Dissecting MCAT Biology Discrete Trap Answers

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Dissecting MCAT Biology Discrete Trap Answers

Session 30

This week on The MCAT Podcast, we go over some MCAT biology questions on Blueprint MCAT (formerly Next Step Test Prep) practice tests where students commonly fall into traps.

In fact, a good chunk of students got these questions right (40%-70%). But in each case, although it’s just a really sizable minority, there was one answer that a lot of students got tricked into choosing. So now we’re going to help you avoid being tricked yourself.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[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 question that requires that students 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,” and in a sense, that’s true. A peptide is an amide, and an amide is a carboxylic acid derivative made from an amine and a carboxylic acid. 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 need to answer the question, and not just select a true statement.

Yes, a peptide bond is a carboxylic acid derivative, but that isn't what makes it stable. Click To Tweet

Again, yes, a peptide bond is a carboxylic acid derivative, but that isn’t what makes it stable. This is crucial because the question specified, “…is inherently stable because.” And of course, there are carboxylic acid derivatives like acyl halides, and to a lesser extent, anhydrides, that are not stable. So answer choice (B), although true about peptides, doesn’t actually answer the question that’s being asked.

Read Questions Carefully on the MCAT

This is both a reading comprehension issue and an “I’m going to fast” issue. Students can get into a panic mode, not read all the answer choices, guess one, and move on. Maybe by the time they get to the end of the question and start reading the answer choices, they’ve forgotten exactly what question was asked.

Make sure you don’t miss an important word like “least” in the question, which you should use the on-screen highlighter to highlight. Make sure you’re answering the exact questions being asked. Use the highlighter to highlight the exact question.

Make sure you're answering the exact questions being asked on the MCAT.Click To Tweet

[Related post: Best MCAT Course (with a Promo Code).]

[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 its own transcription initiation site.

[05:47] Bryan’s Insights:

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

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 the “difference between mRNAs that are polycistronic versus monocistronic.” That is, how could you have multiple genes in one mRNA as opposed to a single gene in mRNA? And that has nothing to do with whether the mRNA 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, when you’re reviewing your practice tests and you see a mistake like this, don’t brush it off. If you have that “it was a stupid mistake” kind of reaction, always dig in a little more. Find out why it was a stupid mistake, so you don’t do it 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.Click To Tweet

[Related episode: How Many MCAT Practice Tests Should I Take?]

[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 in the question, but you have to focus on exactly what the question asks, which is about the process of binding, specifically one kind of RNA binding to another kind of RNA.

Two nucleic acids come together, and like Velcro, they stick to each other. Whether it’s two DNAs, two RNAs, or 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 gene silencing or all the funky little non-coding RNA processes that are hinted at in the question. This was really just a definition question. What do you call it when two nucleic acid strands glom onto each other? Hybridization.

What do you call it when two nucleic acid strands glom onto each other? Hybridization.Click To Tweet

The trap here is answer choice (A), RNA polymerization. The word RNA shows up over and over again in the question, and it sounds like polymerization. Things sticking together—that sounds like making a polymer—and so students pick answer choice A.

But 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 long strand. So when you transcribe mRNA from DNA, you are polymerizing a long RNA strand. Or when your DNA replicates, you are polymerizing new DNA strands.

That’s certainly something that 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 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’re really paying attention, then 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.

[Related episode: What Is the Best Way to Learn MCAT Testing Strategy?]

[12:45] The Biggest Lesson from these Tricky MCAT Questions

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

Probably the biggest lesson in all of standardized testing is to answer the exact question they asked you.Click To Tweet

This is 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. A study came out in 2008 that said that the MCAT is the least content-based test out of all of the higher-level tests. The MCAT is a reading and reasoning test that is only incidentally about science.

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