The Nobel Prize in Physiology or Medicine in 1923 was jointly awarded to Fredrick Grant Banting and John James Macleod "for the discovery of insulin".

In 1921, Fredrick Banting extracted some insulin from a dog's pancreas and injected it into another diabetic dog (with its pancreas removed) and successfully kept the dog alive for 70 days. John Macleod subsequently obtained and purified insulin from the pancreases of cattle, which was then used in an attempt to inject a diabetic human boy named Leonard Thompson.

For many years, the large scale production of insulin involved obtaining and purifying insulin from cattle and pig pancreases. At the time, it took about 3 500 kg of pancreas from 23 500 animals to make only 0.5 kg of insulin. Another main issues with using animal insulin was the allergic reactions in the human patients.

It was only subsequently in 1978 that the human gene was able to be successfully expressed in transgenic E. coli bacteria with the use of recombinant plasmids by Genetech. Genetech also worked on improving the yield of insulin for larger scale production in fermenters. FDA approval for the use of the bacterial production of insulin happened very quickly in 1982.

1. The human insulin gene is isolated from the human chromosome.

• The ends of the gene are cut by a restriction enzyme to generate sticky ends.

2. A bacterial plasmid is isolated from a bacterial plasmid donor.

   • Plasmids are vectors for cloning (making copies) of the insulin gene.

   • The plasmid is cut by the same restriction enzyme, to generate the same sticky ends on the plasmid

3. Thus the sticky ends on the human insulin gene and the bacterial plasmid are complementary to each other, and can bind to each other.

4. The DNA backbone between the bacteria plasmid and insulin gene is repaired using DNA ligase.

   • Recombinant plasmid (plasmid containing human insulin gene) is created

5. The recombinant plasmid is inserted into the bacterial cell through the process of transformation.

   • Bacteria are either heat shocked or applied with an electrical current to make the cell walls and membranes permeable so that the recombinant DNA plasmid can enter the bacterial cell

6. Bacterial cells are first grown on a petri dishes, where bacterial cells are screened.

   • Plasmids used as vectors also contain an antibiotic resistance gene within itself.

   • Successfully transformed bacterial cells can be selected for when they are grown on an agar medium containing that specific antibiotic.

   • Bacterial cells that did not take up the recombinant plasmid will not survive.

7. Successfully transformed bacteria are cultured and multiply in a fermenter; large numbers of transgenic bacteria are cultured.

8. Recombinant DNA plasmid inside the bacterial cells is transcribed into human insulin mRNA, which is then translated by the bacterial ribosomes to synthesise human insulin (protein).

9. Insulin made by the bacterial can be isolated and purified by crystallisation.