Join us as we break down a Bio/Biochem passage from the Blueprint diagnostic exam! The topic is around Giardiasis and the intestinal flagellate protozoa that causes it.
Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.
[02:26] Tips for Answering Bio/Biochem Questions
George says you don’t need to know every little nitty-gritty detail about the science sections. It’s only the questions that they ask you.
If you have a structural idea of what’s being presented in each of these bio and chemistry passages, all you’ve got to do is look at the question. Go back and identify if you need a specific detail. A lot of questions are also pseudo discrete questions.
[04:18] Passage 2 (Questions 6 – 10)
Giardiasis, caused by the intestinal flagellate protozoaGiardia lamblia, is a common intestinal disease. Giardia are unique in theirpossession of two nuclei that are similar in appearance, DNA content and transcription. Usually G. lamblia aretransmitted by contaminated water with cysts. G. lamblia have sevencomplex genotypes, but onlytwo genotypes cause human infections. The illness is transmitted by differentiation into infective cysts. Three knowncyst wall proteins (CWPs) are highly synthesized in a concerted manner during differentiation into cysts.
Now we know they’re talking about a certain disease and a certain organism. Take note of the phrase, “possession of two nuclei” because a lot of genetics questions might come down to maybe ploidy of chromosomes. Then hone in on the idea of the cysts and then the genotypes.
Just keep in mind there are different genotypes. And specifically, there are two genotypes that lead to these cyst forms that lead to human infections.
[06:26] Paragraph 2
Encystation specific vesicles (ESVs) give the organism the ability to modify or partition cyst wall cargo during secretory transport. These organelles aid the accumulation and maturation of cyst wall material (CWM) composed of at least 3 cyst wall proteins (cwp1-3) complexed with β-(1-3)-N-acetyl-D-galactosaminepolymer. CWM protein is eventually deposited in the plasma membrane of the host’s intestinal cells. Trafficking of mature CWM from ESVs to the cell surface, where it forms the cyst wall, is rapid. Duringcyst formation, a round of DNA replication occurs. Then, nuclear replication takes place, with the DNA divided equally among the resulting 4 nuclei, followed by a second round of DNA replication.
Just keep track of new things that are being introduced here. So far, we have this idea of a polymer as well as of cyst formation.
There are a lot of other details but just highlight cyst formation. That way, if you need to come back, you know where to find it.
Then we move on to the figure below. Just get a feel of what it’s showing you and only come back to it if you really need it.
Figure 1 Regulation of cwp abundance within 2-hour transitions during the 14-hour encystation time-course (hpie = hours post initiation of encystations. N = total protein in sample)
[08:36] Paragraph 3
At any given time throughout the parasite’s lifespan,only one cwp gene is expressed.During an infection, however, the particular cwp gene that is transcribedregularly alternates in a process termed antigenic variation. Bothexpressed and silent cwp genes are located in the nuclear periphery, but in different specific regions. In expressed cwp genes, promoter histone H4 is acetylated on arginine 6 and trimethylated on arginine 2; in contrast, silent cwp genes include a trimethylated histone H4 at arginine 6. During the homologous recombination in cyst nuclei, type II topoisomerases assist in chromosome replication and tissue development for Giardia differentiation into dormant cysts. In addition, the levels of cyst formation and the cwp1-3 gene expression are increased by Topo II overexpression.
Just keep in mind that “only one cwp gene is expressed.” Zone in on the infection stage. Highlight “infection” and “regularly alternates.” Also, highlight expressed vs. silent, and if you need to come back, you can look for those differences.'The MCAT is good at dressing things up core concepts into big, ugly new scenarios.'Click To Tweet
George reassures students that you’re not expected to know anything about Giardiasis or G. lamblia, specifically. But they’re going to bring in this novel organism to throw you off.They’re putting these things in to throw you off.
So when you’re reading, look for those transition words. There’s that one contrast mentioned in the passage between expressed vs. silent.
If you go in with a structural approach, you can remember where the key details are so that if you do need to refer to them, you know where they are. If the question doesn’t ask you about it, there’s no value in figuring out all the specific details and understanding it. If they don’t give you points for it, you don’t need to know it.
In this passage, there are a lot of core concepts like expression, DNA, proteins, how pathways interact, as a lot of that fits in here. And then relate it back to your core concepts. You might be asked things about expression.
