Biology Cell Checkpoints

[mpatel24]

(1.) The level of active mitotic CDK rises and falls during the cell cycle. Which of the following best explains why the levels of the active mitotic CDK rise?
(a.) Cyclin is degraded by APC, removing MPF inhibition
(b.) Active MPF tht already exists int he cell stimulates APC, which phosphorylates cyclin
(c.) New cyclin is produced, binds CDK, and then undergoes a series of phosphorylation and dephosphorylation reactions to gnerate active CDK
(d.) Checkpoint proteins trigger a MAP kinase cascade which activates transcription factors such as jun, that promote transcription of CDK protein.


(2.) The level of active mitotic CDK rises and falls during the cell cycle. Which of the follwing best explains why the levels of active mitotic CDK fall
(a.) Transcription of cylin stops after activation of p53
(b.) Polyubiquitination of APC triggers degradation of CDK
(c.) Active CDK stimulates a phosphatase that dephosphorylates CDK.
(d.) APC degrades securin, which allows separin to degrade cohesions holding sister chromatids together, which allows anaphase to begin
(e.) Active CDK activates anaphase promoting complex, causing polyubiquitination of cyclin.

Hey, I found these two questions dealing with checkpoint signals during cell division in a forum a couple years ago, but could not find an answer. Can you guys help me find the answer: I say the 1st one is C or D and the second one is B. Is the second one not e, because cdc20 phosphatase activates CDK.

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[1TB4RKSB4CK]

Where is this question from? Looks like these questions should be answers by extrapolation from the passage.

You are right for the second question via PoE and kinase general action.

Ummm.... I thought the first one was D.

 

[Euxox]

Usually these kind of questions wold come with a passage -- they are too advanced to be discretes. This level of cell biology knowledge is not expected on the MCAT, so trying to learn the requisite cell biology knowledge is heading down the wrong path. You need to use whatever passage these questions came with and learn how to quickly extract information from the passage and apply it to find the answer.

 

[mpatel24]

Thanks for your guys help. I have a test in my biology 400 level class in a couple of days, so I thought i would search up some topics that we covered in class that will apply to the mcat. Another SDner posted these questions.

BTW, for the second one, I just looked through my lecture notes and found out that APC only degrades cyclins and securins but only inactivates, NOT degrades CDKs. Also the answer for B is worded incorrectly, shouldn't it be polyubiquination BY APC, not OF APC.

I know I am going way too detailed, just curious for clarification.

 

[1TB4RKSB4CK]

hmmm nope
APC would be able to transfer those ubiquitin molecules for degradation using the proteasomes I think...

Okay, a little bit of checking (no way I can answer a discrete like that) and I'm sure the first question is D
there would be a stimulation of Ras from the mitogens>MAP kinase activated>starts a cascade>production of the regulator gene to code for a transcription factor(I don't recall the pathway)>ultimately increases CDK activity through those factors
I think the genes in those interphase cycles G1 and S are stimulated after a checkpoint.