Regulation of Cell Division Flashcards
The restriction point - when is it, what happens
It is late G1, before S phase.
It checks if the environment is favorable and then it will pass the restriction point. (Via G1 Cdk)
From there it will be committed to complete the cell cycle.
If environment is unfavorable, the cell enters the G0 phase until conditions are favorable.
What are the four Cdks and what do they do?
G1 Cdk - promotes passage through the restriction point
G1/S Cdk - commits cells to replication
S Cdk - initiates replication
M Cdk promotes mitosis.
Describe S-Cdk activity and how it is controlled.
ORC is a multi-protein complex that contains the polymerase and binds to the origin.
It will recruit Cdc6 at the beginning of G1. At the end of G1, all origins have this complex and are ready for the signal. (Cdc6 is usually present in low levels but increases throughout G1.)
Cdc6 binding will lead to the recruitment of Mcm proteins (helicases) and is responsible for the poised nature of the pre-RC.
S-cyclin becomes expressed in late G1, S-cyclin forms a complex with Cdk which phosphorylates the pre-RC, activating it for replication.
After the complex begins, the Cdc6 is phosphorylated and this results in its dissociation and degradation. S-Cdk also phosphorylates Mcm to be exported from the nucleus.
All of which is to prevent re replication.
S-Cdk dissociates Cdc6 and exports Mcm to prevent rereplication, what are additional controls preventing re replication?
How is the cell cycle control system reset?
S-Cdk activity remains high during G2 so Cdc6 is always phosphorylated.
M-Cdk, also phosphorylates Cdc6 and Mcm.
Resetting: at the end of mitosis, all Cdk activity is reduced to zero.
That allows enough time for the Cdc6 and Mcm to dephosphorylate, be produced and enter the nucleus.
What does M-Cdk do and how is it regulated?
(3 proteins.
- M cyclin is gradually increased during the G2 and M phases.
- CAK - Cdk activating kinase
- Wee1 - Cdk inhibitory kinase.
Cdk binds M-cyclin, at which point it is only partially active. CAK adds a phosphate which is overpower by the Wee1 inhibitory phosphate.
At this point it is poised for activation.
Then comes Cdc25- a phosphatase which cleaves the inhibitory phosphate.
What does M-Cdk do?
It will phosphorylate/activate proteins that are responsible for
- assembly of the spindle for chromosome rearrangement,
- chromosome condensation,
- breakdown of the nuclear envelope
What are the two feedback loops caused by increase in M-Cdk activity.
Active M-Cdk inhibits Wee1 causing more M-Cdk to just go directly to activation (never inhibited)
M-Cdk phosphorylates more Cdc25 activating the phosphatase.
How is M-Cdk and exit from mitosis regulated?
M-Cdk performs its own degradation.
M-Cdk will phosphorylate the proteosome and kill itself.
It is all a matter of kinetics, M-Cdk will have a weaker affinity for the proteosome leading to a lag in its degradation.
Describe G1 phase and the 3 mechanisms to ensure absence of Cdk activity.
G1: absence of Cdk activity.
- Ubiquitin mediated degradation
- Cyclin kinase inhibitor accumulation (CKI) which sits on cyclin-Cdk complexes
- decreased cyclin transcription.
What is E2F and Rb?
E2F is a transcription factor that regulates expression of G1/S and S cyclins (entry into S phase)
The Rb protein will bind during G1 and blocks its activity so the cyclins are not transcribed.
When it is time to divide, G1-Cdk accumulates and phosphorylates Rb and Rb dissociates from E2F.
What are the three positive feedback loops that occur once E2F is released from Rb inactivation
- E2F increases its own expression
- E2F expression leads to production of G1/S-Cdk and S-Cdk which phosphorylates more Rb and releases more E2F from inhibiton.
- Activate the proteosomes to degrade CKI’s which will loose inhibition for the cyclin Cdks
Describe the G2 DNA checkpoint.
If the cell detects DNA damage (free DNA ends),
Cdc25 activity will be blocked and entry into M phase will be stalled.
Describe the G1 DNA damage checkpoint.
This checkpoint prevents progression into S phase by inhibiting activation of G1/S-Cdk and S-Cdk.
p53 stimulates expression of several genes including CKI protein p21. Which binds to G1/S Cdk and S-Cdk and prevent passing through the restriction point.
Which protein is most responsible for preventing reinitiation into the M phase?
APC ubiquitin ligase which degrades M-cdk
