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Biology -- Quick Hormone Question
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As a peptide I would think....
That's what I thought too. I'll C&P the questions in.
The passages says "Stilbestrol is a crystalline non-steroid with estrogenic effects often superior to those of the estrogen, estradiol."
Which of the following constituents of oviduct cells would first show increased activity on contact with stilbestrol?
A.Golgi bodies
B.Mitochondria
C.Ribosomes
D.RNA polymerase II
Correct Answer/Explanation (highlight):
D. Stilbestrol is a compound with estrogenic effects greater than those of estrogen. Estrogen acts by binding to an intracellular receptor that then regulates the transcription of genes by RNA polymerase II, the enzyme responsible for transcription of all mRNAs (D is correct). Ribosomes and Golgi bodies would play a role after transcription, in the translation and processing of proteins (A and C are wrong). Mitochondria are not directly involved in the activity of estrogen or stilbestrol (B is wrong).
If radiolabeled stilbestrol were administered to the experimental chicks, stilbestrol would be found most heavily concentrated:
A.at the cell membrane of oviduct tissue.
B.in the cytoplasm of oviduct tissue.
C.in the nuclei of oviduct tissue.
D.in the mitochondria of oviduct tissue.
Correct Answer/Explanation (highlight):
C. When stimulated by the addition of a ligand such as stilbestrol, estrogen receptor will localize within the nucleus, where it regulates genes by binding to enhancers and promoters. Radiolabeled stilbestrol would localize with estrogen receptor in the nucleus (C is correct). There is no estrogen receptor in the plasma membrane or mitochondria (A and D are wrong). Some estrogen receptor may be located in the cytoplasm, particularly in the absence of ligand, but it will localize mostly in the nucleus when it has ligand bound (B is wrong).
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It means that it's a substance that activates estrogen receptors but that doesn't have the classic ring structure of a steroid (could be anything...a peptide, an immunoglobulin, or a synthetic chemical). This particular question is probably based (by the guy who was writing it) on the real medical drug diethystilbesterol that increased the risk of OB/GYN cancers in kiddos whose mothers were exposed to it a few decades ago. If you really want to know the chemical structure you could google it but it's irrelevant to the question.
The point is that "stilbesterol" is a substance that activates estrogen receptors and what the question is really asking is whether you know how steroids induce transcription/translation. As such, I'm pretty sure that the answer to #1 is D b/c steroids work by influence transcription of mRNA--> production of proteins. The first step in this should be RNA polymerase II transcribing mRNA (whoa, having to dig way back to undergrad for this one; double check me with the answer book and post it please). I'm pretty sure that the anwer to #2 is C b/c the way that steroid hormones work is by getting into the cell and hanging out in the nucleus were they induce transcription until they are metabolized (most hang out for a while...the fact that they have to work through a transcription/translation cascade is why the effects of steroid hormones are slow acting compared to most other messenging systems in the body).
Again, the take home point is that this set of questions is really asking you "how do steroid hormones work" and recognizing this is the key to getting it correct.
Leonardo Noto
Physican-turned-Author
www.leonardonoto.com or follow me on Twitter @DrLeonardoNoto
I'd argue that it's a great set of questions b/c it requires you to see the forest through the trees. The MCAT and the medical boards love these kind of questions b/c they test critical thinking skills in addition to knowledge. It just takes lots of practice to get good at them!
Leonardo Noto
I would agree if they did not put the word non-steroid. But once you put that word, you make people think that the hormone is hidrophilic...ie the receptor is on the cell membrane. But again because they say the hormone has estrogenic effect, that is a clue that tells you that the receptors are in the nucleus. I hope the people who write mcat do not do stuff like that.
Some steroid receptors are located on the plasma membrane.
http://www.pnas.org/content/100/5/2168
Thyroid hormones are located in the nucleus and those are non-steroid hormones.
I knew the thyroid exception...but did not know that some steroid hormone receptors are on plasma membrane...thanks..
@Leonardo Noto: That makes sense. I actually leaned toward those answers at first seeing as it mimics estrogen, but then the "non-steroid" label caused to me to sway opposite. I guess I failed to consider that "non-steroid" does not necessarily mean peptide but could also be, say, an amino acid derivative like TH.
