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Further Explanation on Question 3 of Chemical and Physical Sciences Section of GS-Free Practice Test

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agileduck

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I was wondering if someone could explain this question in further detail. I understand why it's not answer choice (a) carbamoyltransferase, but I don't understand why it cannot be answer choice (b) argininosuccinate synthase. My thinking is if you have growth of both argininosuccinate and arginine on minimal medium, couldn't both enzymes be functioning correctly (or defective). In order to tell which enzyme was defective wouldn't you need to know what was in the minimal medium, and since they don't give us that you couldn't know the answer, unless you make an assumption of the composition of the minimal medium. Are they expecting us to make that assumption, and if so what should that assumption be? Am I just overthinking this or is my thought process completely off? HELP!


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SpongeBobDude

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It's just my guess, but notice that citrulline is absent. That means that even if argininosuccinate synthase is not defective - we can't prove it. Simply because there is no citrulline present in media to prove that (we would need to see the synthesis step for that - and we can't). So from data given only 1 step of transformation can be confirmed with certainty (like the question stem asks) - from argininosuccinate to arginine.
I understand your confusion and to be honest I think question could have been worded better to begin with, but this kind of stuff is what makes Bio section especially hard for me. Sometimes we have to make decisions from slightest hints of question stems even if question is not worded ideally.
 

Altius Premier Tutor

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I think you are not thinking about the Table in the best way. Each plus sign indicates that when the mutant is "fed" the item indicated (ornithine, argininosuccinate, etc.) the mutant grows. The passage equates growth with the presence of arginine because it is necessary for growth. So, the last column is irrelevant. If we add arginine and that is what is needed for growth either mutant will obviously grow. However, the key is that when we added citruline the mutants DID NOT grow. That means the enzyme that converts citruline to argininosuccinate is likely defective. However, when we add argininosuccinate we DO get growth. That tells us the enzyme that creates arginine is likely functioning. The question asks for an enzyme NOT defective in both mutants, so C is best. Your other answer, B, must be false because the question asks for enzymes that are NOT defective in P or Q and the "no growth condition w/ citruline" suggests that argininosuccinate synthase is INDEED defective.
 
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