In a nutrient medium that lacks histidine, a thin layer of agar containing around 10^9 Salmonella typhimurium histidine auxotroph produces around 13 colonies over a two day incubation period at 37 degrees centigrade. How do these colonies arise in the absence of histidine? The experiment is repeated in the presence of 0.4 micrograms of 2-aminoanthracene. The number of colonies the number of colonies produced over 2 days exceeds 10,000. What does this indicate about 2- anthracene? What can you surmise about its carcinogenicity?

Reverse mutations in the colonies enabled to recover the ability to synthesize histidine. The 2-aminoanthracene is a chemical agent that induce mutations and therefore it is potentially a carcinogen.

Explanation:

A reverse (suppressing) mutation is a type of mutation capable of restoring the activity of previously mutated genes. On the other hand, 2-aminoanthracene is an aromatic amine carcinogen capable of inducing hepatic and skin tumors. In this case, 2-aminoanthracene has been used to induce reverse mutations at loci responsible for the synthesis of histidine.

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