Genetic modification involves these steps:
Type 1 diabetes can be controlled by injecting insulin. The extra insulin allows the glucose to be taken up by the liver and other tissues, which results in cells receiving the glucose they need, and blood glucose levels stay normal.
Previously, insulin was extracted from the pancreas of pigs or cattle, but these do not have an identical composition to human insulin. Bacterial cells have been genetically modified - by inserting the human gene for insulin production into their plasmids so that the bacteria produce human insulin.
This diagram shows how the genetic modification of the bacterium works:
Here the human gene for insulin is inserted into a plasmid. This is a small closed circle of DNA present naturally in bacteria. When the bacterium divides, all the bacteria will carry the plasmid, and produce human insulin.
Sheep and goats have been genetically modified to produce chemicals in their milk that can be used to treat disease. In one example the milk produced contains a protein needed to treat patients with cystic fibrosis.
Research is also exploring the possibility of providing tissues needed for transplants from animals that have been genetically modified so that the tissues are not rejected by the human immune system.
There is no doubt that the genetic engineering of bacteria and yeast to produce insulin has revolutionised the treatment of insulin. Because it is 'human' insulin, it does not have the potential side effects of injecting pig or cattle insulin.
But many people do have concerns about inserting human genes into other organisms. The organism involved is Escherichia coli - E. coli for short - whose not genetically modified form is found in huge numbers in the human large intestine.