MP 30 : Dissecting MCAT Biology Discrete Trap Answers

Session 30

This week, Bryan picked from common MCAT biology questions on the Next Step tests where students fall into traps when reviewing answers. In fact, a good chunk of students got them right (40%-70%). However, in each case, although it's just a really sizable minority, there was one answer that a lot of students got trapped into. So now we're going to help you avoid it.

[01:59] Question #15:

The peptide bond that forms the backbone of proteins is especially stable because it:

  • (A) consists of a triple bond, significantly stronger and more stable
  • (B) is a carboxylic acid derivative
  • (C) would result in proteins that denatured easily if it were unstable
  • (D) exhibits resonance stabilization

[02:27] Bryan's Insights:

This is a content fact that students have to know about peptide bonds, which are amides, and amides have resonance stabilization. Hence, choice (D) is the right answer. In fact, about 75% of students get that right. The trick here, however, is that answer choice (B) says, “is a carboxylic acid derivative,” in the sense that it's true. A peptide is an amide and an amide is a carboxylic acid derivative from amine and a carboxylic acid forming amide. So a lot of students pick this answer, almost the entire other 25% of students.

The lesson you need to learn from this is that you're going to see this trick again in some of these other questions. So you have to be able to answer the question, and not just be true.

Again, yes, a peptide bond is a carboxylic acid derivative but that doesn't mean it's inherently stable. Since the question was “…is inherently stable because.” And of course, there are molecules like acyl halides, to a lesser extent, anhydrides. There are carboxylic acid derivatives that are not stable. So answer choice (B), although true about peptides, doesn't actually answer the question that's being asked.

This is both a reading comprehension issue and an “I'm going to fast” issue where students get into a panic mode and just guess and they don't read all the choices so they move on. Alternatively, by the time they get to the end of the question and starting to read the answer choices, they've already forgotten exactly what question was asked.

Make sure you don't miss a keyword like “least” in the question stem, which you should highlight the question itself to make sure you're answering the exact questions being asked. Use the highlighter in the question.

[04:51] Question #16:

In prokaryotes, genes can exist as operons that are transcribed into a polycistronic mRNA containing multiple genes in a single transcript. In eukaryotes, transcripts exist only as monocistronic mRNA containing a single gene. What fundamental genetic difference is responsible for this distinction?

  • (A) mRNA is transported outside of the nucleus in eukaryotes.
  • (B) Prokaryotic mRNA has a five-prime GTP cap.
  • (C) Prokaryotes use a single start codon for multiple genes.
  • (D) In eukaryotes, each gene has their own transcription initiation site.

[05:47] Bryan's Insights:

Answer choice (D) is the right answer. In eukaryotes each gene has its own initiation site, and so one mRNA is transcribed off it for one gene, and that then is the right answer.

Now again, a very common trap answer is choice (A) mRNA is transported outside the nucleus in eukaryotes. This is true. You move outside the nucleus to make a protein. Also, prokaryotes don't have a nucleus so that is a distinction between the two.

But just like we saw in Question #15, this is an example where the trap answer, although true, is not the answer to the question. The question was specifically getting at this “difference between mRNAs that are polycistronic versus monocistronic.” How could you have multiple genes in one mRNA as opposed to a single gene in mRNA? And that has nothing to do with where the RNA is whether it is in the nucleus or not.

Again, a key lesson on the MCAT: Don't pick an answer choice just because it's true, pick it because it actually answers the question. Moreover, if you have that “it was a stupid mistake” kind of reaction, always dig in and don't just brush it off as a stupid mistake. Find out why it was a stupid mistake so you don't do it again.

[07:35] Question #17:

In miRNA directed gene silencing, a small RNA binds to an mRNA and directs degradation of the mRNA or prevents translation of the mRNA. Which of the following terms describes the process through which binding occurs?

  • (A) RNA polymerization
  • (B) hybridization
  • (C) elongation
  • (D) transcription

[08:15] Bryan Insights:

You will probably notice some yammering on the question but you have to focus on exactly what the question asks, which is the process of binding, specifically one kind of RNA binds to another kind of RNA. Two nucleic acids come together, two single-stranded nucleic acids come together, like Velcro, they stick to each other. Whether it's two DNAs, two RNAs, a DNA and an RNA. It actually doesn't matter. It's all the same thing. It's all answer choice (B) hybridization.

Notice it had nothing to do with the gene silencing, or all the funky little non-coding RNA processes that are hinted at in the question. This was really just a definition. What do you call it when two nucleic acid strands glom onto each other? They hybridize.

