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How bacteria are modified to make a human protein

Biotechnology and genetic modification · Genetic modification · note 2 of 3

Spec 21.3.3
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
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How bacteria are modified to make a human proteinSpec 21.3.3

In short

Bacteria are genetically modified to make human insulin by cutting the human gene out of DNA with restriction enzymes, which leave sticky ends. A plasmid is cut with the same restriction enzymes, and DNA ligase joins the gene into it, forming a recombinant plasmid. This is inserted into bacteria, which multiply and express the gene, making the human protein.

Human insulin can be made by bacteria. The same process can be used for other human proteins.

  1. Isolate the gene. The DNA making up the human gene is cut out of human DNA using restriction enzymes. These cut the DNA leaving sticky ends (short single-stranded overhangs).
  2. Cut the plasmid. A bacterial plasmid is cut open using the same restriction enzymes, which forms complementary sticky ends.
  3. Insert the gene. The human gene is joined into the plasmid using the enzyme DNA ligase, which joins the sticky ends. This forms a recombinant plasmid.
  4. Put into bacteria. The recombinant plasmids are inserted into bacteria (specific details of how are not required).
  5. Multiply. The bacteria containing the recombinant plasmids are grown and multiply, so there are many copies of the gene.
  6. Expression. The human gene is expressed in the bacteria, which make the human protein.
The stages of genetic engineering: a human insulin gene is cut out, a bacterial plasmid is cut open, the gene is joined into the plasmid, the plasmid is put into a bacterium and the bacteria multiply and make insulin. (opens full size in a new tab)
A human gene is cut out, inserted into a plasmid and put into bacteria, which make the protein.
Common mistake:

Restriction enzymes cut DNA, and ligase joins DNA. Do not swap them. The same restriction enzyme is used on the gene and the plasmid so the sticky ends match.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is genetic engineering in agriculture?

Genetic engineering in agriculture means genetically modifying crop plants by inserting genes, often from another species. Examples are herbicide-resistant crops, which can be sprayed to kill weeds without being harmed, insect-resistant crops that need less insecticide, and crops with improved nutritional quality, such as golden rice, which contains a vitamin A precursor.

How is human insulin made by bacteria?

Supplement Human insulin is made by inserting the human insulin gene into bacteria. Restriction enzymes cut the gene out of human DNA, leaving sticky ends, and cut open a plasmid with matching sticky ends. DNA ligase joins the gene into the plasmid, making a recombinant plasmid. The bacteria multiply and express the gene, making insulin.

What do restriction enzymes and DNA ligase do?

Supplement Restriction enzymes cut DNA, leaving short single-stranded overhangs called sticky ends, while DNA ligase joins DNA. In genetic modification, the same restriction enzyme cuts out the human gene and cuts open the plasmid, so the sticky ends match. DNA ligase then joins the gene into the plasmid to form a recombinant plasmid.

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