# PS6KEY - Professor Hinck Bio 110 Fall problem set 6 1 Two...

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Professor Hinck Bio 110, Fall, problem set 6 1) Two intracellular signaling molecules X and Y, are maintained at 1000 molecules/cell. Molecule Y is synthesized and degraded at 100 molecules/sec with each molecule lasting an average of 10 sec. Molecule X has a rate that is 10X slower--that is to say it is synthesized and degraded at 10 molecules/sec with each molecules lasting an average of 100 sec. A signal amplifies the rate of degradation of Y and X 10-fold. After 1 sec, how will the number of molecules of Y and X change? Y: 100 molecule/sec – (100 X 10 (amplification)= 100-1000 = -900 so 1000-900= ANS: 100 molecules left in the cell X: 10 molecule/sec – (10 X 10 (amplification)= 10-100 = -90 so 1000-90= ANS: 910 molecules left in the cell Remember the take-home message from lecture 2) Why do signaling responses that involve changes in proteins already present in the cell occur in milliseconds to seconds, whereas responses that require changes in gene expression require minutes to hours? ANS: In both cases the signaling pathways themselves are rapid. When the pathway modifies a protein that is already present in the cell, its activity is changed immediately, leading to a rapid response. When the pathway modifies gene expression, there will be a delay corresponding to the time it takes for the mRNA and protein to be made and for the cellular levels of the protein to be altered sufficiently to invoke a response, which would usually take an hour or more. 3) How is it that different cells can respond in different ways to exactly the same signaling molecule even when they have identical receptors? ANS: Cells with identical receptors can respond differently to the same signal molecule because of differences in the internal machinery to which the receptors are coupled. Even when the entire signaling pathway is the same, cells can respond differently if they express different effector proteins at the ends of the pathways.

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Professor Hinck Bio 110, Fall, problem set 6 4) Working out the order in which the individual components in a signaling pathway act is an essential step in defining the pathway. Imagine that two protein kinases, PK1 and PK2, act sequentially in an intracellular signaling pathway. When either kinase is completely inactivated, cells do not respond to the normal extracellular signal. By contrast, cells containing a mutant form of PK1 that is permanently active respond even in the absence of an extracellular signal. Doubly mutant cells that contain inactivated PK2 and permanently active PK1 respond in the absence of a signal. In the normal signaling pathway, does PK1 activate PK2 or does PK2 activate PK1. What outcome would you have predicted for a doubly mutant cell line with an activating mutation in PK2 and an inactivating mutation in PK1? Explain your reasoning.
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• Fall '09
• Hinck
• GTP, Professor Hinck Bio

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