Question

A mutation on securin prevents its ubiquitynation and degradation. In a cell containing this mutation, which of the following phenotypes can be predicted?

a) the condensing complex will not be assembled.

b) the cohesin complex will not be assembled.

c) mitotic chromosomes will not be aligned properly in metaphase.

d) sister chromatids will not be separated after metaphase

Answer 1

.

Basic functions

1)securin is an inhibitor of sister chromatid separation during anaphase.

2) chromatin condensation is done by the protein complexes called condensin while the linkage between sister chromatid is maintained by cohesins.

3) Both condensin and cohesin contribute to chromosome segregation during mitosis.

Answer:

d) sister chromatids will not be separated after metaphase.

Explanation:

Till there is transition from metaphase to anaphase,the seperase is present in inactive form and it is kept in inactive state by the securin by the formation of seperase-securin complex.But when the there is a proper alignment of chromosme during metaphase a signal is generated called 'Anaphase signal' by which the APC/C (ubiquitin protein ligase) gets the signal to perform ubiquitynation and degradation of securin which leads to activation of seperase (protease).This activated seperase goes and cleaves the Scc1 cohesin subunit leading to the seperation of sister chromatid in anaphase,

Since ,securin is not degraded it renders seperase inactive which is responsible for seperation of sister chromatids seperation and thus, sister chromatid seperation will be blocked.

Answer will not be (a), (b) and (c) because:

• Mitotic chromosomes proper alignment in metaphase in controlled by Aurora A and Polo-like kinase which are not affected by securin and also the condensin is not affected when securin is inactive In metaphase as there will be no activation of seperase which is responsible for disassembly of cohesin complex,so the answer will not be (c).
• Cohesin complex assembly is not affected by securin when it is not degraded as there will be no activation of seperase which is responsible for its disassembly.
• condensin complex remains intact during the process and is not affected by securin.