Transformation occurs when a cell takes up genetic material, usually exogenous DNA, from its surroundings and undergos genetic alteration. It can occur naturally or be induced artificially. The most common known incidences of transformation occurs in bacteria, such as Escherichia coli. The bacteria needs to have competence before transformation can occur.
Bacterial transformation was first discovered by Frederick Griffith, a British biologist, in 1928. He experimented with a rough strain and a smooth strain of Streptococcus pneumoniae.The rough strain of the bacteria could be killed by the immune system of a mouse. The smooth strain had a outer coat which prevented it from being killed by the immune system, it could infect and kill mice. He discovered that when he injected mice with a rough strain of S. pneumoniae and a heat-killed smooth strain of S. pneumoniae. This "transforming principle" was later discovered by Oswald Avery, Colin MacLeod, and Maclyn McCarty in 1944. They used the enzymes protease, RNase, or DNase on the rough and heat-killed smooth strains before injecting the strains into three saparate groups of mice. They discovered that the the strains that had protease or RNase used on them still killed the mice, but the strains that had DNase used on it did not infect the mice. They concluded that DNA was the cause of bacteria transformation.
Methods of Transformation
Bacteria cells can be transformed using calcium chloride transformation, where cells are exposed to Ca+ (in a CaCl2) and chilled to make the cell permeable to the exogenous DNA. This is because both the DNA and the plasma membrane of the cell are slightly negatively charged, and tend to repel one another. This process allows the plasma membrane to temporarily lose its negative charge. The cells are then incubated with DNA in ice, before being heat-shocked (42°C for 30-120 seconds).
This method of transformtaion is usually used in Recombinant DNA technology to insert the recombinant DNA into a host cell. For instance, transforming host bacteria cells with recombinant circular plasmids. The efficiency of transformation using the calcium chloride method decreases with the size of plasmids. 
- ↑ Hartl, D.L. &amp; Ruvolo, M., 2012. Genetics: Analysis of Genes and Genomes. 8th ed. Jones &amp; Bartlett Learning.
- ↑ Inoue, H.; Nojima, H.; Okayama, H. (1990). "High efficiency transformation of Escherichia coli with plasmids". Gene 96 (1): 23–28
- ↑ Donahue RA, Bloom FR (September 1998). "Transformation efficiency of E. coli electroporated with large plasmid DNA" (PDF). Focus 20 (3): 77–78.