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Plasmids

	Plasmids Continuing research in bacterial conjugation revealed that several other genetic traits other than chromosome were present. The genes for the production of some toxins, for the production of pilus and for the antibiotic resistance were found on DNA fragments in the cytoplasm. The word plasmid was coined for these fragments independent of chromosome. Plasmids are small circular DNA molecules which replicate in cells independently of the chromosome.
Such fragments have been termed by different authors, as sex factors, conjugons, extrachromosomal replicons and transfer factors. For instance, the genes responsible for transfer of resistance 10 some antibiotics are called resistance transfer factors Those responsible for transfer of the property of producing colicins (antibiotics type substances lethal to closely related strain or species of the bacteria producing them) are called the colicin factors (Cf). The plasmids normally contain only about 2 per cent of the total genetic information, and multiply independently of the chromosome. They pass quickly from cell to cell as does the F factor (fertility factor) and are found to be a critical factor in transferable drug resistance (TOR)
Thus by genetic recombination, a pathogenic bacterium may acquire the genes (for resistance to a particular drug) from a harmless (non-pathogenic) organism. For instance 1976 saw the emergence of penicillin resistant gonorrhea organisms, and multiple resistance in pneumonia cocci were located in l981. The plasmids are not essential to the cell growth and may be lost without any harm to the cell. Those plasmids that attach themselves to the chromosome are called episomes. Thus, the F-factor in Hfr cells is considered an episome. The study on plasmids was pioneered by the Stanford University microbiologist, Stanley Cohen who later utilised them in genetic engineering experiments. Plasmids in fact lie at the core of genetic engineering, which offers the potential to increase the range of antibiotics and to increase the availability of any compound in short supply e.g. insulin

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