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 proteinsBacterium 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 resistanceThe 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 t...
