In this episode, we tackle the first bio/biochem discrete question set. We evaluate peptide bonds, solute concentrations, gene transcriptions, and more!
Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.
[01:41] Tips for Handling Discrete Questions
Generally, there are usually two discrete groups in the science passages. This is true for Chem/Phys and psych/social as well. And it’s usually the same number of discrete questions as you would have in a passive question. So you usually see three to four questions per passage, and three to four discrete questions per discrete chunk.
Joya advises in handling discrete questions is to treat it the way you treat running. Because you will be running differently when you’re on flat ground versus when you’re on a hill.
Hills would be the reading comprehension passages in the sciences, for some students, because they find that passages are harder than discrete questions. So you’re in a hill-running mode when you’re on a passage. Then when you get to the discrete, the course is leveling out a bit. There’s no trick in the passage. So you’re going into pure knowledge mode and pure recall.
[04:25] Question 14
The peptide bond that forms the backbone of proteins is especially stable because it:
- consists of a triple bond, which is significantly stronger and more stable.
- is a carboxylic acid derivative.
- would result in proteins that denatured easily if it were unstable.
- exhibits resonance stabilization.
A peptide bond is an amide linkage between the anion group of one and the carboxylic acid group of another. So it’s the C=OH on one end and the NH2 on the other. Then you’re going to lose a molecule of water, and you’re going to get a bond between your C and N. That’s your linkage. It’s an amide linkage which is a peptide bond.
A – You might actually want to draw it out just so you can see what it looks like. And so, you know that you’re going to not have a triple bond, right off the bat. Then both carbon and nitrogen are involved in the bond are bonded to two other things. And that’s too many bonds. So there’s no triple bond and an amide. And for this reason, we cross out A.
B – there is a carboxylic acid involved in a peptide bond. That’s the carboxy terminus of the amino acid. That’s your C=OH. But that doesn’t make things stable. It makes them reactive. Thinking back to your chemistry, no acid halides are super reactive. And so this is a true statement. But it does not answer this question. The carboxylic acid derivative is not a marker of stability, but a marker of reactivity. So B is out.
C – This is giving us the opposite of what we want. Now, the point of the question is that proteins are stable because peptide bonds are stable. Not proteins are unstable because peptide bonds are unstable. So this is just the opposite of everything. So C is out.
D – Just by the process of elimination, this is the correct answer. But if you look at it, resonance stabilization makes sense. There’s a lone pair of nitrogen. There’s a double bond on the carbonyl. There are lots of places for electrons to move around resonance structures, and having more than one of them typically indicates stability.
Resonance gives a more stable compound so resonance stabilization is a thing in and of itself. And then remembering what the peptide bond has which is carbonyl carbon and nitrogen with a lone pair. Those are two very common contributors to resonance. So that all meshes together very nicely and tells us the correct answer is D.
Correct Answer: D
[08:40] Question 15
Drinking ocean water is ultimately fatal to a human because:
- the water has a very low solute concentration relative to the body’s cells, resulting in cell swelling and death.
- the kidney must work very hard to excrete the excessive levels of bivalent ions in the ocean water, causing kidney failure.
- the water has a very high solute concentration relative to the body’s cells, resulting in cell shrinkage and death.
- ocean water contains toxic levels of environmental pollutants that can damage organs or cause cancer, leading to death.
This is that hypertonic-hypotonic isotonic solution question. It’s saying what happens when you stick a cell in a solution that has more soluble than it does. Ocean water is salty.
A – It’s not a low salt concentration, that’s the opposite. So we cross this out. The definition of what would happen is correct, but that’s just not what ocean water is. So it’s true that cell swelling and lysing could happen if I went into a very low salt concentration, but that’s not saltwater.
B – Kidney working hard sounds right, but then we get to bivalent ions. But in salt water, we have sodium and chloride, which are monovalent ions.
C – This is what makes sense.
D – This is very specific to where you are in the ocean.
Correct Answer: C
[11:24] 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?
- mRNA is transported outside of the nucleus in eukaryotes.
- Prokaryotic mRNA has a 5’ GTP cap.
- Prokaryotic ribosomes differ from eukaryotic ribosomes.
- In eukaryotes, each gene has its own transcription initiation site.
A – Prokaryotes do not have organelles that are membrane-bound. There’s no nucleus, so there is no mRNA being transported outside of the nucleus and carry out. But I don’t think that has anything to do with the number of genes per transcript. So this is not relevant. That the mRNA leaves the nucleus and eukaryotes has nothing to do with how the transcript is made, or how many genes per transcript there are. So this is again, just like the last one where it’s true, but it isn’t answering the question.
B – Prokaryotic mRNA does not have a cap. So this is just factually inaccurate. Eukaryotes have the cap. Prokaryotes do not. That’s why bacteria mutate so quickly. And we see viruses and bacteria, and all these kinds of things, transforming rapidly all the time. And we have to keep making new things for them because they don’t have that kind of editing. We have a lot more editing on eukaryotic mRNA.
C – Just like choice A, this is a true statement. Prokaryotes are that 70S ribosomes, they have two subunits of 50S and 30S respectively. Eukaryotes are 80S and they have 60S and 40S respectively. But then again, that doesn’t have anything to do with making mRNA. Ribosomes come later in the process. They’re involved in the translation of a transcript, not the production of a transcript. So again, it’s a true statement, but it does not answer the question.
D – There’s a transcription site, a place where it’s supposed to start for each individual gene. And that makes sense.
Correct Answer: D
[16:22] 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?
- RNA polymerization
All this stuff about directing degradation or preventing translation, that’s completely irrelevant. All they’re telling us is that there’s a little RNA that binds to a bigger RNA. There’s binding between two strands of RNA, a little one and a big one. And then whatever happens after is whatever happens after.
But the real question is, what is it called when two strands of RNA bind together?
A – RNA polymerization is when nucleotides are strung together to make one single-stranded RNA strand. So it’s the thing that’s happening during transcription. So the RNA strand is being polymerized. There’s adding that happens. Polymerization in general is when you add little things together to form one thing. And so that is not about two complementary strands joining. It’s just about that one strand getting made.
B – When we get a hybrid of two strands, it’s what we think of when we have either DNA and a DNA, or an RNA and RNA, or DNA and RNA. It’s a hybrid molecule of two strands coming together. So that’s exactly what’s happening here.
C – Elongation is exactly what it sounds like, which is you make something longer. It’s not again, about two things coming together. It’s about one thing getting longer.
D – Transcription is the process of turning DNA into RNA. It’s not about two types of RNA coming together.
Correct Answer: B
[19:17] Final Thoughts
Your emotional response to a really challenging test is not really indicative of how you did on the test. So just let the score tell you how you did. Don’t void just because a discrete section didn’t go well.
It sounds like a myth. And then when you start doing it in real life, you realize it’s actually possible. When the hill is so long that you can’t see the top of it, don’t look for the top, look two feet in front of you. And stop thinking about getting through the whole thing or making a really good score or any of that. Just focus on one foot in front of the other.
Answer every question on the MCAT because there’s no point in leaving them blank. Maybe you can come back later. Because sometimes you learn something from a passage that actually answers a discrete question. It happens.
And finally, always remember that whatever happened in the past is in the past. You’re just going to move forward. And sometimes that means you don’t think about anything but the question in front of you. Because when you think too big, you think too far ahead. Then it starts to feel unmanageable when something goes awry, or something doesn’t go according to plan. So scrap the plan one step at a time.“Everybody has a plan until they get punched in the face. And then it's just about making it through whatever you need to get through for the next six questions.”Click To Tweet
Remember, you do have strengths even if they’re not apparent in a given moment.
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