This week we’re starting a new series, breaking down questions from Next Step Test Prep‘s full-length 10. We go from the very beginning until the end. We start with the first passage and question 1. We can’t cover every question each isn’t specifically suited for a podcast. So we’re skipping questions but you’re going to get a lot of them.
The Premed Playbook: Guide to the MCAT is finally available in Paperback ($9.99) and in Kindle ($4.99) formats. Also, stay tuned for The Premed Playbook: Guide to the Medical School Personal Statement, coming out soon!
Check out Session 288 on The Premed Years Podcast where we had the Director of Admissions for the University of Illinois Medical School. We talked all about how they take an application from A-Z, who gets accepted, who doesn’t, and why. We discuss how they look at everything along the way. You will find all of other podcasts on MedEd Media.
If you want to get access to Full Length 10, use the promo code MCATPOD to save some money upon check out. Basically, these are grab bag questions that we’ll be covering over the course of several months in this podcast. We will really walk you through the Full Length 10.
[03:55] Passage 1, Question 1
Sodium dodecyl sulfate, also known as sodium lauryl sulfate, is an anionic surfactant commonly used in household cleaning and personal hygiene products. In biochemistry and biotechnology, its most frequent use is to denature or linearize proteins and negatively charged the resulting fragments. Proteins treated with SDS can be linearized in a similar fashion to proteins subjected to acid-catalyzed hydration. The mechanism of action is different. However, scientists utilizing SDS for this purpose often run into difficulties with proteins that have greater hydrophobic content, typically those found in the context with regular exposure to surfactants.
Question 1: Which of the following proteins would likely be most accurately analyzed using SDS-PAGE? (This is discussed in the passage but a good MCAT student should be able to recognize your SDS-PAGE backwards and forwards before you even walk in on test day.)
[06:10] Answer Choices:
If we’re looking for most accurately, we want the ones that have very little hydrophobic content.
- (A) Protein A: a small protein with many hydrophilic surfaced structures typically found free-floating in the bloodstream of mammals.
- (B) Protein B: a large protein found in the internal portions of the phospholipid bi-layer of human white blood cells.
- (C) Protein C: a large protein found almost exclusively in reptilian adipose tissue.
- (D) Protein D: a small protein observed experimentally in the surviving portion of bacterial populations that have been exposed to high doses of surfactants.
[08:10] The Breakdown
The inside of a phospholipid bi-layer, the inside of the cell membrane, is the fatty portion, the hydrophobic portion. And C is the adipose tissue, it’s fatty, hydrophobic. So both B and C are hydrophobic. It doesn’t matter what the question says. When two answer choices say the same thing, they’re both wrong.
A is hydrophilic, while B and C imply hydrophobic. So B and C are out. Then D mentions high doses of surfactants. The passage says that scientists run into difficulties with surfactants. So D is out as well.
[09:05] Question 2
Question 2: Chiral manipulation of which of the following amino acids could theoretically result in a meso compound.
- (A) I only
- (B) II only
- (C) II and III only
- (D) None of the above
[09:28] The Breakdown
The reference to amino acids came a little bit later in the passage (not mentioned above). But you just have to know your amino acids from outside knowledge. This is when you should be able to know the answer even without seeing the passage in front of you.
Again, on one level, there’s no such thing as high yield. You have to know everything. (*Except for one high-yield thing – amino acids.) Every single MCAT given has at least one, if not two to four questions that just ask about amino acids.
In this case, you have to know what a meso compound is. Then you have to know your amino acids so you know which one could be a meso compound.
A meso compound is a compound that has two chiral centers with an internal plane of reflection. So if you put a mirror in the middle of the molecule, it would reflect on to itself. Classically, the human body is like a meso compound where if you put a mirror down the middle of the human body, one side would reflect onto the other side.
As a rule, amino acids only have one chiral center. Therefore, normal amino acids don’t fit into this definition. The only amino acids that have a second chiral center are isoleucine and threonine. Students should know these are the two that have the side chain that have a chiral carbon in them. even so, they wouldn’t still have an internal plane of reflection because the other chiral center wouldn’t map in the reflection way back onto the main chiral center of the alpha carbon.
So no amino acid could be a meso compound. So the correct answer here is D, none of the above. If you had no idea at all and if you were completely lost, you should guess answer choice B only. Proline is one of those weird amino acids. You’d get it wrong here but that’s a decent guessing strategy.
[12:10] Question 4
Question 4: This passage states that scientists utilizing SDS-PAGE often run into difficulties with proteins that have a greater hydrophobic content, typically, those produced by organisms in context where surfactants are regularly encountered. Which of the following is a reasonable explanation of why this might be the case?
- (A) The negatively charged sulfate head on the end of SDS cannot easily penetrate hydrophilic regions of the proteins to disrupt non-covalent interactions, thereby inducing higher variability in the ratio of bound SDS to the target protein molecule.
- (B) The negatively charged sulfate head on the head of SDS cannot easily penetrate hydrophobic regions of the protein to disrupt non-covalent interactions, thereby inducing higher variability in the ratio of bound SDS to the target protein molecule.
- (C) The negatively charged sulfate head on the end of SDS cannot easily penetrate hydrophilic regions of the protein to disrupt non-covalent interactions, thereby inducing higher variability in the ratio of unbound SDS to the target protein molecule.
- (D) The negatively charged sulfate head on the end of SDS cannot easily penetrate hydrophobic regions of the proteins to disrupt non-covalent interactions thereby inducing higher variability in the ratio of unbound SDS to the target protein molecule.
[14:00] The Breakdown
Sating it’s a charged molecule, whether it’s negative or positive, you should be able to analyze what it can penetrate versus cannot. Choices A and C say that negatively charged cannot penetrate hydrophilic. And then choices B and D say negatively charged cannot penetrate hydrophobic.
Think about a table salt. Things that are charged like to dissolve in water, just like table salt dissolves in water. So negatively charged can penetrate a hydrophilic region. Salt charge things. They love water and so they’re hydrophilic. Hence, right off the bat, A and C are out because charged things can go to hydrophilic regions.
Whereas, B and D got it right. So now we look at your bound and unbound. This is just a simple recollection of the mechanism, how you run an SDS-PAGE. The SDS binds to the protein and coats it. Then you take that solution with proteins, wearing an SDS coat. And those are the things you run through the gel. So you only care about the SDS that actually got coated on the protein. So the answer here is B.
[18:30] About Next Step Test Prep
If you’re on the market for full length practice tests for MCAT scores, go check out Next Step Test Prep. Their full length exams are getting awesome reboot where you get more in-depth answers, actual access to snippets of information from their textbooks to give you more knowledge so you don’t have to go bouncing around to different resources.
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