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e diel, 10 qershor 2007

Genetic engineering using bacterium species

Genetic engineering differs from conventional plant breeding. In conventional plant breeding half of the genes of an individual come from each parent, whereas in genetic engineering one or a few specially selected genes are added to the plant genome.

Moreover, conventional plant breeding can only combine closely related plants.
Genetic engineering permits the transfer of genes between organisms that are not normally able to cross breed because they are not genetically compatible. The transferred genes are called transgenes. They can come from another plant species, or even from a completely different organism (e.g., bacterial genes). These transgenes are then replicated and inherited in the same way as natural plant genes.

When the desired trait is found in an organism that is not sexually
compatible with the host, it may be transferred using genetic engineering.

There are 2 ways of genetic engineering, i.e. biologically, and physically.

Biologically, in plants, the most common method for genetic engineering uses the soil bacterium Agrobacterium tumefasciens as a vector. Researchers insert the desired gene or genes into the bacterium and then infect the host plant. The desired genes are transmitted to the host along with the infection. This method is used mainly with species such as tomato and potato.

In the most common transformation technique for these crops, physical means are used. The desired gene is coated on gold or tungsten particles and a “gene gun” is used literally to shoot the gene into the host at high velocity. Once the DNA reaches the cell nucleus, it inserts itself at random into one of the host chromosomes and can express the desired character. The genetically modified plant is then grown from the transformed cell.

Overview of how transgenic crops are created:

When the
bacterium infects the plant, it penetrates the plants cells and transfers its modified DNA to the plant.

Three distinctive types of genetically modified crops exist:
(a) “distant transfer”, in which
genes are transferred between organisms of different kingdoms (e.g. bacteria into plants);
(b) “close transfer”, in which genes are transferred from one
species to another of the same kingdom (e.g. from one plant to another); and
(c) “tweaking”, in which genes already present in the organism's
genome are manipulated to change the level or pattern of expression.

Once the
gene has been transferred, the crop must be tested to ensure that the gene is expressed properly and is stable over several generations of breeding.

A number of economically valuable characteristics have been introduced into plants by
genetic engineering. Most of the genetically modified crop plants used so far have transgenes that provide resistance to herbicides or insects. To improve crop production and soil management, research is now exploring how to increase the variety of transgenic characteristics to include resistance to drought, heat, cold, acid soils, and heavy metals.

Transgenic plants can provide food with enhanced nutritional content. For example, genetically modified “Golden Rice” contains two daffodil genes and one bacterial gene that together result in elevated levels of provitamin A.

These techniques could be applied to improve many characteristics in other crop species.

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