Complete question:
You are studying a biochemical pathway in the mold Neurospora where enzyme 1 converts the initial substrate into intermediate substrate A; enzyme 2 converts intermediate substrate A into intermediate substrate B; enzyme 3 converts intermediate substrate B into intermediate substrate C; and enzyme 4 converts intermediate substrate C into the end product, an amino acid that is essential for growth. You isolate a mutant that is unable to grow on minimal media. Which data would provide the strongest support for the hypothesis that this mutation occurred in the gene that codes for enzyme 2?
a. The mold can grow on rich medium plus intermediate substrate C, but not on rich medium plus intermediate substrate B.
b. The mold can grow on minimal medium plus intermediate substrate B, but not on minimal medium plus the initial substrate.
c. The mold can grow on minimal medium plus intermediate substrate B, but not on minimal medium plus intermediate substrate A.
d. The mold can grow on rich medium plus intermediate substrate D, but not on rich medium plus intermediate substrate C.
e. The mold can grow on minimal medium plus intermediate substrate A, but not on minimal medium plus intermediate substrate C.
Answer:
c. The mold can grow on minimal medium plus intermediate substrate B, but not on minimal medium plus intermediate substrate A.
Explanation:
Let us first diagram the pathway for a better understanding.
The normal organism produces four enzymes that convert substrates in the medium that allow it to survive, grow, and reproduce.
initial substrate ------------> A ----------------> B ----------------> C ----------------> AA
<em>Enzyme 1 Enzyme 2 Enzyme 3 Enzyme 4</em>
Any mutation on the Neurospora´s enzymes will not let the organism grow on the minimal medium because it will not be able to convert the minimal substrate into the following one, because the mutated enzyme will not accomplish its original function.
- So if the mutation occurs in enzyme 1, the organism will <em>not be able to convert the initial substrate into intermediate substrate A</em>. And the rest of the reaction will not be possible either because of the lack of substance A.
initial substrate -----X----> A -------X-----> B ------X--------> C ------X-------> AA
<u> </u><u><em>mutated Enzyme 1 </em></u><em> Enzyme 2 Enzyme 3 Enzyme 4</em>
<em />
- If the mutation occurs in enzyme 2, the organism will <em>not be able to convert the intermediate substrate A into intermediate substrate B</em>. And the rest of the reaction will not be possible either because of the lack of substance B.
initial substrate ------------> A -------X--------> B -------X------> C --------X-----> AA
<em>Enzyme 1 </em><em>mutated Enz. 2</em><em> Enzyme 3 Enzyme 4</em>
<em />
- If the mutation occurs in enzyme 3, the organism <em>will not be able to convert the intermediate substrate B into intermediate substrate C</em>. And the last reaction will not be possible either because of the lack of substance C.
initial substrate ------------> A ----------------> B -------X--------> C -------X------> AA
<em>Enzyme 1 Enzyme 2 </em><u><em>mutated Enz. 3</em></u><em> Enzyme 4</em>
<em />
- And if the mutation occurs in enzyme 4, the organism <em>will not be able to convert the intermediate substrate C into the essential amino acid</em>.
initial substrate ------------> A ----------------> B ----------------> C -------X-------> AA
<em>Enzyme 1 Enzyme 2 Enzyme 3 </em><u><em>Mutated Enz. 4</em></u>
<u><em /></u>
But, if we artificially add to the medium the substrate that should be produced by the original enzyme (and that is not converted because of the mutation), then the organism will grow and survive because the other enzymes will be able to produce the essential amino acid.
So, if enzyme 2 is the only enzyme mutated, the organism will not be able to live in a medium with substrate A because the mutated enzyme will not convert the substrate A into B. There will <em>not be B substrate in the medium, and the other enzymes will not produce the essential amino acid</em>. So if the mutation occurs in enzyme 2, substrate A is useless to the organism because it will not survive.
But if we add the intermediate substrate B to the medium, the organism will survive. In a medium with substrate B, all the other reactions will be possible, and the organism will get the essential amino acid. The artificial addition of B substrate will replace the function of the original enzyme 2 -which is the one that converts A into B-.