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Transgenic Animals

 

How Transgenic Animals are Generated

Copyright Molecular Station 2012

Transgenic Plants

In nature, plant cells often live in close association with cerain bacteria, which may provide a convenient vehicle for introducing cloned DNA into plants.  Agrobacterium tumefaciens, for example, attaches to the cells of dicotledonous plants and causes the formation of plant tumours known as galls.  This bacterium introduces a circular DNA molecule called the Ti (tumour-inducing) plasmid into the plant cell in a manner similar to bacterial conjugation.  The plasmid DNA then recombines with the plant DNA.  Since the Ti plasmid has been isolated, new genes can be inserted into it using recombinant DNA techniques and the Ti genes causing tumours can be disrupted.  The resulting recombinant plasmid can then transfer desired genes into plant cells.
An especially useful characteristic of plants for transgenic studies is the ability of cultured plant cells to give rise to mature plants.  meristemativ (growing) cells from dissected plant tissue or cells within excised parts of a plant will grow in culture to form callus tissue, an undifferntiated lump of cells.  Under the influence of plant growth hormones, different plant parts (roots, stems, and leaves) develop from the callus and eventually grow into whole fertile plants.  When an agrobacterium containing a recombinant Ti plasmid infects a cultured plant cell, the newly incorporated foreign gene is carried into the plant genome.  A. tumefaciens readily infects dicots (petunia, tobacco, carrot) but not monocots; reliable techniques for introducing genes into monocots are still being developed.  Direct introduction of DNA by electroporation has been successful in rice plants, (which are monocots), and the future looks bright for the manipulation of other commercially important monocotyledonous crop plants.  Also available for gene transfer experiments are cells of a tiny, rapidly growing memberof the mustard family called Arabidopsis thaliana.  This plant appears to be well suited to geentic analysis of a variety of developmental and physiological processes.  It takes up little space, is easy to grow, and has a small genome, and genes defined by mutations can be cloned by positional cloning strategies. 

Transgenic Fruit Flies

Foreign DNA can be incorporated into the Drosophila germ-line genome by the technique of P-element transformation.  This technique makes use of a segment of the P element, a highly mobile DNA element, which can transpose from an extrachromosomal element into a chromosome.  Generally, this procedure results in incorporation of a single copy of the transgene into the Drosophila genome.  In contrast, transgenic mice carry multiple copies of the transgene incorporated into their chromosomes.  In both organisms, however, the chromosomal insertion site is highly variable.
Flies that develop from injected embryos will carry some germ cells that have incorporated the transgene; some of their progeny will carry the transgene in all somatic and germ-line cells, giving rise to pure transgenic lines.  Individuals carrying the transgene are recognized by expression of a marker gene (e.g., one affecting eye color) that is also present on the donor DNA.  Although the transgenes in Drosophila and mice insert in chromosomal sites different from the position of the corresponding endogeneous gene, they usually are expressed in the right tissue and right time during development.

Copyright Molecular Station 2012

 

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