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(recombinant-DNA technology) The isolation of useful genes from a donor organism or tissue and their incorporation into an organism that does not normally possess them. For example, genes from a donor may be incorporated into the DNA of a microorganism, which, by its own replication, will produce many copies of the foreign gene and hence substantial quantities of the gene's products. These products, which may be enzymes, hormones, antibodies, etc., may have commercial or medical value. Genetically engineered microorganisms are already proving commercially successful, e.g. in the production of human insulin. However the manipulation of eukaryotic organisms is considerably more difficult. A possible method of introducing foreign genes into higher plant cells involves using the tumour-inducing plasmid that is associated with the bacterium Agrobacterium tumefaciens, the causative agent of crown gall disease. Once introduced into a plant cell by the bacterium part of this plasmid becomes incorporated into the plant genome and disrupts the control of cell division. Often cell differentiation is also disrupted leading to the development of disorganized tumorous tissue. However certain strains of A. tumefaciens contain plas-rnids that do not cause disorganized growth. If new DNA sequences are incorporated into such plasmids and the plasmid introduced into a culture containing totipotent plant cells then a genetically transformed plant could be obtained. All dicotyledons are susceptible to A. tumefaciens but monocotyledons are not naturally infected. Other natural vectors that have been tried in plant genetic engineering are the caulimoviruses, which include the cauliflower mosaic virus. There are considerable difficulties to be overcome in other methods not employing natural vectors because, if the introduced genes do not also cause an identifiable disease, there is the added problem of introducing mutant marker genes so transformed tissues may be distinguished.
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