Viruses:
About a century ago at time of Louis Pasteur and Robert Koch the word virus was generally referred to as a poison associated with disease and death. Present notion of virus is entirely different. Now they are recognized as particles of nucleic acid with protein coat. They replicate in living cells and cause many diseases such as influenza, hepatitis, small pox and AIDS. The branch deals with the study of viruses is known as virology. The word virus is derived from Latin word venom meaning poisonous fluid. It can be defined as non cellular infectious entities which contain either RNA or DNA normally encased in proteinaceous coat and reproduce only in living cells. They utilize biosynthetic machinery of host for its own synthesis and then transfer themselves efficiently to other cells. In 1796 Edward Jenner first vaccinated 8 year old boy with material removed from cowpox lesion on hand of milkmaid. After six weeks boy was inoculated with pus from small pox victim but he did not develop disease. Later Jenner used material for vaccination from cowpox lesions and successfully vaccinated 23 persons.
In 1884 one of Pasteur associates Charles Chamberland found that bacteria can not pass through porcelain filters while agent responsible for rabies ( a disease which is transferred to humans by bite of rabid dogs, foxes, cats, bats and other animals) can pass through these filters. In 1892 Ivanovsky discovered that agent which caused tobacco mosaic disease was filterable. He obtained bacteria free filtrate from ground up infected plants and placed it on healthy leaves of tobacco. He observed that filtrate produced disease in healthy plants. After that presence of similar filter passing ultramicroscopic agents was seen in victims of many diseases including foot and mouth disease and yellow fever. Filterable agents were first purified in 1935 when Stanley was successful in crystalizing tobacco mosaic virus.
Characteristics: Viruses are extremely small infectious agents which can only be seen under electron microscope. They range in from 250 nanometer of poxviruses to 20 nanometer of parvoviruses. They are 10 to 1000 times smaller than most bacteria so they can pass through the pores of filters from which bacteria cannot pass. They can reproduce only in animal and plant cells or in microorganisms where they reproduce by replication( a process by which many copies or replicas are formed). Thus they are obligate intracellular parasites. They lack metabolic machinery for synthesis of their own nucleic acid and protein. They depend on host cell to carry out these vital functions. During reproduction in host cells they may cause diseases.
Structure:
Complete, mature and infectious particle is known as virion. Virion are composed of central core of nucleic acid either DNA or RNA which is also known as genome and is surrounded by protein coat called capsid. Capsid gives definite shape to virion. Capsid is made up of protein subunits known as capsomeres. Number of capsomeres is characteristics of particular virus. For example 162 capsomeres are present in capsid of herpes virus and 252 in the capsid of adenovirus which cause some common colds. In some animal viruses nucleocapsid is covered by another membrane derived from host cell called envelop. Animal and plant viruses may be polyhedron, helical, enveloped or complex.
Bacteriophages:
Bacteriophages are the viruses that infect bacteria, were discovered independently by Twort in 1915 and D, Herelle in 1917. Twort discovered that bacterial colonies sometimes undergo lysis and this lysis can be transferred from one colony to other. Even highly diluted material from lysed colony can transfer lytic effect. However heating the filtrate destroyed its lytic property. From these observations he concluded that lytic agent might be a virus. D,Herelle rediscovered this phenomena in 1917 and used the word bacteriophages meaning bacteria eater.
Bacteriophages occur in two structural forms having cubical or helical symmetry. In general appearance cubical phages are regular solid or icosahedral and helical phages are rod shaped. Many phages consist of head and tail. In those cases heads are polyhedral but tails are rod shaped.
Life cycle of bacteriophages:
Earlier researches on phages were mainly on limited number of phages that infect Escherichia coli. Of these best known phages are T phages. Among T phages, T2 and T4 phages are mainly used in phage studies. Overall structure of T4 studied by electron microscopy resembles that of tadpole consisting of head and tail. Head is elongated pyramidal, hexagonal prism shaped structure to which straight tail is attached. Within head double stranded DNA molecule is present. Structure of phage tail is more complex than head. A layer of distinct protein forms inner tube or core which is enclosed in sheath made up of another type of protein. On one side of sheath is collar and on other side is end plate. To end plate six tail fibers are attached which are the structure for attachment.
Bacteriophages replicates only inside bacterial cell. First step in replication is its attachment to host cell at receptor site on cell wall of bacterium. During attachment weak chemical union between virion and receptor site takes place. Next step is penetration, tail releases enzyme , the lysozyme to dissolve a portion of bacterial cell wall. Tail sheath contracts and tail core is forced into the cell through cell wall and cell membrane. Virus injects its DNA into cell just as syringe is used to inject vaccine. Protein coat which forms phage head and tail structure of virus remains outside the cell. Immediately after entering in host cell, viral nucleic acid takes control of host biosynthetic machinery and induces host cell to synthesis necessary viral components(DNA and protein)and starts multiplying. About 25 minutes after initial infection approximately 200 new bacteriophages are formed and bacterial cell burst, it undergoes lysis. Newly formed phages are released to infect other bacteria and another lytic cycle begins. Phage which cause lysis of host cell is known as lytic phage.
All infections of bacterial cells by phages do not result in lysis. In some cases viral DNA instead of taking over control of host machinery becomes incorporated into bacterial chromosome. Phage in this state is called prophage and this process is known as lysogeny. In this condition bacterium continues to live and reproduce normally. Viral DNA being part of bacterial chromosome passes to each daughter cell in all successive generations. Sometimes the viral DNA gets detaches from host chromosome and lytic cycle starts and this process is called induction. Lysogenic bacteria are resistant to infection by same or related phages. Phage which cause lysogeny is called lysogenic phage.
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