The trap here is again, answer choice (A) RNA polymerization. The word RNA shows up over and over again in the question stem, and it sounds like polymerization- ‘oh yeah things sticking together, that sounds like making a polymer,' and so students pick answer choice A. Again, this is just a definition question. You have to know what RNA and DNA polymerization is, which is when individual nucleic acid monomers get strung together into a whole big long RNA strand. So when you transcribe mRNA from DNA, you are polymerizing a long RNA strand. Or when you do a replication for it and your DNA replicates, you are polymerizing new DNA strands.

That's certainly something that happens as it happens all the time, but it's not what the question asked about. The question was much more straightforward and it basically gave us the exact definition of hybridization. We just had to pick that out of the answer choices.

[10:14] Question #44:

Several samples are analyzed for nucleotide composition. Which of the following compositions most likely represents single-stranded piece of DNA? Which of these is a single-stranded piece of DNA?

  • (A) 17% A and 17% T, 33% G and 33% C
  • (B) 29% A and 14% U
  • (C) 4% A, 4% U
  • (D) 12% A, 12% T, 30% G, 46% C

[11:22] Bryan's Insights:

If you really paid attention then at this point, you would have immediately eliminated choices (B) and (C), which have U, since the question said DNA and uracil is only found in RNA. Uracil is not found in DNA.

The correct answer is (D). The trick with (A) is that when the A's and T's match each other, both 17%, and when the G's and C's match each other, both 33%, that's double-stranded.

In fact, this is a question that over 60% of students get wrong because they pick A right off the bat because they've been so conditioned to make the A's and T's and G's and C's all match each other. But again, they didn't answer the exact question they got asked.

[12:45] The Biggest Lesson

Probably the biggest lesson in all of standardized tests whether you're taking the MCAT, or the USMLE, or anything is to answer the exact question they asked you.

Another reason we always stress that even though you have to know the content for the MCAT, the MCAT is not a content-based test. This is proof to the study that came out in 2008 in a science article suggesting that the MCAT is the least content-based test out of all of the higher level testing. It is a reading and reasoning test that is incidentally about science rather than a recall test.

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Transcript

Introduction

Dr. Ryan Gray: The MCAT Podcast, session number 30.

A collaboration between the Medical School Headquarters and Next Step Test Prep, The MCAT Podcast is here to make sure you have the information you need to succeed on your MCAT test day. We all know that the MCAT is one of the biggest hurdles as a premed, and this podcast will give you the motivation and information you need to know to help get you the score you deserve so you can one day call yourself a medical student.

Welcome back to The MCAT Podcast, I am your host Dr. Ryan Gray, and I'm sorry for my voice, I'm sick. I've been sick for a long time. I have an almost three year old who's in preschool, and she brings home lots of bugs, and so I've been dealing with a raspy throat, so I apologize if it's bothering you, but I will fight through to get you this episode today. So session 30 here at The MCAT Podcast, we're going to jump right in with Bryan.

Alright last week we had a biology passage, this week we're going back to discrete questions. I know a couple weeks ago you said you cherry picked some of the more common questions that students got right, what are you going to do with us this week, Bryan?

Bryan Schnedeker: So with this set of questions what we're trying to illustrate is trying to avoid falling into trap answers. So each one of the questions here is a question that a good chunk of students got right, anywhere to 40% to 70% got right, but in each case there was one answer that a lot of students got trapped on. There was a really sizable minority that got trapped by a particular trap answer, and so we want to think about what's tempting about that, and how can we avoid it?

Dr. Ryan Gray: Alright.

Bryan Schnedeker: You want to hit us with question number 15 here, Ryan?

Question #1

Dr. Ryan Gray: Oh you're going to make me do these again, alright let's go. So 15. ‘The peptide bond that forms the backbone of proteins is especially stable because it A) consists of a triple bond which is significantly stronger and more stable. B) is a carboxylic acid derivative. C) would result in proteins that de-natured easily if it were unstable. Or D) exhibits resonance stabilization. I have no idea.

Bryan Schnedeker: Sure, yeah. No patient presents with an inflamed peptide bond, right? So you don't need to know that once we're actually doctors. So in this case this is a content fact that students have to know about peptide bonds, which is that they are amides, and that amides have resonance stabilization. So answer choice D is the right answer there, and in fact most students get that right, about 75% of students get that right. The trick here is answer choice B says, ‘is a carboxylic acid derivative,' and here's why that's a trick. It's true, a peptide is an amide, and an amide is a carboxylic acid derivative from amine, ano carboxylic acid forming an amide, and so a lot of students pick that. I mean almost the entire other 25% of students picked answer choice B, that it's a carboxylic acid derivative. And so the lesson that this question is teaching us, and we're going to see this again in some of these other questions, is it has to answer the question, not just be true. So yes a peptide bond is a carboxylic acid derivative, but that doesn't mean it's inherently stable, right? The question was, ‘is stable because.' And of course there's molecules like acyl halides, to a lesser extent, anhydrides. There are carboxylic acid derivatives that are not stable. So answer choice B, although true about peptides, doesn't answer the question they actually asked us.

