Genetic Engineering Of Cotton For Insect Resistance

Essay, Research Paper

GENETIC ENGINEERING OF COTTON FOR INSECT RESISTANCE

The DNA code mostly contains instructions for protein synthesis. The code is read in groups of three nucleotides and each triplet of nucleotides codes for one of the twenty amino acids which link together in a polypeptide chain to form a protein. The code is universal, so the same code applies in nearly all living organisms. Some triplets have special functions and direct protein synthesis to start or stop. Protein synthesis occurs in ribosomes where a copy of the gene coding for a protein (mRNA) is translated to produce a protein. Some proteins may be consist of several polypeptide chains and the genes required to do this are collectively called a transcription unit.

Fig. 2 Diagram showing how genes code for proteins

Bacterium also contain small circular loops of DNA called plasmids which are not essential to the bacterium but can be useful in certain environmental conditions such as resistance to antibiotics. Because bacterium are prokaryotic and don’t have a nucleus plasmids are easy to obtain in pure form and can be introduced into other cells. Plasmids are also capable of independent self-replication, which makes them useful in multiplying useful DNA.

Bacteria also produce restriction enzymes, which can cut DNA at specific base sequences. Different restriction enzymes cut different base sequences and some make staggered cuts which leaves unpaired DNA (“sticky ends”) and other cut leaving no unpaired DNA (“blunt ends”).

Techniques used in genetically engineering cotton for insect resistance

The first step in inserting the Bt gene into the cotton plant is determining the Bt protein’s amino acid sequence. Using the principles of the genetic code it is possible to construct a complementary DNA sequence called and oligonucleotide using an automated DNA synthesiser.

This oligonucleotide can then be used as a DNA probe to isolate the DNA from the Bascillus thuringiensis. It is made radioactive and when inserted into the bacteria it hybridises (attaches to the complementary base pairing) with the DNA sequence that codes for the Bt protein. The DNA binding to the probe becomes radioactive so it can be detected by x-ray film.

Fig. 3 DNA probe production

The gene is then isolated from the bacterium by using restriction enzymes and multiplyed in the bacterium E. coli through gene cloning. The gene is first inserted into a plasmid from E. coli containing a gene coding for resistance to the antibiotics kanamycin and neomycin. The plasmid is cut with the same restriction enzyme as used to cut the Bascillus thuringiensis’ DNA. The restriction enzyme cuts both the DNA and the plasmid leaving sticky ends on the resulting fragments that enable the Bt gene to be incorporated into the plasmid. The complementary ends pair and the enzyme DNA ligase is used to join them together.

Fig. 4 Bt gene insertion into E. coli plasmid

The plasmid is then introduced into the E. coli cells by transformation. The E. coli cells that take-up the new plasmid then can be identified by their resistance to the antibiotics kanamycin and neomycin. The E. coli replicates the plasmids so that a single cell may contain hundreds of identical copies.

After the plasmids containing the Bt gene have been multiplied the Bt toxin gene is then isolated again and is inserted into a plasmid of the bacterium Agrobacterium tumafacien using the same techniques as used to insert the Bt gene into the E. coli. This plasmid is then put back in the Agrobacterium, which transfers the Bt gene into the cotton plant cell. The bacteria does this by infecting the plant cell causing a tumor to form and while infecting the plant part of the plasmid is transferred into the plant’s nucleus.

Fig. 5 Bt gene insertion into cotton plant cell

Biological implications of genetically engineering cotton for insect resistance

The transgenic cotton plant produced by this genetic technique has an altered genotype, which leads to it having an altered phenotype. The plant can then produce the Bt Toxin in its leaves through protein synthesis. This then crystallises and when an insect eats the protein it reacts in the insect’s gut and kills the insect within 24 hours.

This altered genotype and phenotype will increase the chances of survival of the cotton plants against the cotton budworm (Helicoverpa) and the native budworm (H. puntigera). The protein produced by the plant is only toxic to these pests and will only be activated in the gut of these pests. The gene shouldn’t transfer into other plants that are related to cotton or disturb natural ecological systems. It is possible, however, that the gene may enter a wild strain of cotton may and this would increase the survival chances of the cotton in the wild.

The genetic application will ultimately decrease the survival chances of the two types of budworm, but if they are continuously exposed to the toxin they may eventually develop resistance to the toxin. A mutation causing resistance to the toxin could occur in the budworm enabling it to survive the toxin. This mutant strain would breed successfully because it would have no other competition and can pass the gene to future generations. The Bt cotton would therefore have an indirect impact on the genotype of the cotton budworm

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