What are transgenic organisms? Genetic engineering often involves the moving of DNA from one organism to another. This has been possible because the genetic code is universal. The result is a transgenic organism - one that contains DNA from a different type of organism. There are many reasons for doing this:
Making bacterial factories for human proteins If a human gene - like the one that makes insulin - can be inserted into a bacterium, the bacterium could make lots of protein product (insulin). They would act like factories, producing huge amounts of pure pharmaceutical products like insulin. 
Diabetics cannot make their own insulin because they have a defective gene, so they have to be given daily injections of insulin that was extracted from animals like pigs. There is always the risk of such extractions from living animals carrying other pathogens - HIV has certainly been passed through blood transfusions. So, genetically engineered bacterial factories - often called biofactories or biofermentation - can make pure protein product in large amounts. One problem with making such bacterial factories is that bacteria have different mechanisms for switching genes on and off, compared to eukaryotes. So, if a human gene is placed inside a bacterium it has to be put near (just after) a promoter and operator. Then the gene can be switched on by adding the appropriate inducer chemical. 
Another problem is that bacteria have no way of removing introns from the RNA that is transcribed from DNA. So, genes are usually got from mRNA, which is copied as cDNA by reverse transcriptase. This means they have no introns. Some genes do not work well in bacteria and need the cellular apparatus of eukaryotes. Often this is because the protein needs to be modified after synthesis. Fortunately, there is a harmless and easily grown eukaryote cell called yeast. Yeast is a single-celled fungus that has plasmids (very rare in higher cells) so it is used in such cases. In fact, insulin is now produced using yeast cells. Similar methods of using bacteria or yeast have produced human growth hormone, factor VIII for blood clotting, erythropoetin hormone (causes the body to make red blood cells), interferon (aids the immune response to some cancers), antibiotics and chymosin (replaces renin to clot milk in the dairy industry). Vaccines stimulate our immune system so that it will respond faster if it encounters a particular pathogen again. We say it is developing immunity. Live vaccines are more effective in producing immunity, but they can have side effects. The genes that produce proteins on the surface of pathogens have been isolated and engineered into bacteria or yeast. The bacteria produce the coat proteins and they work as a vaccine, without side effects. Immunity to hepatitis B, whooping cough, tetanus and diphtheria have been developed in this way. Click here to find out about transgenic plants |
