Transformation occurs naturally in some species of bacteria, and can also be done artificially. Bacteria that are capable of being transformed, whether naturally or artificially, are called competent.
Transformation is one of three processes by which outside genetic material may be got into bacterial cells. The other two are conjugation (transfer of genetic material between two bacterial cells in direct contact), and transduction (injection of foreign DNA by a bacteriophage into the host).
Transformation may also used to describe the insertion of new genetic material into nonbacterial cells, such as animal and plant cells. Introduction of foreign DNA into eukaryote cells is usually called "transfection".
Transformation was first demonstrated in 1928 by British bacteriologist Frederick Griffith. Griffith discovered that a harmless strain of Streptococcus pneumoniae could be made virulent after being exposed to heat-killed virulent strains.
Griffith thought some "transforming principle" from the heat-killed strain was responsible for making the harmless strain virulent. In 1944 this transforming principle was identified as being genetic by Oswald Avery, Colin MacLeod, and Maclyn McCarty. They isolated DNA from a virulent strain of S. pneumoniae and using just this DNA were able to make a harmless strain virulent. They called this uptake and incorporation of DNA by bacteria 'transformation'. See Avery–MacLeod–McCarty experiment.
The results of these experiments were at first sceptically received by the scientific community. Not until the discovery of other methods of genetic transfer (conjugation in 1947 and transduction in 1953) by Joshua Lederberg were Avery's experiments accepted. Transformation did not become routine procedure in laboratories until 1972 when Cohen successfully transformed Escherichia coli by treating the bacteria with calcium chloride. This created an efficient and convenient procedure for transforming bacteria and opened the way for biotechnology and research.
In 1907 a bacterium that caused plant tumors, Agrobacterium tumefaciens, was discovered and in the early 1970s the tumor inducing agent was found to be a DNA plasmid called the Ti plasmid. By removing the genes in the plasmid which caused the tumour and adding in new genes researchers were able to infect plants with A. tumefaciens and let the bacteria insert their chosen DNA into the genomes of the plants.
Not all plant cells are susceptible to infection by A. tumefaciens so other methods were developed including electroporation and micro-injection. Particle bombardment was made possible with the invention of the Biolistic Particle Delivery System (gene gun) by John Sanford in 1990.
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