viruses

INTRODUCTION:

The life form which exists without a cellular structure is known as acellular or non-cellular life. The primary candidates for non-cellular life are viruses. Majority of biologists consider viruses are non living because they are not capable of autopoiesis

(ability of reproduction) without host. The other examples of acellular life are viroid which are smallest infectious agents consisting solely of short strands of circular single stranded RNA without protein coat. The prions are infectious agents composed entirely of protein, capable of multiplying itself and transferable from one host to another.

Viruses Discovery and Structure:

A virus is a biological agent that reproduces only inside the cells of living host Viruses can infect all type of life forms i.e., from animals and plants to microorganisms Including bacteria.

In 1884 the French microbiologist Charles Chamberland made a filter paper for the cause of tobacco mosaic disease. In his experiment he proved that tobacco mosaic

filtration of bacteria. In 1892, Russian biologist Ivanovsky used this filter to determine disease was not caused by bacteria but caused by other infectious agent which can pass

through filter paper. He called these filterable viruses. His view was confirmed by

American virologist W.M. Stanley in 1935, when he observed tobacco mosaic virus under Electron Microscope. In the early 20" century (1915, 1917) Twort and Herelle discovered bacteriophages (viruses that infect bacteria). Since then thousands of species of viruses

have been discovered and microbiologists speculate that there are millions of species of viruses still to be discovered.

Viruses Living or Non Living Viruses

show the characteristics of both living and non-living things. The living characteristics of viruses include: They have their own genetic material. They undergo mutation.

Can reproduce inside host cell by using host metabolic machinery. Get destroyed by ultraviolet radiations and chemicals. Occur in different varieties or strains. The non-living characteristics of viruses include: They are non-cellular particles. Generally lack enzymes and co-enzymes and depend upon host enzymes and

coenzymes for their metabolic activities. Can be crystallized and stored in laboratory. Do not respire and use the energy of host for their activities. Therefore, depending upon the ambivalent(fluctuating) nature of characteristics possessed by the Viruses; they are considered on boundary line between living and non living things.

Classification of Viruses

Viruses may be classified on the basis of morphology, type of host they infect, presence or absence of outer covering and types of nucleic acid. Classification of viruses based upon structure ( morphology): On the basis of capsid:

Some viruses have helical capsid such as tobacco mosaic virus (TMV). Many have polyhedral capsid, contain a glycoprotein spike at each vertex, such as adenovirus. Viruses possess an outer envelope studded with glycoprotein spike, such as Influenza viruses. Viruses like bacteriophage possess complex capsid consisting of a polyhedral lead and tail apparatus.

{{__On the basis of genome (DNA and RNA):__Double stranded DNA viruses (dsDNA Viruses).{{

Bacteriophages are ubiquitous

e.g., Paroviruses (small viruses of vertebrates and invertebrates) cause rash. is more than any other Double stranded RNA viruses (dsRNA viruses), organism on earth. e.g., Reoviruses, cause diarrhoea. Single stranded RNA reverse transcribing viruses template for DNA ( SERNA-RT viruses), e.g., HIV (retrovirus).

Bacteriophages are ubiquitousviruses found wherever bacteria exist. It is estimated that number of Bacteriophages

}}==Classification of viruses on the basis of host they infect:==}}

Bacterophages attack bacteria. Plant viruses which cause more than 2,000 types of plant diseases such as TMV, Potato yellow dwarf virus. Animals viruses cause many diseases to animals and human such as mouth and foot disease in livestock, parovavirus causes mumps and measles. Rous sarcoma virus causes cancer.