In an experiment employing the methods of the Ames test, two strains of Salmonella are used. Strain A contains a base-substitution mutation, and Strain B contains a frameshift mutation. Four plates are prepared to test the mutagenicity of the compound ethyl methanesulfonate (EMS). Plate 1 is a control plate with Strain A and S9 extract but no EMS. Plate 2 is also a control plate and contains Strain B and S9 extract but no EMS. Plate 3 contains Strain A along with S9 extract and EMS, and Plate 4 contains Strain B, S9 extract, and EMS. Characterize the expected distribution of colony growth on the four plates. Defend your growth prediction for each plate.

Key Concepts

Ames Test

The Ames test is a widely used method for assessing the mutagenic potential of chemical compounds. It employs specific strains of bacteria, typically Salmonella, that are sensitive to mutations. By observing the growth of these bacteria in the presence of a potential mutagen, researchers can infer whether the compound causes genetic changes that lead to increased colony formation.

Types of Mutations

Mutations can be classified into various types, with base-substitution and frameshift mutations being two primary categories. A base-substitution mutation involves the replacement of one nucleotide with another, potentially altering a single amino acid in a protein. In contrast, a frameshift mutation results from the insertion or deletion of nucleotides, shifting the reading frame and often leading to a completely different and nonfunctional protein.

S9 Extract

S9 extract is a metabolic activation system derived from the liver of rodents, used in mutagenicity testing to simulate the metabolic processes that occur in living organisms. It contains enzymes that can convert pro-mutagens into their active forms, allowing researchers to assess the mutagenic effects of compounds like ethyl methanesulfonate (EMS) under conditions that mimic biological metabolism.