Question

1. A common feature of cell-cycle regulation is that the events of one phase ensure progression into a subsequent phase. In S. cerevisiae, mid- and lateG1 cyclin-CDK’s catalyze progression through G1, and multiple B type (S and M) cyclin-CDK complexes catalyze progression through S, G2, and M. Name three ways in which the activities of mid- and late-G1 cyclin-CDKs promote the activation of S-phase and mitotic cyclin-CDK complexes.

Answer 1

Cyclins control the events of cell cycle by working in coordination with enzymes called cyclin dependent kinase(Cdks).Cdks are kinases which phosphorylate target proteins which in turn gets activated or inactivated.

During G1 phase, the transition from G1 to S phase is controlled by growth dependent cyclin dependent kinase which initiates DNA replication. One important regulatory protein is RB (retinoblastoma) which is potent inhibitor of G1-S transcription.Phosphorylation of Rb by Cdk releases the inhibitor from transcription factor to activate G1-S genes which include G1 cyclins.

This also results in positive feedback loop which commits cell to new cell division cycle and activates G1-S transcription.

In yeast, early G1 transcription is inhibited by Whi5 binding to SCB-bibding factor which forms a complex at target promoter.Cln3-Cdk phosphorylated Whi5 which relieves inhibition and thereby resume transcription. This indices nuclear export of entire complex and starts transcription. This activation results in accumulation of Cln1 and Cln2 proteins which bind to Cdk and inactivate phosphorylation of Whi5. This is the mechanism of positive feedback which plays a crucial role in transition from G1-S phase in cell cycle.