# 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

## 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

- [gene](https://www.gradenine.co.uk/glossary/gene)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-gene-is-a-length-of-af0595f0) · Published by Druglandscape Ltd.