Answers
Answer a-
The cdc2-13 complex regulates replication and mitosis by promoting cdc25 activity and also promotes amino-acid synthesis during G1 phase. Activity of this fusion proteins can be regulated by reversible association with Rum1, an cdk inhibitor as shown in the following figure.
Taken from- Gérard, Claude, et al. "Cell cycle control by a minimal Cdk network." PLoS computational biology 11.2 (2015).
The low activity state of cdc2-13 fusion proteis is achieved by Rum1-dependent inhibition and their degradation depends on APC. Whereas the intermediate fusion protein activity is maintained by Wee1-dependent inhibitory mechanism.
Cells with this fusion proteins pass first cell cycle stages but die at G1 phase of next cycle may be due to higher level of inhibition of this fusion protein by Rum1 at the G1 stage.
Answer b-
When cells are injected with cdc2-13 fusion proteins cells can directly enter M phase. It is shown by Coudreuse, D. & Nurse, P. that, cells with higher CDK activity can enter mitosis from any stages of cell cycle. Higher CDK activity does not even activate the S phase checkpoint.
This suggests that M phase progression is independent of passage through S phase checkpoint which requires intermediate or low CDK levels.
.