Virus : Definitions, history, Characteristic


Virus : Definitions, history, Characteristic

"Viruses are ultramicroscopic, obligate, intracellular parasites, self-replicating, non-cellular organisms. A virion or infection particle of a virus always consists of a nucleic acid core of genetic material surrounded by a protein coat and is termed a nucleoprotein (DNA or RNA).

FROSCH saw PLANT VIRUS and ANIMAL VIRUS for the first time and TWORT discovered BACTERIOPHAGE.

Their detailed studies have been made by electron microscope. The virions of most bacteriophages contain DNA and a few contain RNA. Virions of some animal viruses contain DNA and others have RNA. Cauliflower mosaic virus has double-stranded DNA.

History of viruses - The study of viruses is called Virology, and the specialists in the field of virology are called Virologists. The term virus is derived from Latin, meaning "venom or poisonous fluid" and the name was given to the causative agents of infectious diseases by Pasteur. Yellow fever (the first viral disease of man) and smallpox, like diseases of the virus, have been known since the early days of human history, but their cause was known quite late. Adolf Meyer (1886) observed mottling in tobacco plant leaves and named it Mosaic krankhet i.e. mosaic.

Characters of viruses

1. Viruses are ultramicroscopic infectious particles, ranging from 20 - 350 nm in diameter.

2. They are nucleoproteins. They have any one nucleic acid either DNA or RNA, but never both together.

3. Viruses are obligate, intracellular parasites and non-cultivable on inanimate culture media. Easily propagated in living culture media.

4. The outer shell or capsid is mostly of proteins. Lipoviruses have an additional covering of lipids or lipoproteins.

5. These contain no metabolic enzymes or proteins synthesizing machinery of their own and use host machinery for the synthesis of their proteins.

6. These replicate but do not grow, but their nucleic acid directs the host cell to make various parts of the virus and then to assemble these parts into complete, infectious particles, virions.

7. They are transmissible from diseases to healthy plants.

8. They possess the capacity for variation and adaptation.

9. They are highly resistant to acids, alkalies, salts, and high temperatures.

Nature of viruses
Viruses exhibit both living and non-living properties, so they are called links between living and non-living.

(a) Characters of non-livings
Characters of non-living viruses are given below-

1. They do not have protoplasm or (cell wall and cell membrane).
2. They do not have an enzyme system.
3. They do not respire.
4. They can be crystallized.
5. Attempts to culture viruses in different types of culture media have failed.

(b) Characters of living beings
Characters of living viruses are given below-

1. They replicate inside the living cells.
2. Nucleic acids present in their body and capable of synthesizing protein for their coat. They use the ribosomes of the host for the purpose.
3. Nucleic acids show gene mutations as chromosomes of living organisms.
4. They cause diseases like bacteria and fungi.

No comments:

Post a Comment