Breaking Down More Bio Discrete Questions for the MCAT

Session 66

Today, Bryan and I discuss more biology discrete questions as well as some tips to help you do well on the MCAT.

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[01:25] Learning Terminologies

Question #13: A bacterium has a faulty lac operon in which there is a structural defect in the operator. In this bacterium,

(A) There’s a mutation in a segment of DNA that binds a promoter.
(B) A missense mutation is found in the gene that codes for their repressor.
(C) There’s a structural problem with a segment of DNA that binds a repressor.
(D) There will be no proteins available capable of digesting lactose.

Bryan’s Insights:

Lac is a protein for lactose. And this is one of those terminology stuff when it comes to the lac operon, the classic example of gene expression regulation.

The issue students run into is a bunch of “word salad” where words like represser, operator, and promoter can get jumbled around.

Bryan recommends to use flashcards or a study sheet, so you know all the terminology. And if you know the terminology, this would be a relatively straightforward question.

Since it says defect in the operator, you should know that the operator is a little DNA segment that binds with a repressor. Lac operon is a repressible system. So this leads us to answer choice (C).

[03:40] The Process of Elimination

Question #14: Unlike the cells from which human organs are composed, the cell of a unicellular organism such as algae…

(A) Has a genome where nearly all materials codes for protein.
(B) Typically utilizes mitosis for cellular division.
(C) Can perform catabolic reactions to gain energy from macromolecules.
(D) Contains membrane-bound organelles to execute cell functions.

Bryan’s Insights:

This is an example of the process of elimination to approach the question. Answer choices B, C, and D are human functions. So the correct answer is (A).

You have to be willing to choose answer choices that feel really uncomfortable or you are not thrilled with. But sometimes you have to pick them because they’re the last man standing.

In humans and higher organisms, there’s tons of introns and the majority of A’s and C’s and T’s and G’s in the body are just freeloaders. To say that all materials code for protein is true of a bacteria, algae, or something tiny. But this is not true for people.

[06:40] Eukaryotes vs. Prokaryotes

Question #15: Some eukaryotic cells are covered with small, ciliary projections used for absorption while others contain larger flagella used for propulsion. These cellular structures are composed of:

(A) Microfilaments
(B) Microtubules
(C) Intermediate filaments
(D) Myosin

Bryan’s Insights:

Microfilaments are not for cilia and flagella. They work with myosin to do muscle stuff. Both eukaryotes and prokaryotes can have these little projections of the cells.

But remember that flagella are made of flagellin. But in eukaryotes, cilia and flagella are made of microtubules so the answer is (B).

The MCAT also wants you to to remember that for eukaryotes, it’s the 9 + 2 arrangement. This means nine pairs of microtubules in a little hollow cylindrical shape with two more microtubules in the middle providing the backbone.

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