Diagnostic Chem/Physics Section Q#21

sumbeeson
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Joined: Wed Aug 30, 2017 12:33 pm

Diagnostic Chem/Physics Section Q#21

Postby sumbeeson » Fri Sep 01, 2017 2:44 pm

Could you please elaborate on why option (C) is incorrect? Based on the passage information I thought phosphorylated beta catenin could be found in the cytoplasm and the nucleus. How does the contrast between rates of degradation tell us where phosphorylated beta catenin is ending up? Thank you for your help!
NS_Tutor_Andrew
Site Admin
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Joined: Mon May 23, 2016 1:47 pm

Re: Diagnostic Chem/Physics Section Q#21

Postby NS_Tutor_Andrew » Fri Sep 08, 2017 6:37 pm

Hi sumbeeson,

Excellent question! The way I'd suggest thinking about it would be that the fact that the nuclear levels of 32P decline quickly indicates that the phosphorylated beta-catenin isn't effectively making it into the nucleus. Combined w/ passage information telling us that degradation happens in the cytosol, we can conclude that the phosphorylated form is being degraded.

On a strategy note, though, note that the question stem asks you to explain a possible contrast (phosphorylated vs. non-phosphorylated form being degraded), which means that the correct answer choice should at least imply a contrast too. For these explanation/interpretation questions, it's common to have an incorrect answer choice be an empirically correct statement that doesn't really connect to any explanation, so watch out for answer choices like C that simply make an observation about the findings without implying some kind of contrast.

Hope this helps, and keep up the great questions!
Andrew D.
Content Manager, Next Step Test Prep.
Snazia
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Joined: Fri Mar 09, 2018 9:57 pm

Re: Diagnostic Chem/Physics Section Q#21

Postby Snazia » Sun Mar 11, 2018 7:33 pm

So I still don't understand why A is correct. The answer choice says the levels are decreasing quicker in the nucleus than in the cytoplasm. But the total change in phosphorylated beta-catenin is the same for both, so don't they decrease in the same rate? This thought was the only thought that made me change my answer from A to C.
NS_Tutor_Andrew
Site Admin
Posts: 520
Joined: Mon May 23, 2016 1:47 pm

Re: Diagnostic Chem/Physics Section Q#21

Postby NS_Tutor_Andrew » Mon Mar 12, 2018 5:46 pm

Hi Snazia,

Once more, thanks for the excellent question! I think that "decrease faster" in the answer choice refers to the idea that the nuclear 32P levels become negligibly low more quickly after the experiment starts (by about 40 min) than the cytoplasmic 32P levels. I do see your point, though, although looking very carefully over the graph it does seem like the change in signal divided by change in time is slightly different for the two graphs -- looking at the 30 min time mark for instance, it seems like the cytoplasmic 32P signal decreased from about 1.5 to about 0.6 (for a change of 0.9) while the nuclear signal decreased from 1.2 to 0.2, for a change of 1.0. That said, that's a pretty small difference and I think the issue has more to do with what the answer choice means by "decrease faster." I think the key thing with this question remains recognizing that C cannot be the best answer because it does not describe a contrast between the nuclear and cytoplasmic 32P levels, which would be required to explain the observation introduced in the question stem.

Hope this is helpful!!
Andrew D.
Content Manager, Next Step Test Prep.

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