As we have seen, researchers studying the cell cycle made good use of rapidly-di

Question

As we have seen, researchers studying the cell cycle made good use of rapidly-dividing cells like fertilized sea urchin eggs. Although these eggs have large maternal stores of proteins and mRNAs, they still depend on the synthesis of some new proteins, including cyclins. Thus, if you inhibited protein synthesis in early sea urchin embryos, you would expect them to fail to enter the next M phase. However, inhibiting protein degradation can also cause defects in the cell cycle. If you inhibit protein degradation in early embryos, the cells become stalled in M and fail to move forward toward S phase. What is the best explanation of this result?

A) Cyclin-dependent kinases like Cdk1 are rapidly degraded soon after their Cdk-cyclin complex reaches its peak of activity. This degradation, which depletes the amount of active Cdk1-M cyclin complex to zero, is necessary for cells to exit M.

B) A number of protein factors inhibit the activation of the Cdk2-S cyclin complex, and these must be degraded before the cell can proceed toward S phase.

C) The turnover of tubulin through degradation is essential for the mitotic spindle to move the chromosomes to opposite ends of the cell. Without chromosome separation, the cell cannot go into telophase and exit M.

D) Cyclins like the M cyclin are rapidly degraded soon after their Cdk-cyclin complex reaches its peak of activity. This degradation, which depletes the amount of active Cdk1-M cyclin complex to zero, is necessary for cells to exit M.

Explanation / Answer

The correct option for this explanation is (D) Cyclins like the M cyclin are rapidly degraded soon after their Cdk-cyclin complex reaches its peak of activity. This degradation, which depletes the amount of active Cdk1-M cyclin complex to zero, is necessary for cells to exit M.

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