2. You are conducting research on the cell cycle in amphibian embryonic cell and
ID: 186425 • Letter: 2
Question
2. You are conducting research on the cell cycle in amphibian embryonic cell and determine that the length of the cell cycle is 30 minutes. Your research has shown that the amount of cyclin B varies over the cell cycle. You add a drug that blocks the cell cycle and obtain the results shown below. Cells with drug added 50 Control cells without drug 0 30 60 90 120 time after fertilization (minutes) From these data, cells treated with the drug are arrested in which state of the cell cycle (G1, S, G2 or M)? (2 points) M yha s A. From the data, propose a simple molecular explanation for how the drug acts to block the cell cycle. (2 points) B.Explanation / Answer
A) The cells treated with the drug are arrested in the M-phase of the cell cycle. Cyclin B is the mitotic cyclin which is necessary for the progression of cell into and out of the M-phase of cell cycle. At the end of S-phase the phosphatase cdc25c dephosphorylates tyrosine15 and this activates the cyclin B/CDK1 complex. Upon activation the complex is shuttled to the nucleus where it serves to trigger for entry into mitosis.However, degradation of Cyclin B is required for the cell to progress out of the mitosis. In the graph given above, the concentration of Cyclin B increases with time in the cells that are treated with the drug. This suggest that the drug is inhibiting the degradation of cyclin B, thus arresting the cell in M phase.
B) Cyclin B degradation is necessary in celll for it to progress out of Mitotic phase of cell cycle. As can be seen from the data that the drug blocks the degradation of Cyclin B. One of the components required for the degradation of Cyclin B is Anaphase Promoting Complex (APC). APC is inactive in the interphase, but becomes active towards the end of Mitosis. Activation requires phosphorylation of APC by protein kinase Cdk1/CyclinB. It is possible that the concerned drug blocks phosphorylation of AP by cyclin B, thus causing it to remain inactive hence, inhibiting the degradation of cyclin B which causes the cell cycle to be arrested in M phase only.
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