Discovery of Virus, Characteristics and life cycle bacteriophage

 Discovery of virus:

     Viruses were first discovered in 1892 by Dimitri lvanovsky marking a significant breakthrough in the study of microorganisms like viruses. He saw in filter paper that there are other agents whose can pass through paper while bacteria cannot pass. It means that their size are more smaller than bacteria. He named it filtrable agent. With time many scientist trying to discover these agent.
A scientist Martinus Beijerinck named it virus.
The term 'virus' originates from Latin word 'venome', which means poisonous fluid, reflecting its harmful nature. This name is because it causes disease. 
In 1935 Stanley proves that it is not a fluid. He crystalized tobacco mosaic virus and contain protein and nucleic acid. 
In 1926 Thomas Millton said that it live as parasite.

Characteristics of viruses:

  • Viruses are very small agents as they are 10 to 1000 times smaller than that of bacteria. They are 250nm to 20nm.
  • They can easily pass through pores of filter paper.
  • They are infectious agents that causes disease.
  • They lives as parasite and totally depend on host.
  • They only can reproduce on host body by replication. They cannot replicate on artificial media.
  • A viruses cannot exist without host, as they relay entirely on host cell for survival and reproduction. 

Is virus living or non-living?

    The debate on whether a viruses are living or non-living entities remain one of the most intriguing question in virology. Nobody can classified virus in living and non-living. Virus is display characteristics similar to living as well as non-living. 
Virus is similar to living organisms in that they contain genetic material like DNA and RNA. DNA essential for replication, is composed of protein and nucleic acid. Protein is the major part of living cell. Approximately 90% of the viral structure is compose of protein, making it a crucial component in virus morphology.  Viruses can evolve and reproduce using host's metabolic machinery.
Viruses are similar to non-living. However, they lack cellular organelles, cytoplasm and ribosomes and don't have their own metabolic activity. Viruses are also smaller than cell.
It is a big debate on virus to classified. They are not included in any kingdom of classification.

Structure and composition:

    Viruses are composed solely of protein and nucleic acid. They have a three-dimensional icosahedral(20 sides) head and a tail. The head, called capsid, is composed of many protein molecules. These protein units known as capsomeres. Inside the capsid is nucleic acid in the form of DNA and RNA. Most viral capsids have a sheath of lipoprotein around capsid. Without this envelope it is called naked virus. Shape of capsid is different in different viruses. It may be helical, complex and polyhedron(having many faces). Its tail is generally rod shape and help in injection to the host body.

Classification of virus:

    Viruses are different in their morphology and host. Virus are classified on the base of different DNA and RNA and replication method, as well as their host range. Some are TMV(tobacco mosaic virus), poliovirus and bacteriophages etc.  The life cycle of bacteriophage is described bellow.

Life cycle of a bacteriophages:

    Bacteriophages infect Escherichia coli. Best phage for study is called T phase. They are also classified in T2 and T4 phage. T4 phage is resemble to tadpole, a head and a tail. Its straight tail is attached to hexagonal prism-shaped and pyramidal head(capsid).  Tail of bacteriophage is more complex than its head. Inner tube of tail is called core, compose of protein. Outer sheath is compose of other type of protein. Its bottom layer is called base plate and upper one side is called collar.
  

Replication and attack on bacteria:

  • The first step for replication is attachment between virion and receptor site of bacterial cell wall.
  • Tail contract and penetrate into cell. It inject DNA in cell through tail as syringe of injection.
  • After entering nucleic acid control metabolic machinery of bacteria to synthesis DNA and protein. 
  • After synthesis of viral components it start multiplying and bacterial cell under goes lysis(break down or burst). About 200 daughter virus formed in 15 minutes. The phage which is cause lysis called lytic phage. 
Sometime nucleic acid become a part of bacterial chromosome and bacteria normally live and reproduce. This passes to each daughter cell. It is called lysogeny.  

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