Immediately after entering the host cell, the viral nucleic acid takes the control of the host’s biosynthetic  machinery and induces the host cell to synthesize necessary viral components (DNA, proteins), and starts multiplying. About 25 minutes after initial infection, approximately 200 new bacteriophages are formed, bacterial cell bursts, i.e., it undergoes lysis. Newly formed phages are released to infect the bacterial and another cycle, the lytic cycle begins (fig. 5.6). the phage which causes lysis of the host cell is known as lytic or virulent phage.

All infections of bacterial cells by phages do not result in lysis. In some cases viral DNA, instead of taking over the control of host’s machinery, becomes incorporated into the bacterial chromosome. Phage in this state is called prophage and this process is known as lysogeny.

In this condition the bacterium continues to live and reproduce normally. Viral DNA being the part of bacterial chromosome passes to each daughter cell in all successive generations. Some times, however, the viral DNA gets detached from the host’s chromosome and lytic cycle starts. This process is called induction. Lysogenic bacteria are resistant to infection by the same or related phages. The phage which causes lysogeny is called temperate (lysogenic) phage.


Fig. 5.6 Replication of a bacteriophage. After adsorption and penetration, the virus DNA undergoes prophage formation (1) In the lysogenic cycle, phages can exist harmlessly as a prophage with in the host cell for long perods of time. Each time the bacterial chromosome is replicated, the prophage also is replicated, and hence all daughter bacterial cell are “infected” with the prophage. Induction involves either a spontaneous or environmentally induced excision of the prophage from the bacterial chromosome. (2) A typical lytic cycle, involves synthesis and maturation synthesis and maturation of phage and new phages are released.

Classification Of Viruses

Virus morphology and nucleic acid properties are most important for classifying plant, animal and bacterial viruses. The genetic material may be DNA or RNA naked, enveloped or complex. On the basis of morphology viruses are classified into rod shaped (T M V), spherical (poliovirus) and tadpole like bacteriophage viruses etc. fig 5.3 and 5.4

Some Viral Diseases

There are many diseases which are caused by viruses. Only those are being mentioned here which are common in Pakistan.

Small pox: smallpox, which is caused by pox viruses (the DNA enveloped virus) is an ancient disease that is known to have occurred as epidemic in china as early as the twelfth century B.C. until the early twentieth century, small pox was a common disease throughout the world. In small pox, raised fluid-filled vesicles are formed on the body which become pustules later on and form pitted scars, the pocks. By 1950’s immunization and other control measures had largely decreased the danger, but it is still present In the third world countries where many people are affected.

Herpes simplex: herpes virus (DNA virus) is responsible for this disease. It is naturally occurring disease of mankind. In this vascular lesions in the epithelial layers of ectodermal tissues are formed. Most commonly this disease occurs in the mouth, on the lips, and at other skin sites.

Influenza: influenza viruses are enveloped RNA viruses. Influenza is wide spread disease in man and occurs in epidemic form.

Mumps and measles: mumps and measles viruses belong to group Paramyxoviruses. They are large, enveloped, RNA viruses. Mumps is highly contagious, wide spread, but seldom fatal. About 60% of adults are immune to it. Measles is one of the commonest diseases of the childhood and adult human population is equally susceptible the world over. This disease develops immunity in its victim.

Polio: poliomyelitis, caused by polio virus, is found all over the world. It occurs mostly in childhood. The age at which primary infection occurs varies with social and economic factors. The polioviruses are the smallest known viruses land contain RNA in spherical capsid. Some common human viral dieases are shown in Fig 5.7


Fig. 5.7 Some common human viral diseases

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