Question

An investigator is designing a screen to search for new mutants in uracil biosynthesis (uramutants). The
investigator decides to screen for mutants by growing them on 5-fluoro-orotic acid (5'-FOA). 5-FOA is a
toxic agent that kills strains capable of synthesizing uracil. After treating cells in a yeast culture with a
mutagen, the investigator cultures half the cells on YPD and the other half on plates containing 5-FOA. Which
of the following results should the investigator expect?

The number of cells on the FOA plates will be much lower than that on the YPD plates, but these are not likely to be ura mutants.

The number of cells on both kinds of plates will be similar. Only the ura mutants will survive on the FOA plates.

The number of cells on the FOA plates will be much lower than that on the YPD plates, but these are likely to be ura mutants.

The number of cells on both kinds of plates will be similar. The ura mutants will not survive on the FOA plates.

Answer 1

ANSWER

The answer for the above question is that the number of cells on the FOA plates will be much lower than that on the YPD plates, but these are likely to be ura mutants.- IIIrd statement.

Explanation

1. In the screening of URA mutants, URA3 encodes orotidine-5’phosphate decarboxylase, an enzyme which is required for the biosynthesis of uracil.

2. Ura3^- (or ura5^-) cells can be selected on media containing FOA.

3. The URA3⁺ cells are killed because FOA appears to be converted to the toxic compound 5-fluorouracil by the action of decarboxylase, whereas ura3^- cells are resistant.