Mutant Saccharomyces cerevisiae cells that lack the gene encoding securin can divide more or less normally by mitosis, without significant chromosome segregation defects. Cells harboring a nondegradable version of securin, on the other hand, arrest in metaphase as expected, since they cannot activate separase to enter anaphase. Similarly, cells lacking Cdc20 arrest in metaphase, since they cannot activate APC/C. Finally, cells lacking both securin and Cdc20 arrest in anaphase: they manage to separate sister chromatids, but do not progress much further. These results suggest that in wild-type cells, …
A) degradation of securin is necessary to trigger sister-chromatid separation.
B) degradation of securin is sufficient to trigger sister-chromatid separation.
C) Cdc20-APC/C is NOT necessary for sister-chromatid separation.
D) Cdc20-APC/C is NOT necessary for later events in anaphase.
E) All of the above.

Question

Quizplus · Accepted Answer

Both Cdc20 activation and securin destruction are necessary for sister-chromatid separation because cells in which either of these fail to occur are observed to arrest in metaphase. Securin degradation does not appear to be sufficient for sister-chromatid separation, as the cells lacking securin are more or less healthy, meaning other mechanisms also control the separation. Cdc20 appears to be necessary for anaphase beyond sister-chromatid separation, since the double knockouts still arrest in anaphase despite lacking securin.

Mutant Saccharomyces Cerevisiae Cells That Lack the Gene Encoding Securin