Biology Questions

MCAT_Retaker
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Biology Questions

Postby MCAT_Retaker » Sat Mar 17, 2018 3:18 am

For this question, how come the answer isnt 2? If one of the homologous chromosomes has a mutation, cant the answer be 2 as well? The question says one chromosome, so a spermatagonium has only one chromatid which is also called a chromsome. If one chromsome is mutated, that means one chromatid and when thats replciated, thats a chromosome with two chormatid, both which will also be affected. But the other chromatid is also fine?
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NS_Tutor_Andrew
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Re: Biology Questions

Postby NS_Tutor_Andrew » Mon Mar 19, 2018 6:07 pm

Hi MCAT_Retaker,

Yes, I think you're right. I'd revise the terminology slightly, though -- what's happening is that the mutation only occurs in one of the homologous chromosomes in the pair in question (in the diploid spermatogonium). So, only two of the spermatids would have the mutation, because the other two spermatids would each get a chromatid from the other (non-mutated) homologous chromosome.

Hope this clarifies things! We'll fix this question ASAP.
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Re: Biology Questions

Postby MCAT_Retaker » Tue Mar 20, 2018 3:24 am

Yea, i picked 2 at first too. I wonder why no one else brought it up. :/.
Also for this question, how come choice IV is not included. Technically, it does increase with Nef present.
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Re: Biology Questions

Postby NS_Tutor_Andrew » Tue Mar 20, 2018 11:58 am

Hi MCAT_Retaker,

For the passage question, the idea is that for VSV-G, the error bars overlap between the Nef+ and Nef- conditions -- or at least come very close to doing so, especially in comparison to the other 3 viruses. Keep in mind that the question does not narrowly ask "for which viruses does infectivity increase in Nef+ conditions?", but instead asks for which viruses the evidence is strongest that Nef mediates viral efficiency, which is a stronger statement with implications for biological causality. In that context, the dramatic difference between VSV-G and the other 3 viruses, plus the overlapping (or near-overlapping error bars) in the VSV-G results, is sufficient to conclude that the evidence is much stronger for those viruses than for VSV-G.

Parenthetically, for the MCAT it's usually best to assume that a difference with a P-value >0.05 or with overlapping error bars isn't a "real" difference. This statement would make a biostatistician cry, and even as someone who isn't a biostatistician but is interested in stats, it kind of makes me cringe, because the question of what is a "real" or "meaningful" difference is way more nuanced (see this essay if you're interested and have time to kill), but the reality is that many physicians and even biomedical researchers use the simple heuristic of saying that P<0.05 (or other measures of significance) = a real difference and that P>0.05 = no difference, to the point where you can see researchers say things like "the incidence of XYZ thing under study was not different in groups A and B (P=0.62)" even though if you look at the incidence numbers, they were different, just not significantly so. This is personally a bit of a pet peeve of mine, but it's very much a real thing, and is a habit of mind reflected on the MCAT.

This was a long answer to a simple question, but hopefully it helps shed some light on the underlying issue beyond just this question!
Andrew D.
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NS_Tutor_Andrew
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Re: Biology Questions

Postby NS_Tutor_Andrew » Tue Mar 20, 2018 11:58 am

Hi MCAT_Retaker,

For the passage question, the idea is that for VSV-G, the error bars overlap between the Nef+ and Nef- conditions -- or at least come very close to doing so, especially in comparison to the other 3 viruses. Keep in mind that the question does not narrowly ask "for which viruses does infectivity increase in Nef+ conditions?", but instead asks for which viruses the evidence is strongest that Nef mediates viral efficiency, which is a stronger statement with implications for biological causality. In that context, the dramatic difference between VSV-G and the other 3 viruses, plus the overlapping (or near-overlapping error bars) in the VSV-G results, is sufficient to conclude that the evidence is much stronger for those viruses than for VSV-G.

Parenthetically, for the MCAT it's usually best to assume that a difference with a P-value >0.05 or with overlapping error bars isn't a "real" difference. This statement would make a biostatistician cry, and even as someone who isn't a biostatistician but is interested in stats, it kind of makes me cringe, because the question of what is a "real" or "meaningful" difference is way more nuanced (see this essay if you're interested and have time to kill), but the reality is that many physicians and even biomedical researchers use the simple heuristic of saying that P<0.05 (or other measures of significance) = a real difference and that P>0.05 = no difference, to the point where you can see researchers say things like "the incidence of XYZ thing under study was not different in groups A and B (P=0.62)" even though if you look at the incidence numbers, they were different, just not significantly so. This is personally a bit of a pet peeve of mine, but it's very much a real thing, and is a habit of mind reflected on the MCAT.

This was a long answer to a simple question, but hopefully it helps shed some light on the underlying issue beyond just this question!
Andrew D.
Content Manager, Next Step Test Prep.
MCAT_Retaker
Posts: 52
Joined: Thu Feb 01, 2018 3:58 am

Re: Biology Questions

Postby MCAT_Retaker » Wed Mar 21, 2018 2:30 am

Thank you. I also feel like for this question, the third statment is contradicting choice C. A little bit of salt disrupts DNA but a lot of it doesnt? ???
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NS_Tutor_Andrew
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Re: Biology Questions

Postby NS_Tutor_Andrew » Thu Mar 22, 2018 1:45 pm

Hi MCAT_Retaker,

What's going on here is not that the salt disrupts DNA, but that the absence of salt does - in other words, salts help stabilize DNA (keep in mind that salts/ions are always present in physiological conditions), and removing them can destabilize DNA. The lecture outline here goes into a bit more detail, which might be helpful.

Hope this clarifies the question :)!
Andrew D.
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Re: Biology Questions

Postby MCAT_Retaker » Thu Mar 22, 2018 9:46 pm

Thank you.

Basic question. D is false, because enzymes can make reactions occur more easily, right?
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Re: Biology Questions

Postby NS_Tutor_Andrew » Fri Mar 23, 2018 12:33 pm

Personally, I think the biggest problem w/ D is that it's not a precisely-defined statement -- what does "easy" mean w/ regard to a chemical reaction? I could imagine real-world scenarios where you could use "easy" to mean "fast", "thermodynamically favorable", "does not require a particularly sensitive experimental setup", or even "uses cheap, easily accessible reagents". But I think that within the context of the question that's being asked, you're getting at the right idea.
Andrew D.
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MCAT_Retaker
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Re: Biology Questions

Postby MCAT_Retaker » Tue Apr 17, 2018 4:39 am

Is this a mistake?
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