ROLE OF BACTERIOPHAGES IN GENETICS AND MOLECULAR BIOLOGY
INTRODUCTION AND DISCOVERY:
A bacteriophage is any one of a number of viruses that infect bacteria. Bacteriophages are among the most common biological entities on Earth. The term is commonly used in its shortened form, phage.Typically; bacteriophages consist of an outer protein capsid, enclosing genetic material. The genetic material can be ssRNA, dsRNA, ssDNA, or dsDNA along with either circular or linear arrangement. Bacteriophages are much smaller than the bacteria they destroy. In retro respect, there are a few reports in the literature that hint at the presence of bacteriophages. Hankins (1896) reported that the waters of Jumna and Ganges rivers in India had antiseptic activity against many kinds of bacteria and against the cholera vibrio in particular. In 1915, British bacteriologist Frederick Twort, superintendent of the Brown Institution of London, discovered a small agent that infected and killed bacteria. He believed that the agent must be one of the following: 1. A stage in the life cycle of the bacteria;
2. An enzyme produced by the bacteria themselves; or
3. A virus that grew on and destroyed the bacteria.
Twort's work was interrupted by the onset of World War I and shortage of funding. Independently, French-Canadian microbiologist Félix d'Hérelle, working at the Pasteur Institute in Paris, announced on September 3, 1917, that he had discovered "an invisible, antagonistic microbe of the dysentery bacillus".D'Hérelle called the virus a bacteriophage or bacteria-eater (from the Greek phagein meaning to eat). He also recorded a dramatic account of a man suffering from dysentery who was restored to good health by the bacteriophages. He found that the antigenic properties and host-range specificity of phages appeared to be characteristic of given “races” of phages. Thus from the beginning, there were hints that phages might be fruitful organisms for genetic study. Duckworth (1976) provides a detailed historical account of the discovery of bacteriophages. ROLE OF PHAGES:
The early history of bacteriophages might suggest that their study would immediately be applied to the treatment of diseases caused by bacteria. This was not the case; instead antibiotic therapy became the mainstay of treatment for bacterial diseases. However, the importance of the bacteriophage in the advancement of biological science cannot be overstated. Bacteriophages were intensively studied in the decades after their discovery, and came to play a leading role in the advancement of the basic science of microbiology and the new biology of molecular genetics. In the 1940's, and onward, it was the laboratory study of phage biology that directly yielded major insights into bacterial genetics, molecular biology, and the exact manner in which viruses reproduce and spread. These discoveries include:
1. Mutations arise in the absence of selection (Luria and Delbruck 1943).
The Luria–Delbruck experiment (1943) (also called the Fluctuation Test) demonstrates that in bacteria, genetic mutations arise in the absence of selection, rather than being a response to selection. In their experiment, Luria and Delbruck inoculated a small number of bacteria into separate culture tubes. After a period of growth, they plated equal volumes of these separate cultures onto agar containing phage (virus). If virus resistance in bacteria were caused by a spontaneous activation in bacteria—i.e., if resistance were not due to heritable genetic components, then each plate should contain roughly the same number of resistant colonies. This however was not what Delbruck and Luria found. Instead, the number of resistant colonies on each plate varied drastically. Luria and Delbruck proposed that these results could be explained by the occurrence of a constant rate of random mutations in each generation of bacteria growing in the initial culture tubes. Based on these assumptions Delbruck...
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