Exogenous DNA is DNA originating outside the organism of concern or study. The introduction of exogenous DNA into a cell is called transformation . This can take place naturally or artificially. Methods of artificial transfection include chemical methods, including calcium phosphateprecipitation, DEAE-dextran complexation and lipid-mediated DNA transfer; physical methods, including electroporation, microinjection, and biolistic particle delivery ; and using recombinant, lab manipulated viruses as vectors. Transformation is a naturally occurring process in bacteria. Bacteria need to be in a certain physiological state to successfully take up exogenous DNA, which is described as one of competence. Some bacteria are naturally competent, but usually only for a brief time at a certain stage of their growth cycle. Bacteria can also be made competent through a variety of chemical treatments including exposure to calcium ions, or a mixture of polyethylene glycol and dimethylsulfoxide, which make the cell membrane more permeable, leading to the uptake of the exogenous DNA. Another treatment method is the utilisation of electricity as the membrane-permeabilizing agent. Finally, liposome-mediated transformation can be used. In this method DNA is coated with lipid. Fusion of this lipid and the membrane lipid can occur, facilitating the entry of DNA. Transformation of bacteria, plant cells and animal cells has important research and commercial functions. Targeted introduction of exogenous DNA is used to identify genes because the introduced DNA can act cause a mutation or altered expression of the gene into which it inserts. This technology, known as insertion mutagenesis, often employs retroviruses as the vectors of DNA delivery. Such insertion mutagenesis has been often used to identify many oncogenes in specific locations in tumor cells. The use of exogenous DNA to transform cells has spawned the discipline of transgenesis: the use of recombinant DNA techniques to introduce new characters into organisms which were not present previously. The range of organisms that have been created through transgenesis range from bacteria to mammals, including sheep and monkeys, and they have a variety of uses. These include the study of developmental genetics, disease processes and gene regulation. Transgenic farm animals can produce human pharmaceuticals, and increased milk or meat production. Tissues and organs from transgenic animals can be used in transfusions and transplants with a lesser chance of immune rejection.
