User:BattIe5tar/Exogenous DNA

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Exogenous DNA is DNA originating outside the organism of concern or study. Exogenous DNA can be found naturally in the form of partially degraded fragments left over from dead cells. These DNA fragments may then become integrated into the chromosomes of nearby bacterial cells to undergo mutagenesis. Exogenous DNA can also be artificially inserted into the genome, which revolutionized the process of genetic modification in animals. By microinjecting an artificial transgene into the nucleus of an animal embryo, the exogenous DNA is allowed to merge the cell's existing DNA to create a genetically modified, transgenic animal.

History
In 1928, bacteriologist Fredrick Griffith observed exogenous DNA alongside bacterial transformation in the species Streptococcus pneumoniae. In further tests, physician Oswald Avery was able to isolate and confirm that the DNA used in the experiment originated from outside the cell and integrated itself into the cell's genome. Repeated experiments proved exogenous DNA integration was possible in other species of bacteria, prompting studies to extend to mammal cells. The technology for the injection of exogenous DNA into organisms was discovered by Lin in 1966. He was able to use a fine glass needle to insert laboratory-produced DNA into mouse zygotes without breaking their nuclei. In 1976, the first successful delivery of exogenous DNA into mice was performed by Jaenisch using the Moloney leukemia virus.

Transformation
The integration of exogenous DNA with the genome of a cell is called transformation (transfection in animal cells). 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 utilization of electricity as the membrane-permeabilizing agent (electroporation or electro transformation). 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.

Methods of artificial transfection include:

(a) chemical methods, including calcium phosphate precipitation, DEAE-dextran complexation and lipid-mediated DNA transfer;

(b) physical methods, including electroporation, microinjection, and biolistic particle delivery (gene gun);

(c) and using recombinant, lab manipulated viruses as vectors to alter embryos and sperm cells.

Transgenesis
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 (a method named pharming), 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.

Sperm Cells
Using transgenesis to genetically modify animals has spawned a new division of using exogenous DNA to modify sperm cells. Epididymal sperm cells were shown to react to exogenous nucleic acids, allowing for DNA to reversibly bind to the spermatozoa through ionic interactions. The ability of sperm cells to locate and internalize exogenous DNA was then used to transfer foreign genes into an oocyte during fertilization to create transgenic animals. However, a low efficiency rate hinders this technique due to the low uptake of exogenous DNA by sperm cells compounded with the low fertilization rate of the oocyte.

mitochondira genetic manipulation

By successfully altering mouse embryos with foreign viruses, this research would lead towards using exogenous DNA to alter sperm cells.