# 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.

> OCR A-Level Biology A (H420) — 6.3 Manipulating genomes · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Eukaryotic genes contain introns (non-coding sequences) that bacteria cannot remove
2. The bacterial cell would therefore produce a non-functional / incorrect protein
3. mRNA is reverse transcribed into complementary DNA (cDNA) using reverse transcriptase
4. The cDNA contains only the exon / coding sequences and can therefore be correctly expressed in bacteria

## Key terms

- [genomic DNA](https://www.gradenine.co.uk/glossary/genomic-dna)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-gene-from-a-225aa0b2) · Published by Druglandscape Ltd.