Final answer:
The spontaneous mutation rate reflects mutations in the absence of a mutagen, assessed by control plates. Plating on nutritionally complete media ensures the observation of auxotrophic mutants is accurate. The Ames test compares reversion rates in bacteria to assess mutagenicity.
Step-by-step explanation:
The rate of mutations that occur in the absence of a mutagen, such as the number of colonies on the negative control plate, refers to the spontaneous mutation rate. This rate can be determined by comparing the growth of control colonies not exposed to a mutagen with those that have been exposed. In the absence of a mutagen, the colonies on the control plate represent the background level of spontaneous mutations.
To answer the specific question posed, cells are plated on a nutritionally complete plate in addition to nutrient-deficient plates when looking for a mutant to ensure that any lack of growth on the nutrient-deficient plates is due to the absence of specific growth factors, indicating the presence of an auxotroph, rather than a failure in the cell transfer process.
In bacterial genetics, an experiment to determine the mutation rate would involve isolating colonies post-exposure to UV-C light and then comparing the frequency of auxotrophic mutants to the control frequency. The Ames test is a related method that evaluates the mutagenicity of compounds by analyzing the reversion rates of auxotrophic bacteria in the presence of the test compound compared to natural mutation rates.