The purpose of the Ames test is to assess the mutagenicity of a chemical compoun
ID: 265557 • Letter: T
Question
The purpose of the Ames test is to assess the mutagenicity of a chemical compound. Briefly, an auxotrophic his- E. coli strain is grown in rich media containing a possible mutagen and a rat liver extract. Then, the mixture is plated on -His media and the mutagenic ability of the chemical compound is assessed by appearance of his+ revertant colonies A. It is also possible to produce his- mutant colonies from a prototrophic E. coli strain. Why did Bruce Ames choose to assay for reversion of a his- auxotroph "back" to the his+ prototrophic phenotype? Write one reason why it is advantageous to assay for his+ revertants rather than new his- mutants. (3 points) B. Suppose you have obtained two new his- auxotrophic E. coli strains. To determine if either strain will be useful for the Ames test, you test the ability of well-known mutagens to produce his + revertants. Your results are summarized in the table below. lested for reversion by... (Yes - Revertants obtained, No - No revertants obtained) Table his- auxotrophic strain DNA depurination agents DNA intercalating agents Yes Yes What type of DNA mutations are likely present in Strains A and B? (2 points) Strain A Strain B Explain why depurination agents fail to produce his+ revertants of Strain A. (3 points) C. Occasionally, true mutagens do not score positively in the Ames test. Imagine you perfornm the test using the two his- strains shown in the table above. What kind of DNA mutation would be unlikely to produce his+ revertants in both Strains A and B? (2 points)Explanation / Answer
Answer:
A) Why did Bruce Ames choose to assay for reversion of a his- auxotroph "back" to the his+ prototrophic phenotype? Write one reason why it is advantageous to assay for his+ revertants rather than new his- mutants?
His- are histidine dependent bacteria whereas His+ are capable of growing in a histidine-independent media. Ames chose to assay for reversion of a his- auxotroph back to his+ prototrophic phenotype because it is easy to screen colonies of bacteria that have the his+ reversion. It can be achieved by growing bacterial cells in a media that lacks Histidine amino acid which is crucial for bacterial growth. The strains that revert to His+ state are able to grow in the media lacking histidine and can be identified easily.
B) Suppose you have obtained two new his- auxotrophic E.coli strains. To determine if either strain will be useful for the Ames test, you test the ability of well-known mutagens to produce his+ revertants
What type of DNA mutations are likely present in Strains A and B
Strain A: In strain A the revertants were obtained in the presence of DNA intercalating agent. It most likely contained frameshift mutation
Strain B: In strain B the revertants were obtained in the presence of DNA depurination agents. It most likely contains transversion mutations.
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