NS FL6 B/B #21 please explain

[sapientnarwhal]

@NextStepTutor_1

So the passage notes that SREBP-1a and 1-c are both produced from the same gene. Thus I reasoned that alternative splicing occurs. I dismissed B, as I thought that both must have the same transcriptional start sites (TSS), regulatory upstream elements, promoter elements (because they are from the same gene).

However, the explanation seemingly erroneously points out that alternate promoter regions exist in the exons that encode the different isoforms. It is possible I may have encountered this knowledge in my molbio classes, but to my knowledge this information is inaccurate. The pre-mRNA sequences should be identical for both 1a and 1c until alternative splicing occurs.

I was hesitant to select D, yet it was at the very least supported by KO data, and at the most weakly refuted by the data from the OE experiment.

UPDATE: I found a few papers detailing alternative TSS. I don't think this question is fair, as the knowledge required to feel remotely comfortable with this answer is far beyond the scope of the MCAT.

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[NextStepTutor_1]

Hi @sapientnarwhal -

It does actually seem to be the case that SREBP-1a and SREBP-1c have alternative transcriptional start sites, for what it's worth. I see your point about that being a somewhat exotic mechanism, but answer choice D is clearly wrong (SREBP-1c overexpression doesn't reduce cholesterol synthesis, so that choice is not supported), and the prerequisite knowledge for understanding what the hypothesis in B means is within the scope of the MCAT. Since the question asks for which statement is most reasonable, and B both explains the phenomenon and is consistent with passage/scientific information, it's the best option in this context. Your point is definitely taken that alternative splicing is a more familiar mechanism, but I do think that this question is answerable within the scope of MCAT content. Thank you, though, for the very perceptive question, and I will make sure to bring this to our FL team.