Question

5. Securin is not an essential protein in yeast which has only 3 chromosomes but it is in humans which has 48 chromosomes. Based on these observations explain how it is possible for yeast to survive without securin and hypothesize what might kill human cells in the absence of securin.

Answer 1

Although securing gene are essential for viability in fission yeast , budding yeast cells can. Survive without securing at low temperature and initiate anaphase with fairly normal timing, indicating that mechanisms other than securin degradation must regulate separate activation.  

But in case of human ... Explain in image ..

Securin is a protein involved in control of the metaphase-anaphase transition and anaphase onset. Following bi-orientation of chromosome pairs and inactivation of the spindle checkpoint system, the underlying regulatory system, which includes securin, produces an abrupt stimulus that induces highly synchronous chromosome separation in anaphase. Securin is initially present in the cytoplasm and binds to separase, a protease that degrades the cohesin rings that link the two sister chromatids. Separase is vital for onset of anaphase. This securin-separase complex is maintained when securin is phosphorylated by Cdk1, inhibiting ubiquitination. When bound to securin, separase is not functional. In addition, both securin and separase are well-conserved proteins (Note that separase cannot function without initially forming the securin-separase complex). This is because securin helps properly fold separase into the functional conformation. However, yeast does not appear to require securin to form functional separase as anaphase occurs in yeast with a securin deletion mutation.

APC Clb5 Cdk1 (Remove phosphate) - (Add phosphate) Securin Cdc14 Anaphase Separase