Session 217

In this week’s episode, we jump back into the Bio/Biochem passages after a round of discretes. This passage is full of genetics and DNA content.

We’re joined by Madeline from Blueprint MCAT. If you would like to follow along on YouTube, go to premed.tv. If you haven’t yet, check out their new live online course. For other premed resources, find them on Meded Media.

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

[02:49] General MCAT Tips

Take note that we’re going to be reading this paragraph by paragraph. And then after we read the paragraph, we’re going to be going back and highlighting the things that we think are most important.

This technique is helpful because we only highlight the nuggets of information that are going to be important within that paragraph.

[03:18] Passage 6 (Questions 31-34)

Paragraph 1

Note: Important words to be highlighted are in bold.

Recent research was conducted into organellar DNA and cytoplasmic inheritance. Laboratory mice were observed over a few generations. First-generation female individuals, as well as all of the offspring of these individuals, presented with subacute necrotizing encephalomyelopathy and lactic acidosis characteristic of Leigh syndrome, a cytoplasmically inherited disorder.

Some of the mice were transfused with genetically modified erythrocytes which contained genetic code resulting in secretion of the compound EPI-743 into the blood plasma. EPI-743 has been shown to lessen the severity of Leigh syndrome. Other mice were genetically engineered to contain this genetic code in gametes. Production of these gametes was observed at multiple stages through optical microscopy, and the figures below reproduce these observations.

The researchers used the observations contained in the above to track time-based movement of and interactions between homologous chromosomes. Earlier research has indicated that a protein lattice known as the synaptonemal complex is responsible for physical connection of paired homologous chromosomes at points known as chiasmata, and that this synaptonemal complex is responsible for helping to limit double-strand breaks during meiosis to occurring between homologous chromosomes rather than between sister chromatids.

Figure 1 Observation using optical microscope, time = 1 hour

The passage then goes to present these four different figures (Figure 1 through Figure 4) showing the results they got from this time-movement experiment where they follow things at 1 hour, 2.5 hours, 5 hours, and 7 hours) just to see the movement of these chromosomes.

[07:10] Question 31

What is the most likely transmission mechanism for the passage of Leigh syndrome from parent to child?

1. Cytoplasmically inherited RNA
2. Cytoplasmically inherited transcription errors
3. Mitochondrial DNA
4. Nuclear DNA

Reworded question: How do we pass down our information from parent to child?

Thought Process:

We inherit DNA. So you can look at this through the process of elimination. You just have to look at DNA, and that it has to be somewhere in the cytoplasm. So you know it’s going to be C.

A and B – Neither of these really talked about DNA so we can cross both out.

And you’re looking now at C and D, which has to do with either mitochondrial DNA or nuclear DNA.

C – Your mitochondrial DNA comes from your mother’s. And then if you look at that other part of what we highlighted in the first paragraph. It says, “first-generation female individuals, as well as all their offspring of those individuals,” so it does mention female. So then you know your mother is going to give you the mitochondrial DNA.

D – It says “cytoplasmically inherited” disorder and nuclear DNA isn’t in the cytoplasm. The only thing between those two that can be in the cytoplasm is your mitochondria.

Correct Answer: C

[11:40] Question 32

The researchers participating in the experiments described above were assigned to conduct new experimentation, also on Leigh syndrome-positive mouse specimens. As a precursor step, it was decided to create a new population of Leigh syndrome-positive mice. What would be the option most likely to be successful in creating this population?

1. Induction of intentional mutations in the mitochondria of pre-fertilization ova
2. Induction of intentional mutations in the mitochondria of pre-fertilization sperm cells
3. Induction of intentional mutations in the mitochondria of either type of gamete
4. Induction of intentional mutations in the mitochondria of synaptonemal complexes

Reworded question: How do we create Leigh syndrome-positive mice?

Thought Process:

When they are talking about Leigh syndrome in that first paragraph, it says that the first generation has to be female, and then you go to the offspring. So that’s the only point where they really talked about the creation of Leigh syndrome. And so, really lock on to those words of first-generation female, and then all the offspring.

A – Ova is going to be your eggs, which are from your female. And we already said that this disease is going to be mitochondrial DNA, which only can come from the female. So it’s already looking pretty good.

B – Sperm cells are from the father or the male participant.

C – So here again, sperm cells wouldn’t make sense. We want it to be in the female cell, and specifically, in their mitochondria. So it can’t be either type. It has to be the female. C is basically saying A and B are both correct. But we already said B is incorrect. So C is not the correct answer either.

D – We already talked about how if we want to induce this cell type, we need to have an inheritance pattern from the female. Nothing in D talks about that. That being said, synaptonemal complexes are made out of the chromosomes and have nothing to do with the mitochondria. And because of that, this one also cannot be right.

Correct Answer: A

[14:22] Question 33

Which of the following represent differences between growth of cell cultures containing cytoplasmic organelle disease versus genetic transmission of nuclear DNA from parent to offspring?

