Could someone please explain this AP Biology question? I know the answer is C but I do not understand why. Doesn’t Smo prevent gene transcription? If you “block its activity” then wouldn’t it cause gene transcription, whereas the goal of treating cancer is to prevent transcription? Please help!

  • $\begingroup$ Reread the line: “When Hedgehog is absent, Ptc inhibits another protein known as Smo”; Hedgehog is present in the cancer cells, so what you want to do is to mimic Hedgehog’s absence. The question is hard because there is a lot of inhibitory and disinhibitory activity in this pathway, so that’s what you have to do, try to follow all those +s and -s; it might be helpful to draw the whole thing out as a block diagram, and to take that approach with similar questions in the future. $\endgroup$– Bryan Krause ♦Jan 31, 2017 at 16:22
  • $\begingroup$ Thank you! This was actually helpful, I think the information was worded a little weirdly – “When Hedgehog is absent, Ptc inhibits another protein known as Smo, which, in turn, blocks the activation of a group of proteins…” I think I was thrown off by the end of this sentence, because the way the sentence is constructed makes it seem like when Smo is inhibited it blocks the activation of a group of proteins, when it was actually the opposite idea! $\endgroup$ Feb 1, 2017 at 1:58

1 Answer

I’m just going to outline the alternatives, I’ll leave validating why choice C to you.

Choice A. If the new molecule acts in the same fashion as Hh, the results will be identical. If the new molecule bound to Pct but did not effect a response, that might work.

Choice D. Addition of Ci into the nucleus initiates transcription which seems to be the issue here.

Choice B. Best wrong answer! Can you inject enough stem cells to make a difference? Will the cells have some adverse effect? More importantly, the binding of messengers to receptors is a reversible process. The Hh will be able to detach from Ptc on the stem cells and eventually interact with Ptc on the native cells.

So, what effect will Hh have if the Smo in unable to respond?

  • $\begingroup$ Thank you! This was great! Especially your justification for B and explanation that binding to receptors is a reversible process. Now I why that choice was incorrect! 🙂 $\endgroup$ Feb 1, 2017 at 2:01

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