It was the first example of genetic exchange in bacteria to have been discovered. This was first demonstrated in an experiment conducted by Griffith in 1928. The presence of a capsule around some strains of pneumococci gives the colonies a glistening, smooth (S) appearance while pneumococci lacking capsules have produce rough (R) colonies. Strains of pneumococci with a capsule (type I) are virulent and can kill a mouse whereas strains lacking it (type II) are harmless. Griffith found that mice died when they were injected with a mixture of live non capsulated (R, type II) strains and heat killed capsulated (S, type I) strains. Neither of these two when injected alone could kill the mice, only the mixture of two proved fatal. Live S strains with capsule were isolated from the blood of the animal suggesting that some factor from the dead S cells converted the R strains into S type. The factor that transformed the other strain was found to be DNA by Avery, McLeod and McCarty in 1944.
Transformation is gene transfer resulting from the uptake by a recipient cell of naked DNA from a donor cell. Certain bacteria (e.g. Bacillus, Haemophilus, Neisseria, Pneumococcus) can take up DNA from the environment and the DNA that is taken up can be incorporated into the recipient's chromosome.
The steps involved in transformation are:
1. A donor bacterium dies and is degraded.
2. A fragment of DNA (usually about 20 genes long) from the dead donor bacterium binds to DNA binding proteins on the cell wall of a competent, living recipient bacterium.
3. Nuclease enzymes then cut the bound DNA into fragments.
4. One strand is destroyed and the other penetrates the recipient bacterium.
3. The Rec A protein promotes genetic exchange (recombination) between a fragment of the donor's DNA and the recipient's DNA.
Some bacteria are able to take up DNA naturally. However, these bacteria only take up DNA a particular time in their growth cycle (log phase) when they produce a specific protein called a competence factor. Uptake of DNA by Gram positive and Gram negative bacteria differs. In Gram positive bacteria the DNA is taken up as a single stranded molecule and the complementary strand is made in the recipient. In contrast, Gram negative bacteria take up double stranded DNA.
Flash animation of conjugation in gram positive bacterium
Significance: Transformation occurs in nature and it can lead to increased virulence. In addition transformation is widely used in recombinant DNA technology.
For more information, visit www.microrao.com/micronotes/genetics.pdf
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