AAMC Section Bank C/P #72

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sfsn

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I have a question about the portion underlined in red. I agree that 1M NaCl is much higher than 10 mM MgCl2. But it doesn't have Mg2+, even though it's as stable as those conditions that have Mg2+. So why is it so stable even though it lacks Mg2+?

I guess it's not because it has Cl- (not the right answer) - so why is it?
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Fix the temp at 50C (or any value) and compare data points.

100 mM KCl: most unfolding (mRNA denaturation)
100 mM KCl + 10 mM MgCl2: less unfolding than above. MgCl2 must have helped to stabilize the mRNA. Since Cl- is in the ion concentration above, the stabilization must be from Mg2+.
300 mM KCl + 10 mM MgCl2: only slightly more unfolding, but MgCl2 still helps to stabilize the mRNA.
1 M NaCl: also low unfolding, but we had to add a lot more salt to get the same stabilizing effect.

Answer choices: K+ and Cl- both denature the mRNA (eliminate choices). Mg2+ helped. I'm assuming choice D was Na+, which also helped but less effectively per mol.

(I realize that I didn't explicitly answer your question as to "why". Tbh, idk. Just interpret the data and move on :))
 
So could I think of it like this: because we added SO much more salt in the 4th condition (10x (=1/0.1) more than the 100 mM, 3.33x (=1/0.3) more than the 300 mM), the 4th condition is too different from the other three and not so good for comparing/figuring out what led to stability? So we won't consider it to be as relevant as the other three?
 
Even with a lot more salt, the stabilization effect was about the same as with just a bit of MgCl2.

It's like if you study using princeton materials for 2 weeks and kaplan materials for 200 weeks, but still get the same MCAT score. Then you would tell your friend to just use princeton materials because it's more effective.
 
Oh I get it now. A lot of NaCl had the same effect as a tiny bit of MgCl2. The question stem is asking for the most effective ion, so Mg2+ is correct. Thanks!

shudder, the thought of studying for the MCAT for four years..
 
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