The Ames Test - Online Tutor, Practice Problems & Exam Prep

The Ames test is a scientific screening experiment used to determine if a chemical is a mutagen, which induces mutations in DNA. It involves control and experimental plates with His- bacteria on minimal media lacking histidine. If the test chemical is a mutagen, it will induce mutations, resulting in many His+ colonies. Conversely, few colonies indicate the chemical is not a mutagen. The relationship between the number of colonies and mutagenicity is direct; more colonies suggest greater mutagenic potential.

The Ames Test

In this video, we're going to begin our lesson on the Ames test. The Ames test is a scientific screening experiment designed to determine if a chemical is a mutagen. Recall from our previous lesson videos that mutagens are chemicals that induce mutations in the DNA. The Ames test solely determines whether a chemical is a mutagen. Below, we have some text and an image that shows an example of how the Ames test can be conducted. In any scientific experiment, there are control groups and experimental groups. We have the control plate on the left-hand side and the experimental plate on the right-hand side. The sole purpose of this Ames test is to determine if a test chemical is a mutagen, testing whether a chemical is a mutagen.

In this example of the Ames test, minimal media is used, and this minimal media lacks the amino acid Histidine. There is no Histidine available on this minimal media agar plate. This means that only histidine prototrophs or, in other words, His⁺ cells can grow on this minimal media, while His^- cells cannot grow. On this minimal media plate that lacks histidine, both in the control plate and the test plate, His^- bacteria are added. The expectation is that these His^- bacteria will not be able to grow, as these His^- bacteria require histidine to grow and the minimal media lacks histidine. All His^- bacteria should die and not grow.

Within cells, mutations can occur spontaneously or naturally, or they can be induced. The only way for cells to appear on this plate is if the His bacteria mutate back to His⁺ bacteria. The expected mutagen that we're going to add is expected to reverse the mutation in the His^- cells and reconvert these His^- cells back into His⁺ cells. When you add chemicals to the control plate, you add a non-mutagenic chemical, a chemical known not to be a mutagen. Despite this, some colonies will form on this minimal media plate. These little blue colonies are His⁺ colonies, forming from spontaneous mutations. On the control plate, few His⁺ colonies form, and they occur only due to spontaneous mutations that naturally occur.

On the test plate, the same procedure is followed; His^- bacteria are added, and a test chemical is added to these His^- bacteria to determine if it is a mutagen. The results will dictate whether or not the test chemical is a mutagen. If the results are similar to the control, then you can conclude that your test chemical is not a mutagen. If the chemical is not a mutagen, then you would see the formation of very few His⁺ colonies, forming only from naturally spontaneous mutations. However, if the test chemical added to these His^- is indeed a mutagen, the results will show many His⁺ colonies forming, both from natural spontaneous mutations and induced mutations caused by the chemical mutagen added.

This method allows us to determine if the test chemical is a mutagen by the number of colonies that form. If many His⁺ colonies form, then the test chemical is a mutagen. Conversely, if not many His⁺ colonies form and the plate looks like the control plate, then the test chemical is not a mutagen. The number of colonies also reflects the mutagenicity of the mutagen; the more His⁺ colonies that grow, the greater the mutagenicity of that mutagen. This concludes our brief introduction to the Ames test and how it is used to determine if a test chemical is a mutagen or not. We'll be able to get some practice applying these concepts of the Ames test as we move forward. I'll see you all in our next video.

Ames test is a method to:

• A. Test error rate in DNA replication.
• B. Characterize mutation types.
• C. Test mutagenicity of potential hazardous chemicals.
• D. Test proofreading capacities of DNA polymerases during replication (or PCR).

During an Ames test, if the test plate has only a few colonies grown on it like the control plate, this means that:

• A. The chemical is a carcinogen.
• B. The chemical is a mutagen.
• C. The chemical does not cause mutations.
• D. The cells are resistant to mutations.
• E. None of the above are correct.

In the Ames test, why do some His+ colonies form even when a mutagenic chemical is not used?

• A. Some bacterial cells will have spontaneous mutations which turns them from His- to His+.
• B. Some bacterial cells will have induced mutations which turns them from His- to His+.
• C. The minimal media induces mutations changing some of the His- bacteria into His+ bacteria.
• D. The cells used to start the experiment were not entirely His- cells.

Here’s what students ask on this topic:

What is the Ames test and what is its purpose?

The Ames test is a scientific screening experiment designed to determine if a chemical is a mutagen, which means it can induce mutations in DNA. The test involves using control and experimental plates with His- bacteria on minimal media that lacks histidine. If the test chemical is a mutagen, it will induce mutations that convert His- bacteria to His+ bacteria, resulting in many His+ colonies. Conversely, if few colonies form, the chemical is not a mutagen. The number of His+ colonies directly correlates with the mutagenicity of the chemical, meaning more colonies indicate a higher mutagenic potential.

How does the Ames test determine if a chemical is a mutagen?

The Ames test determines if a chemical is a mutagen by observing the growth of His+ colonies on minimal media plates that lack histidine. His- bacteria are added to both control and experimental plates. The control plate receives a non-mutagenic chemical, while the experimental plate receives the test chemical. If the test chemical is a mutagen, it will induce mutations that convert His- bacteria to His+ bacteria, resulting in many His+ colonies. If the number of His+ colonies on the experimental plate is significantly higher than on the control plate, the chemical is considered a mutagen.

What is the role of histidine in the Ames test?

In the Ames test, histidine plays a crucial role in determining whether a chemical is a mutagen. The test uses minimal media that lacks histidine, meaning only His+ bacteria can grow on it. His- bacteria, which require histidine to grow, are added to the plates. If a test chemical induces mutations that convert His- bacteria to His+ bacteria, these His+ bacteria will grow and form colonies. The presence of many His+ colonies indicates that the test chemical is a mutagen, while few colonies suggest it is not.

What are the control and experimental groups in the Ames test?

In the Ames test, the control group consists of a plate with His- bacteria and a non-mutagenic chemical. This plate serves as a baseline to account for spontaneous mutations that naturally occur, resulting in a few His+ colonies. The experimental group consists of a plate with His- bacteria and the test chemical. If the test chemical is a mutagen, it will induce mutations that convert His- bacteria to His+ bacteria, resulting in many His+ colonies. Comparing the number of His+ colonies between the control and experimental groups helps determine the mutagenicity of the test chemical.

What does a high number of His+ colonies indicate in the Ames test?

A high number of His+ colonies in the Ames test indicates that the test chemical is a mutagen. The test involves adding His- bacteria to minimal media plates that lack histidine. If the test chemical induces mutations that convert His- bacteria to His+ bacteria, these His+ bacteria will grow and form colonies. A high number of His+ colonies suggests that the chemical has a high mutagenic potential, as it has induced a significant number of mutations. Conversely, few His+ colonies indicate that the chemical is not a mutagen.