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We Break Down Discrete Biology MCAT Questions

We Break Down Biology Discrete Questions for the MCAT

Session 28

This week, we are breaking down discrete biology questions from the MCAT. These questions are taken directly from the Next Step MCAT Full-length Exams.

Everyone knows that biology questions on the MCAT can really test your application and recall of information. But what we’re going to look at today are the more straightforward questions, questions that 80% (or even more) of the students are getting right in the Next Step Test Prep system.

These are not necessarily easy questions—they’re only easy if you know the answer. But the point of this is just to illustrate that in the competitive world of MCAT prep, you can’t leave any of these points on the table.

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

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[02:22] MCAT Biology Question #1:

When normal human cells are grown in culture, they will divide a limited number of times, typically 50 rounds of mitosis. After this number is reached, the cells become apoptotic. This cell death is a result of:

  • (A) Decreasing number of membrane-bound organelles per cell
  • (B) Decreasing number of non-membrane bound organelles per cell
  • (C) Decreasing levels of growth hormone
  • (D) Chromosomal telomeres shortening after each round of division

[03:07] Bryan’s Insights:

The correct answer is (D), and this is just an important bio/biochem genetic fact that the aging process in cells is due to telomere shortening.

There is actually a lot of interesting science behind the prevention of telomere shortening to hopefully allow us to lengthen our lives. Why can’t we just go in there and make our telomeres longer? Of course, uncontrolled chromosomal alteration and lengthening is what we know as cancer. So you can’t just go in there and muck about with no problems.

The aging process in cells is due to telomere shortening.Click To Tweet

[03:57] MCAT Biology Question #2:

A student finishes an experiment involving several bacteria which are highly pathologic in humans. She was to dispose of the agar plates and micropipette tips she used. Which of the following procedures should she carry out?

  • (A) Microwave all materials for more than 60 seconds.
  • (B) Wipe down on materials with 100% ethanol solution.
  • (C) Place all materials in a biohazard bag and autoclave the bag.
  • (D) Place all materials under UV light for 90 seconds.

[04:48] Bryan’s Insights:

This is one of the classic lab procedure questions where you have to know the mechanics of life in the lab. This is one of the hallmarks of the new version of the MCAT as opposed to the old version, which was more textbook-based. Basically, a lot of these questions want to assess if you have spent time in the lab recently and you know how it works.

In this case, if you ever have biohazards you need to dispose of, you absolutely have to autoclave it. There is no second-best choice or alternative to it. Every lab in the universe has an autoclave for a reason, and it’s to dispose of biohazards.

Hence, the right answer is (C). Place all the materials in a biohazard bag and autoclave the bag.

Every lab in the universe has an autoclave for a reason, and it's to dispose of biohazards.Click To Tweet

[Related episode: Breaking Down an Organic Chemistry MCAT Passage.]

[05:36] Biology MCAT Question #3:

In a population of Amish people, the frequency of the recessive autosomal allele for polydactyly is 1.2%. What percent of the population are carriers for this gene?

Bryan’s Insights:

This is actually the hardest one of the set of questions we’re going to look at. A little less than 70% of the students got this right. This is what they call a Hardy-Weinberg Equilibrium. There are a bunch of percentages listed as answers, and we’re told that the recessive allele has a frequency of 1.2%.

Since all of the recessive and all the dominant alleles in the universe have to add up to 100%, that means the frequency of the dominant allele is 98.8%.

All of the recessive and dominant alleles in the universe have to add up to 100%. So if you're given the percentage of one, you can find the other.Click To Tweet

The extra bit is remembering what the mathematical term is for the carriers in the Hardy-Weinberg Equation, which is usually expressed like this: p2 + 2pq + q2 = 1 or 100%.

And so to be a carrier, to be a heterozygote, to have both of the genes, that is that term in the middle of the equation, usually written as 2pq, where 2 is just the number two, p as the percentage of dominant, and q as the percentage of recessive.

In this case, the dominant was 98.8% or 0.988, while the recessive was 1.2% or 0.012. With MCAT calculations, you don’t have to go into great detail, so you can just round things off. So 2pq is 2 x 0.988 x 0.012. And that 0.988 can be rounded off to 1. Now the equation becomes 2 x 1 x 1% roughly. In other words, this gives us a total of 2%.

You can ignore all those other decimals and all the fiddly bits and just say, it’s going to be about 2% of carriers. And when you look at the answer choices, they’re really spread out like (A) 0.01%, (B) 1%, (C) 2%, (D) 97%. (There are decimals here, but we’re just going to ignore them.) Since answer choice (C) is the only one that’s anywhere close, a little over 2%, the correct answer is C.

[08:39] When Can You Round Off Calculations on the MCAT?

When to round off calculations on the MCAT entirely depends on how spread out the answer choices are. You will see some physics problems where the answer choices are literally orders of magnitudes apart from each other. In that case, you can afford to round off aggressively.

Here, where the answers were 0.01, 1, 2, and 98, even the two answer choices that were closest together (1 and 2) is a 100% difference. So you can round this off pretty aggressively, too.

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

[09:30] MCAT Biology Question #4:

Species that are capable of both sexual and asexual reproduction will typically prefer sexual reproduction because it:

  • (A) increases the likelihood of each individual offspring surviving.
  • (B) increases the likelihood of beneficial mutations
  • (C) creates more variation in the next generation
  • (D) takes less time to complete

Bryan’s Insights:

The correct answer here is (C). About 10%-15% of students pick (A), but evolution does not work on the level of individuals. Evolution works on the level of whole populations and their likelihood of survival, not one individual necessarily surviving.

Sexual reproduction is beneficial because it creates more variation in the next generation.Click To Tweet

When we think about the genetics on the MCAT, it’s not really that you’re getting an ecology question or an evolution question. You simply need to understand the mechanics of that genetic reshuffling that happens during sexual reproduction.

Why would animals go through so much effort like peacocks growing enormous tails and bowerbirds building those huge nests? The point of these enormous metabolic costs animals put into the dance of sexual reproduction is so that the next generation of animals has more variety and is more adaptable to changing environmental conditions. In fact, that variety is so critically important that sexual reproduction strategies are vastly more successful.

The point of the enormous metabolic costs animals put into the dance of sexual reproduction is so that the next generation has more variety and is more adaptable to changing environmental conditions.Click To Tweet

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