Question

The researchers grow and maintain the Ames test bacteria in liquid media containing histidine. They then prepare plates (semi-solid media) with media that lacks histidine. They also prepare tubes of media that is between the semi-solid and liquid media (soft agar) that contains trace amounts of histidine. They inoculate tubes of the soft agar with the mutant Ames test bacteria and add one of the following - 100 l of (1) chemical A in a solvent, (2) chemical B in a solvent, (3) sodium azide, a potent mutagen or (4) the solvent alone. They then pour the soft agar containing the bacteria and treatment onto a plate of media prepared without histidine. The bacteria quickly use up the histidine in the soft agar. The plates are kept in an incubator and colonies of bacteria are counted after a specified amount of time has elapsed. The experiment is repeated three times.

Which of the following groups is the experiment’s negative control? The group treated with…

A. Chemical A or chemical B in solvent

B. Sodium azide

C. Solvent alone

D. None of the above

Answer 1

Answer: Option C is correct.

Explanation:
A negative control ensures that none of the reaction conditions can adversely affect the result.
In the given case, only solvent is used as a negative control to ensure that the solvent as such has no effect on the reversal of mutations.

Sodium azide is a known mutagen. It is used as a positive control.
Chemical A and B are test samples.