We’re back with some more Next Step Test Prep full-length breakdown with Clara. This week, we’re covering passage 1 of the biology and biochemistry section of the MCAT. Meanwhile, check out all our other podcasts on the MedEd Media Network.
[01:48] How to Prevent Panic Attack from Biochem Passages
Some passages can really seem dense. The most important thing to do is to just know your content and what you’re supposed to know. Know that everything else is going to be in the passage somewhere.
Clara explains that there are a lot of pseudo-discrete questions which means you can answer even if you’ve never had the passage at all. You will see some of these in Biochem, although it’s a little based on critical reading. It’s based on experiments, processes, pathways, or details where you need to understand what you read.
[03:50] Passage 1:
Bipolar disorder is characterized by periods of both mania and depression and researchers are considering both melatonin therapy and aerobic exercise as treatments for the manic phase of the disease. Myshkin mice and plus, a serginic type, are heterozygous from an autosomal missense mutation in the A3 isotope of the neuronal sodium-potassium pump. And serve as a model for the manic behaviors typical of bipolar disorder. Researchers divided the group of wild-type WT and Myshkin MYK mice into several cohorts and assessed the effects of melatonin treatment and voluntary exercise which was unlimited running wheel access. The effects of both treatments on manic behaviors were assessed along with the effect of melatonin on sleep and that of voluntary exercise on brain drive neurotrophic factor (BDNF) levels in the hippocampus.
[05:15] Question 2
Which of the following could be the mutation seen in Myshkin mice?
- (A) GUU mutated to UAA
- (B) UAU mutated to UAG
- (C) GUU mutated to GAU
- (D) UUU mutated to UGA
Clara first explains that typically, you will see a figure in a biochem passage, but sometimes you have to infer what these results might have been.
Missense mutation is a mutation of one amino acid into a different amino acid. For the MCAT you don’t need to know which exact three-letter abbreviations correspond to which amino acid because these aren’t the amino acid three-letter abbreviations. They’re actually the mRNA code-on sequence that correspond to those amino acids. So we don’t need to memorize all those.
What you should know is that the code-on sequences for the three stop code-ons. And it would help you here. They are UAA, UAG, and UGA. There’s a good, classic mnemonics for this: U Get Away (UGA), U R Away (URA), U R Gone (URG).
If you know this crucial piece of information, you’d notice that A, B, and D are all mutations to stop code-ons. And a mutation to for a stop code-on is called a nonsense mutation. So those are all nonsense mutations. And C here is not. Hence, C has to be the answer here since non-sense mutations are not missense mutations.
[11:00] Question 3
A male Myshkin mouse is crossed with a wild-type female mouse. What proportion of the male offspring would be expected to show the Myshkin behavior pattern?
You don’t need to know anything about dominant vs. recessive because we know what exact genotype we’re looking for. Here, you’re looking for M/+. You can actually do a Punnett square. M/+ goes on one side of the square and then it’s getting crossed with the wild-type which is a genus of ++.
There are certain pieces of information about mutations and genetics that the MCAT expects you to know. And one of these is that + is going to denote wild-type in general and M for Myshkin.
The correct answer here is 50%.
[15:05] Question 4
BDNF is a plasma-soluble protein secreted after being manufactured in the endoplasmic reticulum. BDNF most likely,
- (A) Diffuses through the plasma membrane of target cells and after binding to its receptor, acts directly on euchromatin to regulate transcription.
- (B) Frequently serves as a target in autoimmune diseases due to its close contact with dissolved immunoglobulins.
- (C) Contains external domain especially rich in V, W, and G residues.
- (D) Has its effect through binding receptors on the external leaf of the plasma membrane.
(A) is like a steroid hormone which diffuses through the nonpolar lipid bi-layer. But once they get inside the cell, they need to bind with their receptor to get carried in the nucleus and then act as regulators of transcription. So A is similar to steroid hormones but steroid hormones are nonpolar. And the question states that BDNF is plasma-soluble so it’s probably polar since the plasma is really watery which is highly polar.
The right answer here is D. It talks about binding receptors outside of the membrane which is something a polar substance would do. Since it’s polar, it can’t diffuse through the nonpolar membrane so it needs to bind to the outside of it.
The abbreviations on C are actually amino acids. You really have to know your amino acids. Clara says it’s even the single most important thing to know for the MCAT.
[21:10] Check Out Next Step Test Prep
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