The Virtual Ames Test: Are chemicals, mutations, and cancer linked?

Updated Febrary, 2016 to use HTML5

A lesson in mutation and selection

Life is a collection of chemicals. Thousands of different molecules are required for the cellular functions of every plant, animal, and microorganism. Without chemicals, life would not exist. How, then, has our society developed the notion that chemicals are "bad" and that "natural" things do not contain chemicals? The answer is probably quite complex. Some of the factors that contribute to the answer include the following:

In contrast with the popular view and the position taken by the federal regulatory agencies, some people believe we should focus on the activity of a chemical and how much of it we are exposed to, not whether it was synthesized in a factory or made by a plant, animal, or microbe.

One of the areas in which this debate about chemical safety has become quite heated is in the regulation of chemicals based on their potential for causing cancer. Many chemicals that are naturally found in the foods we eat show potential for causing cancer in the same tests that have been set up for synthetic chemicals, which has led some scientists to question the relevance of these test.

The most famous example of a test that predicts carcinogenicity of chemicals is the Ames Test, developed by a scientist named Bruce Ames. The test has been used extensively for many years to predict the safety of synthetic chemicals. Recently, Ames and some of his colleagues found that chemicals in many "natural" foods (such as broccoli, potatoes, beer, and mushrooms) show just as much potential for carcinogenicity as many of the synthetic chemicals that alarm regulatory agencies and consumers. Ames has been a vocal advocate of applying the same standards to synthetic chemicals and natural foods. The debate is not finished.

The Ames test is based on the following theories and assumptions:


Some definitions that may be useful for this experiment

Streptomycin: the antibiotic that provides the selection; it differentiates strains of bacteria that are sensitive (do not grow on streptomycin) from those that are resistant (grow on streptomycin).

Mutagen: a chemical (or a physical phenomenon, such as X-rays) that causes changes in the sequences of bases in DNA; exposure to a mutagen will increase the frequency of mutants in a population.

Mutagenicity: the potential of a chemical for causing mutations or changes in DNA.

Mutant: an organism that differs genetically from its parent; in this case, bacteria that are resistant to streptomycin.

Spontaneous mutant: a mutant that arises from random changes in the DNA; changes can be caused by mistakes during DNA replication, damage from X-rays or ultraviolet light.

Frequency of mutants: the proportion of the population accounted for by mutants; for spontaneous bacterial mutants, the frequency is typically one mutant for a given trait in every million to 10 million cells.

Carcinogenicity: the ability to cause cancer in animals.


Key Concept

The Ames test is based on the assumption that mutagenicity is associated with carcinogenicity and that mutagenic activity in bacteria is predictive of mutagenic activity in humans.


Your Challenge

Mutations in DNA appear at random in a population of bacteria. A chemical mutagen increases the frequency at which mutations appear. The Ames Test measures the ability of a chemical to increase the mutation rate in bacteria. The mutations are detected by measuring a characteristic that results from changes at the gene level, such as the ability to grow under certain conditions.

In this lab simulation, you will conduct a modified Ames Test.

You will be provided with:

Propose a hypothesis about the types of chemicals that are likely to be mutagens and design an experiment to test your hypothesis.

Key questions

To run the experiments: