A gene is a length of DNA that codes for the production of a specific protein. Explain why a cell in the lining of the stomach, which secretes digestive enzymes, contains genes that code for proteins found only in nerve cells, even though those proteins are never made in the stomach cell.

Cambridge International IGCSE Biology (0610) — 17.1 Chromosomes, genes and proteins · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

All body cells contain the same set of genes / the same DNA
Genes can be switched on or switched off in different cell types
In the stomach cell, the genes coding for nerve‑cell proteins are switched off / not expressed
Only the genes required for the stomach cell's function are switched on, so only those proteins (e.g. digestive enzymes) are synthesised

Examiner tips

  • Use the phrase "genes can be switched on or off" to show understanding of gene expression
  • Mention that the same DNA is present in all cells, but only a subset is expressed in each cell type
  • Show that the stomach cell does not express nerve‑cell genes, so those proteins are never made

Common mistakes

  • Confusing gene presence with protein production; students say the genes are absent instead of switched off
  • Failing to mention that all cells have the same DNA set
  • Using vague terms like "different genes" instead of "genes are switched on/off"

Mark scheme (4 marks)

  1. All body cells contain the same set of genes / the same DNA
  2. Genes can be switched on or switched off in different cell types
  3. In the stomach cell, the genes coding for nerve-cell proteins are switched off / not expressed
  4. Only the genes required for the stomach cell's function are switched on, so only those proteins (e.g. digestive enzymes) are synthesised

Key terms in this question

gene

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