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.

Answer: (A)

Verified step by step guidance

1. Understand the Ames test: It is a biological assay to assess the mutagenic potential of chemical compounds. It uses strains of bacteria that are unable to synthesize histidine (His-).
2. Recognize the role of spontaneous mutations: Even in the absence of a mutagen, spontaneous mutations can occur in the DNA of bacteria, leading to a reversion from His- to His+.
3. Consider the genetic variability: Bacterial populations can have genetic variability, and some cells might naturally revert to His+ due to random mutations.
4. Acknowledge the possibility of contamination: The initial bacterial culture might not be entirely His-, meaning some His+ cells could be present from the start.
5. Evaluate the experimental conditions: Minimal media can exert selective pressure, potentially favoring the growth of any spontaneously mutated His+ cells, making them more noticeable.