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# Mutagens and the Ames test

Mutagens are agents that change DNA; the Ames test screens chemicals for this by checking whether they make bacteria mutate back to growth.

Updated Nov 12, 2026

## Key facts

- Physical mutagens: ultraviolet light (pyrimidine dimers) and ionising radiation (strand breaks).
- Chemical mutagens: base analogues, alkylating and deaminating agents (nitrous acid), intercalating agents (frameshifts), and many carcinogens once the liver activates them.
- Ames test: Salmonella strains that cannot make histidine are exposed to the chemical; more colonies growing without histidine means more reverse mutations.
- A liver extract (the S9 fraction) is added so that chemicals that become mutagenic only after metabolism (procarcinogens) are also detected.
- Most carcinogens are mutagens, so a positive Ames test flags a chemical for more testing; it does not prove that it causes cancer in people.

## In detail

- A conditional mutation shows its effect only under some conditions; the classic case is a protein that works at a lower temperature and fails at a higher one (temperature-sensitive).
- Site-directed mutagenesis changes one chosen base in a cloned gene, to learn what a single amino acid does or to engineer a protein.
- Antimutagens lower the mutation rate, for example by trapping reactive chemicals or supporting repair; antioxidants act partly this way.
- Spontaneous mutations also come from replication errors and from natural base changes, such as the deamination of cytosine to uracil.

**Also called**
Mutagenicity testing, Site-directed mutagenesis

## Clinical notes

- Drugs and industrial chemicals are screened for mutagenicity before use; the bacterial reverse-mutation (Ames) test is a standard first step.
- Aflatoxin, made by Aspergillus on badly stored grain, becomes a mutagen in the liver and is linked to liver cancer.
- Defective DNA repair, as in xeroderma pigmentosum, shows how much repair protects against ultraviolet mutagenesis.

## Test yourself

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## Why is a liver extract added in the Ames test?

Many chemicals become mutagenic only after liver enzymes metabolise them; the S9 extract provides that activation.

## What does a positive Ames test mean?

The chemical causes mutations in bacteria, so it is a likely carcinogen that needs more testing; it does not prove cancer in people.

## What is a temperature-sensitive mutation?

A conditional mutation: the protein works at a permissive (lower) temperature and fails at a restrictive (higher) one.

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## Connections

## Related conditions 1

- [Xeroderma pigmentosum](https://durgical.com/en/conditions/xeroderma-pigmentosum/)

## Related 5

- [Types of mutations](https://durgical.com/en/biochemistry/topics/types-of-mutations/)
- [DNA repair](https://durgical.com/en/biochemistry/topics/dna-repair/)
- [Cytochrome P450 system](https://durgical.com/en/biochemistry/topics/cytochrome-p450-system/)
- [Oncogenes and tumour suppressor genes](https://durgical.com/en/biochemistry/topics/oncogenes-and-tumour-suppressor-genes/)
- [Recombinant DNA and cloning](https://durgical.com/en/biochemistry/topics/recombinant-dna-and-cloning/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Mutagen](https://www.genome.gov/genetics-glossary/Mutagen) (National Human Genome Research Institute (NHGRI), 2026); [Point Mutation](https://www.genome.gov/genetics-glossary/Point-Mutation) (National Human Genome Research Institute (NHGRI), 2026); [What is a gene variant and how do variants occur?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/mutationsanddisorders/genemutation/) (MedlinePlus, US National Library of Medicine, 2026).

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