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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 S phase ? How long does it take?

DNA replicated and synthesised
6-8 hours


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?

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?

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


How do cyclin levels degrade?

Ubiquitins ( small regulatory proteins) attach to these cyclin proteins and signal them for destruction
The PEST box is involved in this


What does CDK4 bind to to become active?

Cyclin D
Activates it by phosphorylation
Cyclin D CDK4 complex can then phosphorylate Rb protein which in turn releases E2F


What phase(s) of the cell cycle are cyclin E levels high?

They rise in late G1 stages due to the E2F transcription factor being active
They remain the same through the S phase and fall by G2 phase


When are cyclin A levels high?

They rise during S phase
Remain the same during G2 phase


What does cyclin A complex to?

CDK2 in the S phase
CDK1 in the G2 phase

Therefore cyclin A complexes to two different CDKs!!


When are cyclin B levels high?

Start to increase at end of G2 phase, high in M phase
Complexes with CDk1 in Last G2 phase


What is the ankyrin repeat?

When CDKs are bound to by CDKIs (inhibitors), the ankyrin repeat is the interaction domain which allows two proteins to stick together. Both proteins, the inhibitor and it's target, will have this domain/ ankyrin repeat


What are CDKIs? What effect do they have on growth?

These are inhibitors of CDKs, they mask the CDKs active site and stop them phosphorylating things.
They prevent activation of CDKs, they inhibit the cell cycle and are therefore growth suppressors
Growth suppression is very important
If a fault in DNA is detected the cell cycle can be paused using CDKIs


What is used to PAUSE the cell cycle for a period of time so that damaged DNA can be repaired?

They're reversible and can dissociate
Therefore once DNA has been repaired the CDKIs can dissociate from the cyclin/ CDK complex and the cell cycle can carry on.


What is p53? What does it do?

It's a tumour suppressor protein
It mediates an increase in CDKIs to PAUSE the cell cycle whilst damages DNA gets repaired.
P53 is only present when damaged DNA is present, as it's stabilised by it, a healthy cell with healthy DNA won't have much p53.
If DNA damage is too great, the p53 can up regulate apoptotic proteins (Bad, Bax)


Will a healthy cell with nice healthy undamaged DNA have much p53 present?

P53 only present when damaged DNA is resent as its stabilised by it.
P53 increases CDKIs which pause cell cycle so damaged DNA can be repaired


Two tumour suppressors you can think of?

pRB and p53!
pRB deactivates E2F transcription factor: less cell growth, pause in cell cycle
p53 activates CDKIs to pause cell cycle, it checks for damaged DNA


Would a mutation resulting in loss of function of CDKIs or p53 be more serious? Why

p53 more serious
It's further upstream in the cell cycle pathway than CDKIs
Damaged cells containing Damaged DNA won't be signalled die if no p53 present
This will result in a tumour
P53 is mutated in 50% of all cancers


pRB is the guardian angel of the genome!

It checks for damaged DNA and calls In CDKIs to pause the cell cycle whilst repair takes place


What are the three members of the Cip1/ Kip2 family of CDKIS?

p21 waf1/cip1
p27 kip1
p57 kip2

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