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Question: this problem deals with the mechanism by which the dna...

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This problem deals with the mechanism by which the DNA damage checkpoint induces cell-cycle arrest before mitosis.
QUESTION 9-1    Briefly describe why it is advantageous for a cell with damaged DNA to delay mitosis.


In fission yeast entry into mitosis is regulated by the phosphorylation status of Tyrsin-15 (=Y15) in Cdc2. This residue can be phosphorylated by the two kinases Wee1 and Mik1, and de-phosphorylated by the phosphatase Cdc25. Mitosis is triggered when the phosphatase activity of Cdc25 overcomes the kinase activity of Wee1/Mik1.
To determine if the checkpoint pathway inhibits mitosis by targeting Cdc2 Y15 phosphorylation, cells with a Cdc2-Y15F mutation were studied. These cell cells are viable but “wee” (small), because Cdc2 cannot be phosphorylated on F15. Y15F cells and wild type cells were synchronized in G2 by elutriation (a technique that allows isolation of the small newborn cells) and irradiated, and the kinetics of completion of mitosis was monitored (Figure 9-1). Unirradiated cells of both strains were included as controls

Y1SF irradiated continuously 0Y1SF unieradiated -wild type irradiated continuously 70 2 60 50 8 40 20 10 60 180 Minutes after elutriation 120 240 300 Figure 9-1 QUESTION 9-2 QUESTION 9-3 QUESTION 9-4 Explain why the unirradiated Y15F cells enter mitosis before the wild type. Describe the effect of irradiation in wild type and Y15F mutant cells. What can you conclude about the role of Cdc2-Y15 phosphorylation in checkpoint arrest?

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