Question

Four mutant strains of bacteria $(1-4)$ all require substance $S$ to grow (each strain is blocked at one step in the $S$-biosynthesis pathway). Four plates were prepared with minimal medium and a trace of substance $S$, to allow a small amount of growth of mutant cells. On plate $A$, mutant cells of strain $1$ were spread over entire surface of the agar to form a thin lawn of bacteria. On plate $B$, the lawn was composed of mutant cells of strain $2$, and so on. On each plate, cells of each of the four mutant types were inoculated over the lawn, as indicated in the figure by the circles. Dark circles indicate excellent growth. A strain blocked at a later step in the $S$ substance metabolic pathway accumulates intermediates that can 'feed' a strain blocked at an earlier step.

What is the order of genes $(1-4)$ in the metabolic pathway for synthesis of substance $S$?

• A. $2\to 4\to 3\to 1$
• B. $2\to 1\to 3\to 4$
• C. $1\to 3\to 4\to 2$
• D. $1\to 2\to 4\to 3$

Answer 1

Answer: (C)

Correct answer is (c) $1\to 3\to 4\to 2$