Biology is a core subject for the MCAT. It infiltrates a lot of different areas, so you need to be prepared for it. We break down some questions today to help!
In this episode Bryan and I cover Biology. We’re covering a grab bag of biology discrete questions to help guide you as you’re preparing for the MCAT to make sure that you’re rocking it.
[01:48] Genomic Library
Question 29: Which of the following is not a difference of between a cDNA library, and a genomic library?
- (A) A genomic library is larger than a cDNA library.
- (B) A genomic sequence contains both coding and non-coding sequences, whereas a cDNA library includes only coding sequences.
- (C) A cDNA sequence is difficult to express in prokaryotic system whereas a genomic sequence can be conveniently expressed in a prokaryotic system.
- (D) A genomic library includes promoters but a cDNA library does not.
Bryan explains that a genomic library is exactly what it sounds like – all the genes, all the DNA in the cell.
The c in the cDNA means “complementary” DNA. And the way you construct it is to take the actual, expressed, final mRNA and then reverse transcribe it back to DNA. Then this makes a little library that can store all of it. So what you’ve got is that the DNA that just codes for the actual RNA that’s going out to do the job.
Since the question asks for “which is not a difference,” looking through the answer choices, you can cross off the three that are a difference. (A) is a difference so cross it off. (B) is again, true. (D) is also true.
The key takeaway here is that for cDNA, only the final expressed mRNA,not the extra junk.
So (C) here is what’s not the difference. And it’s the correct answer choice. Saying that a cDNA sequence is difficult to express in prokaryote is a false. That’s the point of cDNA library. You want to make this particular gene product that is normally a human protein. And getting bacteria and human stuff to play together is difficult. Prokaryote and eukaryotes have different architectures.
Again, cut out all that extra junk in the human genome and just get the actual final expressed mRNA. And then the DNA from that can be put right into a bacteria. This can then be grown in massive quantities to produce huge amounts of a protein you want to study or use therapeutically.
[05:32] ELISA Technique
Question: An enzyme-linked immunosorbent assays (ELISA) can be used to measure the concentration of antigen in solution. During ELISA, an enzyme is conjugated with an antibody which results in the color change and end products that correlate to the amount of antigen present in the original solution. If a control has an absorbance of 0.35 and a sample solution has an absorbance of 0.64, which of the following can be concluded?
- (A) The antibody of the sample solution was downregulated.
- (B) The antigen of the sample solution was upregulated.
- (C) The antigen of the sample solution was downregulated.
- (D) The antibody of the sample solution was upregulated.
From a test strategy point considering how confusing this could be, it might be smart to just guess and go for a bunch of students. The key thing that this question is getting at is first, the difference between antigens and antibodies. They want you to have that kind of basic immunology knowledge.
Then reading the question of how an ELISA works, although you shouldn’t theoretically need that description. ELISA is a very common test or a very common lab procedure. So you should actually walk into the MCAT already knowing what an ELISA is and how it works.
We use them literally everyday, not even just in the lab. A form of an ELISA assay is how a normal pregnancy test works when you buy it off the shelf. That little stick you’re buying is a kind of a consumer-grade ELISA.
As we should walk into the test knowing, an ELISA test is testing for the antigen, the protein we want to test. Remember that the antibodies are what the immune system uses to react against antigens. So ELISA are testing for antigens. And right away, you’re eliminating answer choices A and D. Antibodies are just a step in the process and not the one an ELISA is testing for.
So although it could be confusing, if you just remember that one fact about what an ELISA tests for, you could at least get down to a 50-50 and move one.
And at that point, the amount of light absorbance is a measure of the thing you’re measuring. Since the number went up and there was more absorbance, there was more. So that would mean the antigen in the sample was upregulated, which is answer choice (B).
[09:01] Blotting Techniques
Question #59: A biologist wants to determine whether the high levels of a particular transcript results in increased levels of the associated protein. Which of the following tests could be used to measure protein production?
- (A) Western blot
- (B) Eastern blot
- (C) Northern blot
- (D) Southern blot
This is another thing that you have to walk into the test knowing. They ask about protein so you have to know that western blots test for proteins.
Southern blots test for DNA. This was the first developed some forty plus years ago. Interestingly, this has nothing to do with the direction on the map. The actual name of the guy was Dr.Southern. And he created this technique for testing for DNA.
Then when they later invented the RNA, they just called it a northern blot (like using equivalent analogy). And from there, western became protein and eastern became posttranslational modifications to proteins.
So it’s just this kind of central dogma thing and there should be mnemonics you can use for this.
[11:09] Strategies for Breaking Down Questions
When breaking down questions, you don’t know if the information in the question is, say extra versus critical. Bryan suggests that you read the entire question. Have the whole question in your eyeballs. And then rephrase or reformulate exactly what the question asked you to do. So it’s like two-step process, and not a skip-right-to-the-end-of-the-question process. Because sometimes the question has extra junk and sometimes it’s all essential. So you don’t know until you’ve read the whole thing and then rephrased the wording in your own terms.
[12:15] Next Step Test Prep
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