A pharmaceutical company uses recombinant DNA technology to produce human insulin in bacteria. Explain how a gene encoding human insulin is inserted into a bacterial plasmid so that the insulin protein can be expressed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Recombinant DNA technology allows human genes to be expressed in bacterial host cells. The process involves several molecular tools to cut and join DNA sequences, and requires careful consideration of regulatory sequences to ensure successful protein production.
Model answer (4 marks)
The human insulin gene is first excised from human DNA (or synthesised from mRNA by reverse transcriptase) using a restriction endonuclease that generates sticky ends.
The bacterial plasmid is cut with the same restriction enzyme, giving complementary sticky ends.
The insulin gene and plasmid are joined by DNA ligase, forming a recombinant plasmid.
A bacterial promoter (e.g. lac or T7) is placed upstream of the insulin gene so that bacterial RNA polymerase can transcribe it, allowing protein production.
The bacterial plasmid is cut with the same restriction enzyme, giving complementary sticky ends.
The insulin gene and plasmid are joined by DNA ligase, forming a recombinant plasmid.
A bacterial promoter (e.g. lac or T7) is placed upstream of the insulin gene so that bacterial RNA polymerase can transcribe it, allowing protein production.
Examiner tips
- Use the word ‘cut’ and ‘sticky ends’ for the restriction step; mention ligase for joining; state promoter is required for transcription; keep each point brief to match 4 marks.
Common mistakes
- Confusing restriction enzymes with ligase; forgetting to mention the promoter; writing the insulin gene in the wrong orientation; using eukaryotic promoter instead of bacterial.
Mark scheme (4 marks)
- The insulin gene is cut from human DNA (or produced from mRNA via reverse transcriptase) using restriction endonucleases that create sticky ends
- The plasmid is cut with the same restriction endonuclease, producing complementary sticky ends
- The insulin gene and plasmid are joined by DNA ligase, forming recombinant plasmid
- A promoter sequence must be present upstream of the insulin gene so that bacterial RNA polymerase can transcribe it
Key terms in this question
recombinant DNA technology · plasmid · expressed
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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