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. Links: MedEdMedia Network Next Step Test Prep The Premed Years Podcast

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