Ch. 11 - Gene Mutation, DNA Repair, and Homologous Recombination

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 11 - Gene Mutation, DNA Repair, and Homologous Recombination

Problem 9

Chapter 11, Problem 9

Describe the purpose of the Ames test. How are his⁻ bacteria used in the Ames test? What mutational event is identified using his⁻ bacteria?

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span>The Ames test is a biological assay used to assess the mutagenic potential of chemical compounds. It helps determine whether a substance can cause genetic mutations, which may lead to cancer.

span>In the Ames test, his⁻ (histidine auxotroph) bacteria, typically strains of Salmonella typhimurium, are used. These bacteria cannot synthesize the amino acid histidine due to a mutation in the histidine operon.

span>The test involves exposing these his⁻ bacteria to the chemical being tested, along with a small amount of histidine to allow for a few rounds of bacterial division.

span>If the chemical is mutagenic, it will cause mutations that revert the his⁻ mutation back to his⁺, allowing the bacteria to grow on a medium lacking histidine.

span>The mutational event identified using his⁻ bacteria is the reversion mutation, where the original mutation is reversed, restoring the bacteria's ability to synthesize histidine.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ames Test Purpose

The Ames test is a widely used assay that evaluates the mutagenic potential of chemical compounds. It assesses whether a substance can cause mutations in the DNA of a test organism, typically a strain of bacteria. By identifying mutagens, the Ames test helps in assessing the potential carcinogenic risk of chemicals to humans.

his⁻ Bacteria

His⁻ bacteria are mutant strains of bacteria that lack the ability to synthesize histidine, an essential amino acid. In the Ames test, these bacteria are exposed to the test substance, and if mutations occur that restore their ability to produce histidine, it indicates that the substance is mutagenic. This restoration is a key indicator of genetic change due to the test compound.

Mutational Events

The mutational event identified using his⁻ bacteria in the Ames test is the reversion mutation, where a mutation in the bacterial DNA restores the original function lost in the his⁻ strain. This event signifies that the test substance has induced a genetic change, allowing the bacteria to regain the ability to synthesize histidine, thus providing evidence of mutagenicity.

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Two different mutations are identified in a haploid strain of yeast. The first prevents the synthesis of adenine by a nonsense mutation of the ade-1 gene. In this mutation, a base-pair substitution changes a tryptophan codon (UGG) to a stop codon (UGA). The second affects one of several duplicate tRNA genes. This base-pair substitution mutation changes the anticodon sequence of a tRNAᵀʳᵖ from

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