Transformation

Transformation

	Transformation - Transformation is the process of genetic modification in .which fragments of DNA from one cell enter and are integrated in another cell. Bacterial transformation was discovered by Griffith (1928), who used two strains of Streptococcus (Diplococcus) pneumoniae .This bacterium causes pneumonia in man. It is pathogenic for mice and may prove fatal. The wild type strain (8) has cells surrounded by a polysaccharide capsule, and forms a colony with a smooth surface
The strain is virulent (pathogenic), and if injected into mice causes severe septicemia and death. A mutant strain (R) does not have the capsule, and forms a colony with a rough surface. It is a virulent (nonpathogenic). Heat killed smooth cells are rendered a virulent. Injection of a virulent mutant R bacteria or heat killed virulent 8 bacteria into mice did not cause any harm. Injection of a mixture of the two bacterial types, however, killed some mice. From these animals the live virulent S bacteria could be recovered. A small fraction of the R type acquired the ability to produce capsules and became virulent.
This change induced in the R bacteria was called transformation. Griffith thought that the transforming agent was a protein. In 1944 A very, Macleod and McCarty showed that the substance responsible for transformation was DNA. They found that addition of DNA from capsule producing S cells to a culture of non- capsulated R cells transformed the latter, and enabled them to produce capsules. This transforming ability is removed by DNase, an enzyme which breaks down DNA. It was thus established that DNA was the most important factor in the transformation of genetic information.
In reciprocal transformation, demonstrated by Ephrussi and Taylor between 25 strains of pneumococcus, a part of the bacterial genome was replaced by the newly introduced DNA through recombination.Genetic transformation has been demonstrated in several other genera of bacteria including Haemophilu8, Bacillus, Salmonella, Streptococcus, Rhizobium and Neiaseria. It does not normally occur in E.coli. Certain mutant strains can, however, be transformed by treatment with calcium chloride, which partially removes the cell wall and makes the cell permeable to DNA. Genetic transformation has been reported in higher organisms, including the insects Drosophila, Ephestia and Bombyx, mice and human cells cultured in vitro.

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