Question

Which of the following is mediated by ubiquitination?

A the removal of mitotic cyclins at the end of mitosis

B decondensing of the chromosomes at the end of mitosis

C release of CED-4 protein from mitochondria during apoptosis

D the inhibition of mitotic cyclin-dependent kinases (CDKs) until replication forks are gone

E dissolving the nuclear envelope during mitosis

3 Mitotic cyclin-dependent kinases (CDKs) phosphorylate all of the following EXCEPT

A chromatin associated proteins

B microtubule-associated proteins

C kinetochore proteins

D S-phase cyclin-CDKs

E nuclear envelope proteins

all of the following genotypes of C. elegans would lack apoptosis EXCEPT [1 mark]

A loss of function Ced-3 mutant

B double mutant for loss of function Ced-3 and loss of function Ced-9

C loss of function Ced-4 mutant

Dloss of function Egl-1 mutant

Eloss of function Ced-9 mutant

Answer 1

• Removal of mitotic cyclins at the end of cyclins is mediated by ubiquitination. Anaphase promoting complex (APC) acts as ubiquitin ligase and polyubiquitinates proteins such as cyclins, securin etc. leading to their proteasomal degradation.
• Mitotic cyclin-dependent kinases (CDKs) phosphorylate all of the following EXCEPT S-phase cyclin-CDKs. A kinase phosphorylates other substrates without getting phosporylated.
• Loss of function Ced-9 mutant would lack apoptosis. ced-9 acts as binary switch and if it is turned off, ced-3 and ced-4 are expressed that initiate cell death.