Explain why a gene from a eukaryotic organism cannot be directly expressed in a bacterial host cell when the original genomic DNA is inserted, and describe how this problem is overcome in genetic engineering.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists working in a biotechnology company wish to produce human insulin using bacterial cells. They isolate the human insulin gene directly from a white blood cell and attempt to insert the genomic DNA sequence into a bacterial expression vector.
Model answer (4 marks)
Eukaryotic genes contain introns, which are non‑coding sequences that must be removed by splicing before translation. Bacteria lack the spliceosome machinery, so if the genomic DNA is inserted directly, the bacterial RNA polymerase will transcribe the entire gene, including introns, producing a transcript that cannot be translated into a functional protein.
To overcome this, the gene is first reverse‑transcribed into complementary DNA (cDNA) using reverse transcriptase. The cDNA contains only the exon (coding) sequences, so when it is inserted into a bacterial expression vector it can be correctly transcribed and translated into functional human insulin.
To overcome this, the gene is first reverse‑transcribed into complementary DNA (cDNA) using reverse transcriptase. The cDNA contains only the exon (coding) sequences, so when it is inserted into a bacterial expression vector it can be correctly transcribed and translated into functional human insulin.
Examiner tips
- Mention introns and lack of splicing in bacteria; explain why this blocks expression. Show that cDNA contains only exons and is suitable for bacterial expression. Use terms ‘introns’, ‘exons’, ‘spliceosome’, ‘reverse transcriptase’, ‘cDNA’.
Common mistakes
- Assuming bacterial cells can splice introns. Failing to state that cDNA contains only exons. Using ‘mRNA’ instead of ‘cDNA’ for the inserted sequence.
Mark scheme (4 marks)
- Eukaryotic genes contain introns (non-coding sequences) that bacteria cannot remove
- The bacterial cell would therefore produce a non-functional / incorrect protein
- mRNA is reverse transcribed into complementary DNA (cDNA) using reverse transcriptase
- The cDNA contains only the exon / coding sequences and can therefore be correctly expressed in bacteria
Key terms in this question
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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