Dr. Ryan Gray: Makes sense. They just- is that a reading issue and comprehension issue? Or is it an, ‘I'm trying to go too fast' issue?

Bryan Schnedeker: Yeah both, I mean you hit both of the things that happen there. Students get into a panic mode and they read B, carboxylic acid derivative, and they're just like, ‘Oh whatever that's right,' and they just kind of guess and they don't read all the choices, and so they move on. Or alternatively by the time they get to the end of the question and are reading choice A, they've already forgotten exactly what the question asked. And Ryan, you had asked I think last week or the last before about making sure you don't miss a keyword like ‘least' in the question stem, and the thing I mentioned at the time was you can highlight in the question itself, and you should. And so to make sure you're answering the exact question that they ask you, use the highlighter in the question.

Dr. Ryan Gray: Okay. Alright let's move on.

Question #2

Bryan Schnedeker: Sure. I'll go ahead and read 16. ‘In prokaryotes, genes can exist as operons that are transcribed into a polycistronic mRNA containing multiple genes in a single transcript. In eukaryotes, transcripts exist only as monocistronic mRNA containing a single gene. What fundamental genetic difference is responsible for this distinction?' So what genetic difference is responsible for this distinction between polycistronic mRNA in prokaryotes and monocistronic mRNA in eukaryotes? Choice A, mRNA is transported outside of the nucleus in eukaryotes. B, prokaryotic mRNA has a five-prime GTP cap. C, prokaryotes use a single start codon for multiple genes. D, in eukaryotes each gene has their own transcription initiation site. So here's another one where answer choice D is the right answer. In eukaryotes each gene has its own initiation site, and so one mRNA is transcribed off it for one gene, and that then is the right answer. Now again, a very common trap answer is choice A, mRNA is transported outside the nucleus in eukaryotes. That's true, absolutely true, right? You move outside the nucleus to make a protein, and prokaryotes don't have a nucleus so that is a distinction between the two. But just like we saw in 15, this is an example where the trap answer although true is not the answer to the question. The question was specifically getting at this difference between mRNAs that are poly versus monocistronic. How could you have multiple genes in one mRNA as opposed to a single gene in mRNA? And that has nothing to do with where the RNA is; is it in the nucleus or not? So again just like with 15, and really what we're going to see with all these questions, a key lesson on the MCAT, don't pick an answer choice just because it's true, pick it because it actually answers the question.

Dr. Ryan Gray: Think that's a good distinction and probably for you listening, if you've taken an MCAT before, I'm sure when you go and review those questions on that test and you have, ‘Oh that was a stupid mistake, that was a stupid mistake,' that's probably these types of mistakes that you're missing these questions. So good advice here.

Bryan Schnedeker: Absolutely. Yeah and when you have that, ‘Oh it was a stupid mistake' kind of reaction, I always push students to dig in. Don't just brush it off as stupid mistake. Why was it a stupid mistake so you don't do it again.

Question #3

Dr. Ryan Gray: Yeah. Alright question 17. ‘In miRNA,' is that how you say that? MI? I don't even know anymore.

Bryan Schnedeker: Sure, yeah.

Dr. Ryan Gray: Sure, miRNA. It's amazing how much you forget. ‘In miRNA directed gene silencing, a small RNA binds to an mRNA and directs degradation of the mRNA or prevents translation of the mRNA. Which of the following terms describes the process through which binding occurs? A, RNA polymerization. B, hybridization. C, elongation. Or D, transcription.'

Bryan Schnedeker: Ryan, do you remember these terms?

Dr. Ryan Gray: Negative.

