Where This Chapter Goes Wrong
This chapter reads like a list of success stories, which is exactly why it costs marks: almost every error comes from letting two neighbouring ideas collapse into one.
Two pest strategies, two different logics
Bt and RNAi both protect a crop, and they get treated as one trick. They are not. Bt means the plant has been given a gene from Bacillus thuringiensis so that it manufactures a toxin protein which kills the insect that feeds on it. RNAi puts no toxin anywhere. It introduces a construct producing sense and antisense RNA; these two, being complementary, form a double-stranded RNA matching the parasite's own mRNA and silence it, so the parasite cannot survive in the transgenic host. The tobacco root parasite Meloidogyne incognita belongs to that second story and stays there — a nematode is not an insect and does not answer to a cry gene. RNAi is also described as a cellular defence in eukaryotic organisms generally, not as something peculiar to animals.
Inside the Bt story the commonest reversal is the protoxin. The crystal protein is inactive while it sits inside the bacterium — which is precisely why the bacterium is not killed by what it makes — and is converted into the active toxin in the insect's gut, where the alkaline pH solubilises the crystals. That conversion is regularly run backwards, and the pH regularly swapped. The second Bt subtlety is specificity. The toxins are coded by cry genes, and different cry genes act on different insect groups; the choice depends on the crop and the targeted pest. A plant carrying one pair of cry genes is protected against the pests those genes cover and against nothing else; reading "Bt" as blanket immunity is the mistake.
The insulin discriminator
Insulin is two short chains, A and B, linked by disulphide bridges. In the body it is first made as a pro-hormone carrying an extra stretch called the C peptide, and that C peptide is removed as the pro-hormone matures — present before maturation, absent from mature insulin. No single relationship in this chapter is inverted more often. The recombinant route sidesteps the maturation problem: two DNA sequences were introduced into E. coli plasmids, the A and B chains were produced separately, and only then were they combined by creating disulphide bonds.
How far a therapy actually reaches
Gene therapy is a correction, and a correction is not automatically a cure. When the cells carrying the introduced gene are the patient's own lymphocytes, they are not immortal, so the engineered lymphocytes must be infused periodically. A permanent outcome is spoken of only for a gene taken into marrow cells that produce the enzyme, or introduced at an early embryonic stage. Enzyme replacement and marrow transplantation are likewise described as not completely curative.
Diagnosis carries a parallel confusion about timing. ELISA rests on antigen-antibody interaction: it detects the pathogen's antigens, or the antibodies raised against it — and antibodies take time to appear. PCR amplifies the pathogen's nucleic acid, so it can register very low concentrations, before symptoms are visible. Both count as early detection compared with serum and urine analysis, the conventional route, which is late — by the time it reads positive the pathogen concentration is already very high.
Purposes, and who polices them
Transgenic animals are made for five separate reasons: studying normal physiology and development, serving as models of human disease, producing biological products, testing vaccine safety, and chemical safety (toxicity) testing. Two can sit in the same animal, yet they remain distinct purposes. Keep the two governance terms apart as well. GEAC decides on the validity of GM research and the safety of introducing GM organisms for public services; biopiracy is the use of bio-resources without proper authorisation and without compensatory payment. One is regulation, the other is what regulation exists to prevent.