more complicated structures.
• They are highly pathogenic cause disease in plants, animals, and bacteria.
• They are without protoplasm.
• They grow multiply and undergo mutation only within living host cells.
• They cause infection in host by nucleic acids.
• The genome has four gene, one for the core protein, two for the replicase enzyme and the fourth for a protein that probably enable the virus to spread from cell to cell in the plant.
• They are easily transmitted from one organism to another.
• They are not affected by antibodies.
• They can be crystallized.
• They are perfected obligate intracellular parasites that is they depend upon a specific host for their production and development the cells of animals, plants, and bacteria can serve as host to virus. Outside the host cell viruses exit as individual particles called Virions.

👉On the basis of the characters, given above it is not easy to say that viruses are living or nonliving. They have characters of both.

NOTE: -with few exceptions plant viruses have only RNA and a single coat made of one or a few kinds of proteins.

(a)character of non-livings: -

1. They do not have protoplasm. 
2.  They do not have enzyme system.
3.  They do not have respire.
4. They can be crystallized.
5.  Attempts to culture viruses in different types of culture media (outside the cells) have failed.

(b)Characters of living beings: -

1. They replicate, although inside the living cells.
2.  Nucleic acid present in their body, are capable of synthesizing protein for their coat, although they use ribosomes of the host for the propose.
3. Nucleic acid shows similar gene mutation as chromosome of the living organism.
4. They cause disease lack bacteria and fungi.

 However, on the basis of the above given characters many scientists considered a virus as bridge between living and nonliving or in other words they represent the transitional form of life, lying on the border line between the living and nonliving because viruses lack many features associated with life, they are referred to as particles.

CLASSIFICATION: -

According to Holmes (1948), viruses are classified on the basis of their host into the following kinds: -

(a)Animal viruses ( Zoophagineae): -They infect, fowl, pigeon, parrot, dog, crow and arthropods insect, etc. They've usually DNA or many also have RNA.

(b)Plant viruses (Phytophaginae): - They impact angiosperms, like potato, tobacco, sugarcane, cucurbits, beans and any other higher plants. They have RNA.

(c)Bacterial viruses (Phagineae): - They are commonly called bacteriophages or phases i.e, eaters of bacteria they have DNA.

 

I)TOBACCO MOSAIC VIRUS: - It is the most thoroughly studied rod shaped virus. It was isolated in the crystalline form by W.M.stanley(1935). Nobel prize was awarded to him for this discovery. Electron microscope a study have related that the TMV is a road shaped structure, 18 nanometer is diameter and 300 nanometer long. The protein coat (capsid or shell) consists of 2,130 identical subunits (capsomeres), which are arrange in a helix around a central hole of 4 nanometers in diameter. The protein constitutes 94.5% of virus. TMV contains a single chain of RNA of about 330nm in length and runs the entire length of the TMV rod. It contains about 7300 nucleotides. RNA is not present in the center of the hole but in terms with the protein subunit. It protrudes from one end of the virus. It is 5.5% of the virus. Frankel Conart and Schramm (1955) found that RNA alone is capable of causing infection.