1. Meiosis maintains the genetic integrity of growing cell cultures, while mitosis arranges nuclear DNA into a form which is transmissible to offspring.
2. The process of arranging nuclear DNA in preparation for transmission to offspring is a cyclical process, while the growth of cell cultures is carried out by a noncyclical process.

III. Growth of cell cultures requires transformation of diploid cells into haploid cells, while genetic transmission of nuclear DNA to offspring requires transformation of haploid cells into diploid cells.

1. I only
2. I and II only
3. II and III only
4. None of the above

Reworded question: What’s the difference between nuclear DNA transmission like meiosis, and your mitochondrial transmission? So what’s happening with the DNA in mitochondria?

An example of flagging a question:

If you feel this question is too overwhelming, then this would be a perfect place to flag and move on. Just look at the answer choices real fast and choose the answer choice that comes up the most often. So if you had an extra two or three minutes at the end of the section, you could just come back to it.

This isn’t a hard and fast rule. It’s more meant to save time because it can allow you to basically skip some evaluation steps if you’re able to go through the answer choices a little bit more efficiently.

Thought Process:

I – This has been mentioned twice. Just seeing that “meiosis maintains the genetic integrity” is incorrect right off the bat, you don’t even have to finish reading the sentence. Because meiosis is meant to change things up and to add in some recombination or variation into the genome.

Because of that, we can already cross out answer choices A and B.

II – Meiosis isn’t a cyclical process. In fact, it’s a non-cyclical process. So again, this answer choice is wrong and you don’t even have to read the rest of it because you already know that this first part is incorrect.

So both I and II are incorrect.

And if you look at the answer choices, all of them have to have I or II, except for D. So because of that, you don’t even have to evaluate number III. And you could just choose D and skip your evaluation of III.

This is where that Roman numeral strategy comes in handy is it can help you save some time and some effort in combing through all of those words.

Correct Answer: D

[22:28] Question 34

What aspects separate single-crossover events from double-crossover events?

1. Single-crossover events result in one-way displacement of chromosomal content from one chromosome to another, while double-crossover events always reverse this one-way displacement, resulting in chromosomes identical to the pre-crossover chromosomes.
2. Single-crossover events occur during mitosis when a cell splits into two cells, while double-crossover events can only occur during meiosis when a cell splits into four cells.
3. Single-crossover events affect only the ends of chromosome arms, while double-crossover events can affect segments in the middle of chromosome arms.
4. Single-crossover events only affect one arm of each chromosome, while double-crossover events affect two arms of each chromosome.

Thought Process:

This is a perfect example of how prediction can really help you. Because if you know what single and double crossover events are, then you can go through these knowing what they are.

The single-crossover event is basically where you have two strands of DNA next to each other. And in both their double-stranded forms, they dissociate and two of the strands cross in order to have a combination of the two lines.

The double-crossover is where two things crossover. So we want to look for something that has one thing crossover versus two things.

Let’s comb through the answer choices:

A – Even though the process sounds right, where you have one thing crossing over and then two things crossing over, There are two things in this answer choice that scream, maybe not. First is: “identical to the pre-crossover.” It doesn’t make sense because crossovers are meant to add in genetic variation, so identical doesn’t seem like that would ever be a process.

Another is “always reverse this one-way displacement,” which is also a really strong word. So even if you didn’t quite know what was happening here, those two words just don’t make sense in the context. So we could cross out A based on that.

B – Single-crossover adds in genetic recombination and adding a variation to the next generation. But mitosis doesn’t have anything to do with the next generation. It has to do with who you are as a human being or as a cell right then and there. And so because that first part talks about a single-crossover in mitosis, then that’s just wrong. So we’re going to cross this out too.

C – It doesn’t make sense that once you’ve crossed them over once, the rest of it is going to be different than when it started. But if you pass it over twice, you’re just going to have that weird segment that has been changed. But then everything else is going to be in its natural kind of pairing. So C makes sense.

D – Both single and double crossover events will only affect one arm of each of the chromosomes, neither of them affects both arms. So that’s an incorrect fact as well.

Correct Answer: C

[25:51] Working on Your Weaknesses

The questions were very kind of pseudo discretes, where you have to bring in a lot of outside knowledge. For example, the single-crossover and double-crossover weren’t specifically defined in the passage, as well as mitosis and meiosis. So you have to know what those things are and where they’re important.

For students who skip a lot of content review, but use full-length exams for practice, this is where review comes in handy.

Then they would somehow have an idea of which things they need to learn because they’re obviously important things that the AAMC wants them to know.

A really good reviewing technique is if you get something wrong, cover up all the answer choices and which ones were right and explanations. And try to answer why the correct answer is correct on your own, without using the crutch of whatever test company gives you that information.

And if you’re able to get to the right answer just using logic or reasoning, then you know that there’s a gap in your reasoning skills. However, if you reach a brick wall of content, you suddenly know your issue is content. Then you would know which of them you need to address.

Links:

Meded Media

Blueprint MCAT

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