【多选题】How can the cell-cycle control system pause the cycle in various ways?
A.
The cell-cycle control system can transiently delay progress through the cycle at various transition points to ensure that the major events of the cycle occur only when the cell is fully prepared.
B.
At these transitions, the control system monitors the cell’s internal state and the conditions in its environment, before allowing the cell to continue through the cycle. For example, it allows entry into S phase only if environmental conditions are appropriate; it triggers mitosis only after the DNA has been completely replicated; and it initiates chromosome segregation only after the duplicated chromosomes are correctly aligned on the mitotic spindle.
C.
To accomplish these feats, the control system uses a combination of the mechanisms we have described. At the G 1 -to-S transition, it uses Cdk inhibitors to keep cells from entering S phase and replicating their DNA .
D.
At the G 2 -to-M transition, it suppresses the activation of M-Cdk by inhibiting the phosphatase required to activate the Cdk. And it can delay the exit from mitosis by inhibiting the activation of APC/C, thus preventing the degradation of M cyclin.
【简答题】Scientists say they have found key clues(线索)into how long we will live. One of them is a firm __21___. British researchers believe a firm strong handshake is not just a __22___ of confidence but also ...