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Transformation Mechanism in Gene Recombination |
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Transformation
Mechanism
in
Gene
Recombination
Transformation was discovered by an English bacteriologist, Frederick Griffith in 1928, who made a series of experiments with laboratory mice and two types of pneumonia causing bacterium, Diplococcus pneumoniae. This bacterium has two types of strains. One type has smooth (S), capsulated cells, whereas another type has rough (R) noncapsulated cells. The disease is caused by smooth type of cells only i.e. smooth type cells are pathogenic (virulent) whereas rough type cells harmless or nonpathogenic (avirulent).
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The experiments conducted by him are illustrated. As shown in the figure, when live, harmless (rough type) cells were injected in the body of mice the animal remained healthy. The injection of dead, pathogenic (smooth type) cells into the body of mice also did not cause any disease. In a classic experiment, Griffith mixed live, harmless (rough type) cells with the dead remains of pathogenic (smooth type) cells, and then injected the mixture into the laboratory mice.
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The live cocci taken in the mixture were uncapsulated and formed rough colonies (R) on agar. The dead cocci taken in the mixture originally had a capsule and were taken from smooth (S) colonies on agar. To Griffith's surprise, the mice developed pneumonia and died. On autopsy (examination of tissue of dead animal), he isolated live, capsulated cells that formed smooth colonies on agar.
Apparently the live, harmless rough cocci had been transformed in the mice into live, pathogenic, smooth, cocci. A rough to smooth conversion (RS) had been accomplished, Five years later, James L. Alloway of Rockefeller Institute confirmed Griffith's work using fragments from the dead smooth type cells to transform the rough type cells.
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In 1944, Oswald T. A very, Colin M. MacLeod, and Maclyn N. McCarty, also of Rockefeller Institute found that deoxyribose nucleic acid (DNA) isolated from the fragments could induce the transformation.
At that time, DNA was an obscure chemical with little significance. The work of A very MacLeod and McCarty helped bring it to the force. Their experiments were the first proof that in living organism genetic matter is DNA. The possible mechanism of transformation. Though it takes place in less than 1 % of a population, transformation is an important method of recombination in bacteria.
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A number of donor cells break apart and an explosive release and fragmentation of DNA follows. A segment of double stranded DNA containing about 10-20 genes then passes through the cell wall and membrane of a recipient cell. Only a few competent recipient cells can take up the DNA. After entry into cell, an enzyme dissolves one strand of DNA leaving the second strand to be incorporated.
This strand then displaces a segment from a strand of the recipient's DNA. The displaced DNA is dissolved by another enzyme in the cell. The cell is now transformed. It will display its own traits as well as those coded by the new DNA.
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Transformation may also take place by the incorporation of plasmids to competent cells. In this case, no DNA is displaced. Rather, the plasmid adds genes to those already in the cell and multiplies along with the cell.
Since the 1940's, transformation has also been demonstrated in species of Neisseria, Bacillus, Haemophilus, Azotohacter and Streptococcus. The process involves the transfer of DNA from the fragments of donor cells into the cytoplasm of a live recipient cell. Sections of single stranded or double stranded DNA may be taken up but only a single strand will align with the bacterial chromosome and becomes incorporated into it.
Transformations in bacteria have been observed in the ability to form a capsule, a drug resistance and pathogenicity, and in nutritional patterns. Transformations are not common, however, because the large fragments of DNA molecule can not pass through the recipient's cell wall or membrance. In nature, transformation may increase the pathogenicity of an organism.
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