Plant Biotech Blog

Natural variation in gene expression: The tendency to vary is the only consistent feature of Nature. All species organisms, whether wild or cultivated, show naturally inherent variation in physical, chemical and physiological features, which is also the basis for distinguishing different species, varieties. Each species or variety shows some variation in several features both between and within its populations. Nevertheless, species and varieties have a set of discernible and invariable features characterizing their identity. For example all transgenic Bt cotton varieties contain some quantity of Bt toxin, though the actual quantities of the toxin may vary from one variety to the other, as well as within each variety.

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Genes and gene expression: Genes determine the characteristics and life processes of organisms. Each species contains several thousand genes. A gene, the basic unit of inheritance and diversity, is a segment of DNA containing a specific sequence of nucleotides (the building blocks of DNA). Most genes occur in one of three combinations of two variant forms (for example, AA, Aa or aa). The genotype of an organism is the specific and characteristic combination of variants of all genes the organism carries.

A gene expresses through the synthesis of a protein or an enzyme (most proteins are enzymes), which is the means of functioning of a gene. Gene expression varies with the nucleotide sequence of the gene, its promotor, and the point of insertion of the gene in the DNA of the transgenic variety, the internal cell environment, as well as several external factors in the environment.

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Specific Bt toxin encoding genes were isolated from Bacillus thuringiensis and incorporated into the genetic complements of several crop plants such as cotton, corn, rice, tomato, potato, soybean, and others, to develop transgenic Bt varieties, using complex yet elegant procedures of genetic engineering. This results in a crop variety with a single systemic insecticide that kills specific caterpillars feeding on the respective crop. For each crop the most damaging pest has been targeted, as for example, the bollworms of cotton, and the stem borers of rice and corn. The objective is that, while the Bt toxins take care of the major pests, the rest can be controlled by conventional practices.

The choice of Bt genes depends upon the crop and the targeted pest, as most of the Bt toxins are insect group specific. For example, the proteins encoded by the genes Cry1Ac and Cry2Ab control the cotton bollworms, Cry1Ab controls corn borer, Cry3Ab controls Colarado potato beetle and Cry3Bb controls corn rootworm.

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