PA20293 cell cycle flash cards
What happens in S phase ? How long does it take?
DNA replicated and synthesised
6-8 hours
What is the G1 phase? How long does it take?
Gap 1,
Cell prepares for DNA synthesis
Takes 8-12 hours
Longest stage
What happens in G2 phase? How long does it take?
Cells prepare for division
Takes 4-6 hours ☺
What happens in M phase? How long does it take?
Mitosis, spindle formation
Cell division
4-6 hours
What is key in controlling the cell cycle?
Phosphorylation by cyclin dependent kinases
These associate with cyclins at specific stages
The CDKs must be phosphorylated themselves to be activated, this is achieved through association with kinases, and then they can go off an phosphorylated protein targets involved in cell cycle control
What are pocket proteins?
Pocket proteins regulate availability of key transcription factors
Cyclin/ CDK complexes phosphorylate pocket proteins in G1 phase
Remember in G1 phase cell gets ready for DNA synthesis, therefore it needs to increase numbers of transcription factors.
Which stage is pRB involved in?
G1 phase
CDK4 activates pRB by phosphorylating it
pRB then goes on to release E2F, which activates E2F
Do concentrations of CDKs fluctuate throughout the cell cycle?
NO
Concentrations of CDKs are CONSTANT throughout the cell cycle
But they have NO cyclin activity unless they’re associated with a regulatory cyclin subunit
Do concentrations of cyclins fluctuate through the cell cycle?
Yes
Each cyclin is only present and active during the cell cycle stage it promotes, levels decrease when that certain cyclin is not needed.
Therefore they will restrict activity of the specific CDKs they bind to, so that they can only be active in that cell cycle stage only
In the G1 phase, E2F is made active. What is the role of E2F?
E2F is a transcription factor complex
It goes on to transcribe additional genes required for progression onto next stage
Eg transcription of S phase cyclins
But when E2Fs aren’t active, as they’re bound to pRB, they’re acting as transcription repressors
Cyclin E expression is dependent on the E2F transcription factor
What function as transcriptional repressors in the cell cycle?
When E2Fs are bound to Rb
E2F is a transcription factor,
When it’s bound to Rb it is inactive so transcription can’t occur
When Rb is phosphorylated by cyclin D/ CDK4 complex, what happens?
pRB will release/ let go of E2F
E2F is a transcription factor complex so goes off and initiates transcription
What does Rb stand for?
Retinoblastoma tumour suppressor gene
If a mutation occurs and both alleles of the gene coding for the Rb protein are mutated, the protein becomes inactivated and it results in retinoblastoma cancer, as the cell cycle spans out of control as the Rb can no longer hold on to and inactivate E2F
Without the presence of Rb, E2F would be permanently activated
.
What does cyclin Es expression depend on?
On the E2F transcription factor that is activated in the late stages on G1