Bryan Schnedeker: Okay there you go. So the question stem kind of yammers on for a bit, right? It says this happens, this happens, this happens, yada yada. But we've got to focus on exactly what the question asks. The process of binding, specifically one kind of RNA binds to another kind of RNA. So two nucleic acids come together, two single-stranded nucleic acids come together, like Velcro they stick to each other. Whether it's two DNAs, two RNAs, a DNA and an RNA actually doesn't matter, it's all the same thing. It's all answer choice B, hybridization. And notice it had nothing to do with the gene silencing, or all the funky little non-coding RNA processes that are hinted at in the question. Really this was just a definition. What do you call it when two nucleic acid strands glom onto each other? They hybridize. The trap here is again, answer choice, A, RNA polymerization. The word RNA shows up over and over again in the question stem, and it sounds like polymerization- ‘oh yeah things sticking together, that sounds like making a polymer,' and so students pick answer choice A. And so this is just a definition question. You have to know what RNA and DNA polymerization is, and that's when individual nucleic acid monomers get strung together into a whole big long RNA strand. Right? So when you transcribe mRNA from DNA, you are polymerizing a long RNA strand. Or when you do a replication for it and your DNA replicates, you are polymerizing new DNA strands. And so that's certainly something that happens, and it happens all the time, but it's not what the question asked about. The question was much more straightforward, basically gave us the exact definition of hybridization, and we just had to pick that out of the answer choices.

Dr. Ryan Gray: Okay. Moving on.

Question #4

Bryan Schnedeker: Yeah let's take a look at the last one here, number 44. ‘Several samples are analyzed for nucleotide composition. Which of the following compositions most likely represents single-stranded piece of DNA? Which of these is a single-stranded piece of DNA?' Okay let's look at the answer choices. A says, ‘17% A and 17% T, 33% G and 33% C.' Answer choice B says, ‘29% A and 14% U.' Now if you're paying attention at this point you immediately eliminate this choice because the question said DNA and uracil is only found in RNA, so choice B has got to be wrong. Then you go to choice C. ‘4% A, 4% U,' and now again you eliminate uracil, right? So B and C, easy to eliminate. Uracil is not found in DNA. And then we get to answer choice D. ‘12% A, 12% T, 30% G, 46% C.' So once again answer choice A said, ‘17% A and T each, and 33% G and C each.' And then answer choice D said, ‘12% A and T each, but 30% G and 46% C.' And so-

Dr. Ryan Gray: I know it!

Bryan Schnedeker: You do? Okay. Which one is it?

Dr. Ryan Gray: I think. I think it's D, although A could also be DNA. It says, ‘Which one most likely represents?' So I would have to go with D.

Bryan Schnedeker: Okay so Ryan you're absolutely right, D is the right answer, although when you were reading the question you actually skipped over the most important part of the question which was single-stranded.

Dr. Ryan Gray: Yeah, yeah that's what I meant.

Bryan Schnedeker: Yeah so the trick with A, choice A, is when the A's and T's match each other, both 17%, and when the G's and C's match each other, both 33%, that's double-stranded. And in fact this is a question that over 60% of students get wrong because they pick A right off the bat because they've been so conditioned to make the A's and T's and G's and C's all match each other. But again they didn't answer the exact question they got asked.

Dr. Ryan Gray: Yeah single-stranded. I didn't mention that when I talked about it, but that's why I picked D, single-stranded.

Bryan Schnedeker: Yeah there you go, perfect. So big, big, big lesson. Probably the biggest lesson in all of standardized tests whether you're taking the MCAT, or the USMLE, or anything. Answer the exact question they asked you.

Dr. Ryan Gray: I think it's why we always stress that even though you have to know the content for the MCAT, the MCAT is not a test based- it's not a content based test. And this proves without a doubt, right? This is proof, this shows why we say what we say and why the study that came out awhile ago, was that 2008, that science article that showed that the MCAT is the least content based test out of all of the higher level testing.

Bryan Schnedeker: Yeah absolutely, it is a reading and reasoning test that is incidentally about science rather than a recall test.

Final Thoughts

Dr. Ryan Gray: Alright I hope that was useful breaking down some of those common trap answers, helping you figure out why they are traps, and helping you avoid the trap. I hope you have a great week. If you enjoy this podcast, do me a favor, go tell a friend. If you don't listen to The Premed Years, then you don't know that one of my biggest mottos is collaboration, not competition. So if you're not telling your friend about The MCAT Podcast because you don't want him or her to have the same information, then shame on you. Share this information. Them doing well on the MCAT is not going to hurt you. You are competing against yourself when it comes to the MCAT, when you're applying to medical school, it's not- well it is a curved test so you may argue with me. It's not really curved, it's scaled, we'll put it that way. So anyway, go share this with a friend, with you premed advisor, with anybody. If you would like to leave us a rating and review in iTunes you can do that as well, but I'd prefer if you just shared it with a friend. Anyway I hope you have a great week, come check us out next week here at The MCAT Podcast.

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