IGCSE Biology / Genetic modification
Insulin from bacteria revision notes
Edexcel International GCSE Biology (4BI1), chapter 22: Genetic modification, specification point 5.14. Free lesson.
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Genetically modified bacteria can produce human insulin by using the human insulin gene. This provides a reliable supply of insulin that is identical to human insulin and avoids some problems with insulin from animals.
How the insulin gene is joined to a plasmid
A plasmid is a small circular piece of DNA found in a bacterium. Matching sticky ends allow the human insulin gene to fit into a plasmid, and DNA ligase joins the pieces to make a recombinant plasmid.
How bacteria make and supply insulin
The recombinant plasmid is put into a bacterium, usually E. coli. The bacterium reads the human gene and makes insulin. Growing the bacteria in a fermenter allows them to multiply quickly, producing a large amount of insulin that can then be collected and purified. Bacteria transcribe and translate the insulin gene to produce human insulin. The insulin is extracted from the bacteria and purified.
How modified bacteria are identified
Only a small fraction of bacteria take up the plasmid. A marker gene, often one that gives resistance to an antibiotic, helps identify the modified bacteria. Growing the culture on that antibiotic leaves the bacteria with the marker alive, while those without it do not survive. The marker gene shows which bacteria have taken up the plasmid.
Why human genes work in bacteria
The genetic code is universal: the same triplet of bases codes for the same amino acid in every organism. So a bacterial ribosome can read a human insulin gene and build the same sequence of amino acids, producing the same protein.
Advantages over insulin from animals
Bacterial insulin is made from the human insulin gene, so it is identical to human insulin. Animal insulin is slightly different in structure. This makes bacterial insulin far less likely to cause an allergic reaction, and bacteria can produce large amounts quickly. Bacterial insulin is cheaper once the process is set up and its supply is not limited by the number of animals slaughtered. Using bacteria avoids the religious or ethical objections some people have to insulin from pigs or cattle. Animal insulin is more likely to cause an allergic reaction and is more limited in quantity.