the mRNA has been translated into an amino acid sequence which link together to form specific proteins such as insulin. The Vector (Gram negative E. coli). A weakened strain of the common bacterium, Escherrichia coli (E. coli) (see fig. 10), an inhabitant of the human digestive tract, is the 'factory' used in the genetic engineering of insulin. When the bacterium reproduces, the insulin gene is replicated along with the plasmid, (8) a circular section of DNA (see fig. 11). E. coli produces enzymes that rapidly degrade foreign proteins such as insulin. By using mutant strains that lack these enzymes, the problem is avoided. (9) This makes it possible to sever certain nitrogen base pairs and remove the section of insulin coding DNA from one organism's chromosome so that it can manufacture insulin (See fig. 13). DNA ligase is an enzyme which serves as a genetic glue, welding the sticky ends of exposed nucleotides together. The required DNA sequence can be determined because the amino acid compositions of both chains have been charted. Sixty three nucleotides are required for synthesising the A chain and ninety for the B chain, plus a codon at the end of each chain,signalling the termination of protein synthesis. An anti-codon, incorporating the amino acid, methionine, is then placed at the beginning of each chain which allows the removal of the insulin protein from the bacterial cell's amino acids. The synthetic A and B chain 'genes' (see fig. 15) are then separately inserted into the gene for