Start to anticipate some of the questions you might be asked. And that way, you have the confidence that even if you don’t understand 100% what’s going on, you’re not expected to. All you need to know is the same core elements that you’ve been studying for in your MCAT. And that’s enough. Learning the next step to apply it is something you need to practice.
[13:06] Question 6
What is the expected ploidy of a cyst that has completed an encystation?
There are a couple of content nuggets that you do need to bring into this question. Ploidy refers to the number of sets of chromosomes.
A 2n then means a diploid organism or eukaryotes. You have two sets of chromosomes, so one from mom, one from dad. Protozoa is a diploid organism, which means it has two sets of chromosomes per nuclei. We’re also told in the passage that Giardia are unique because they start regularly with two nuclei.
We start with two nuclei, each with 2n. So technically, we’re at 4n. We do this initial round of DNA replication. So now we’re at 4n each in our two nuclei. So we’re at 8n. We do this nuclear division, so we’re just breaking apart the sacks into four nuclei with 2n each. Then in the second round of DNA replication, all those 2n become 4n. So in the end, we have four nuclei with 4n each. Therefore, the total ploidy would be 16.
Correct Answer: D
[18:03] Question 7
The functions of the ESVs of Giardia most closely resemble what cell organelle?
According to the passage, the ESVs have the ability to modify or partition cell wall cargo during secretory transport. If you paraphrase this, it’s like packing, modifying, changing as you send things off. The whole function of the Golgi apparatus is the packing and sorting center. It’s the place where you add things and remove things.
Compared to other choices, lysosomes are these little packets that break things down. Nucleolus is the center in the nucleus itself. It’s responsible for the RNA transcription and processing, making those ribosomes. Then centrioles are like the fishing reels of the cell when you’re dividing. They’re at the opposite poles and then the microtubules attach and things move along them.
Correct Answer: B
[21:48] Question 8
Given passage information, Giardia most likely transports which of the following proteins external to the organism itself?
Go back to the passage that talked about transport. There’s one line that says, “CWM protein is eventually deposited in the plasma membrane.”
When we talk about which of the following proteins would go external to the organism, the only way it goes external to the organism is it passes everything. Then it ends up at the membrane so that it fuses and is shunted out.
So we need to think of something that goes to the membrane right and we look at CWM, it is not in our answer choices. But we know that CWM is made up of cwp1-3 proteins.
Looking at the other choices, β-(1-3)-N-acetyl-D-galactosamine is part of the cyst wall. It would never make it out to the actual extracellular space. The ESV is the packing and sorting center. It can’t actually be shunted out. Then histones like to bind with DNA. The histone modifications can cause the winding or unwinding of DNA so they would be located in our nucleus. Hence, they wouldn’t be shunted out.
Correct Answer: A
[25:18] Question 9
With which of the following do the arginine side chains found on histone proteins most likely interact?
A.Thymine groups on DNA
B.Adenine groups on DNA
C.Phosphate groups on DNA
D.Oxyribose on RNA
Arginine is one of those basic amino acids, which means at physiologic pH, it’s going to be positively charged. The positive charge wants to interact with negative charges, then phosphate has to be the correct answer because it has a negative charge.
Correct Answer: C
[28:24] Question 10
The data in the passage suggest that which time period during encystation is the level of protein dropping most quickly?
Going straight to the figure, pay attention to the axes, percentage of proteins. We see that in the 4-6 group, it has the highest amount of downregulation. We also look at our figure caption, and it says regulation of cwp abundance. So if we have more downregulation, it’s the percentage of proteins that are downregulated. If it’s more downregulated, you have less abundance, therefore less proteins.
Correct Answer: B
[32:44] Final Thoughts
George advises students that as you’re getting towards the end of the section, try not to worry too much about the last passage. Take a breath.'This test is about thinking and analyzing, not about knowing.'Click To Tweet
If you flagged some questions that you know if you had more time for you could work on, keep them in the back of your mind. If you have more time, then come back.
Make sure you answer every question. Never leave a question blank because even if it’s a guessed correct answer, you still get the point. You get no penalties for getting it wrong. So answer every question. Take a deep breath, take that leap of faith, and start the next passage with a fresh mind and approach.