Session 152

Today, we highlight some of the key concepts for Molecular Genetics. We’re joined once again by Clara from Blueprint MCAT (formerly Next Step Test Prep). if you need more one-on-one help with a tutor, give Blueprint MCAT (formerly Next Step Test Prep) a call, They offer free 15-minute consultation. Call 888-530-6398.

High Yield Topic in Biology: Molecular Genetics

[01:40] What is Molecular Genetics

The first step is to just know what the topic is all about. Molecular genetics is a huge topic that covers things like DNA, RNA, protein structure, etc. The “molecular” is used to distinguish it from classical genetics.

Classical genetics is something MCAT students study a lot since they’re used to it back in college. But molecular genetics tends to come up even more often. So expect to see more of DNA mutations.

[02:38] Question 20

A missense mutation replaces alanine residue located in the interior of a soluble plasma protein with a valine residue. Due to this change, the protein will most likely:

(A) suffer a complete loss of function.

(B) be largely unaffected.

(C) display disrupted folding due to the mistaken inclusion of a polar amino acid.

(D) display disrupted folding due to the mistaken inclusion of an unusually bulky amino acid.

Clara’s insights:

The first step here is to figure out what is a missense mutation. And you should know that it’s a type of mutation where one amino acid in a protein is replaced by another amino acid. There are other types of mutations including nonsense mutation where amino acids are replaced by a stop codon.

The question says they replaced an alanine residue with a valine residue. Note that even if this is a genetics question, it also touches on amino acids.

Basically, figure out the difference between alanine and valine. They’re actually the exact same kind of amino acids, which are both nonpolar. This makes it a conservative missense mutation, where one amino acid is replaced by a really similar amino acid. When this happens, it largely unaffects the function of the protein.

Hence, the correct answer here is B.

To know which amino acids are similar, you have to know the characteristic of their side chains. Side chains can be nonpolar, acidic, or basic, etc. Here, alanine and valine are both nonpolar. And if they’re the same type of side chain then you can assume that one is going to replace the other, it’s not going to make much of a difference.

[Related episode: Looking at MCAT Amino Acid Questions]

[06:40] Question 35

A drug that disrupts hydrogen bonding would mostly directly affect what level of protein structure?

(A) Primary

(B) Secondary

(C) Tertiary

(D) Quaternary

Clara’s insights:

Hydrogen bonding is fundamental. It holds a lot of the structures together, which we see everyday in biochemistry. But the most fundamental type of bonding which holds together the individual subunits of a protein is covalent bonding, specifically peptide bonds. So A here is out.

Tertiary and quaternary structures are really higher level forms of structure. They are impacted by hydrogen bonds. But they’re also held together by other forces.

Tertiary structures are held together in part by disulfide bonds. On the other hand, quaternary structures are similar to tertiary structures except for their interactions between different subunits instead of within the same subunit.

Think of tertiary and quaternary structures as being held together by all these different factors. While the only thing that holds secondary structure together is hydrogen bonding between the actual backbones of the amino acids. 

Back to the question, a drug that disrupts hydrogen bonding will totally ruin secondary structure. So B is perfect here.

[09:43] Question 44

A student stains a cell and views it under a light microscope. He correctly identifies the nucleus and notices dark and light regions within its structure. He then tells his lab partner that the dark areas represent euchromatin and are associated with decreased levels of transcription. What is incorrect about this student’s assessment?

(A) Euchromatin, which he has identified, is actually associated with increased transcriptional activity.

(B) He has actually identified heterochromatin, which is associated with increased levels of transcription.

(C) He has actually identified heterochromatin, but it is otherwise correct.

(D) Nothing; the student is absolutely right.

Clara’s insights:

Euchromatin is actually the really loose version of chromatin. It looks really light under a microscope. This must then be heterochromatin which looks really dense and dark under a microscope.

The correct answer here is C. Since heterochromatin is a really dense version, it means that it’s hard for enzymes to get into the little grooves of the structure.

For instance, RNA polymerase which transcribes our DNA has a lot of trouble getting into heterochromatin. Heterochromatin is absolutely associated with decreased levels of transcription.

The student is actually right except that he messed up euchromatin versus heterochromatin.

[13:42] Final Thoughts

You might see multiple chapters in your biology book that all relate to molecular genetics. These are all important.

Finally, if you need more one-on-one help with a tutor, give Blueprint MCAT (formerly Next Step Test Prep) a call, They offer free 15-minute consultation. Call 888-530-6398.

Links:

Blueprint MCAT (formerly Next Step Test Prep)