Question

An organism is able to live on a culture medium containing nutrient A, by the enzyme catalysed reaction :

$A\stackrel{X}{\to }B\stackrel{Y}{\to }C$.

A mutant failed to survive in this medium but grew when nutrient B was added to it. Which gene of this mutant was defective?

• A. Only X
• B. Only Y
• C. X and Y both
• D. Neither X or Y

Answer 1

The correct option is A Only X

According to the question, the wild type organism is able to live on a culture medium containing nutrient A because he carries all the enzymes required for catabolism of this nutrient. The mutant type can not survive on medium containing A. This means that it has mutation in the gene encoding the enzyme needed for conversion of A into B (gene X). It can survive on the minimal medium plus B because it has all the functional enzymes needed for the catabolism of B into C, i.e., gene "Y" is not mutated. Hence, it has mutation in "X" gene. Option A is correct answer.