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. This process of altering bacteria is known as transformation. Bacteria may also undergo artificial transformation through chemical and biological processes. The introduction of exogenous DNA into eukaryotic cells is known as transfection. 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. The creation of transgenic animals also leads into the study of altering sperm cells with exogenous DNA. 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. Applications 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. To successfully take up exogenous DNA, bacteria need to be in a state 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. These treatments typically involve making the targeted cell membrane more permeable towards accepting exogenous DNA, one such example being exposing the bacteria to a calcium ion solution, or a mixture of polyethylene glycol and dimethylsulfoxide. Another treatment method is the utilization of electricity (electroporation or electro transformation) to create holes in the cell membrane for the