Millonig - Regulation of Cell Division Flashcards
Restriction point
Point at the end of G1 that measures the favorability of the environment. If favorable, then the cells pass through the restriction point regardless of what happens later. So, if they become unfavorable later it is too bad.
- If conditions are bad then cells remain in Go.
Cyclin-dependent Kinases (Cdks)
They increase or decrease at different stages of the cell cycle. They lead to the phosphorylation of different proteins. Cdk activity is dependent upon cyclins, which exist for different parts of the cell cycle. We have a S-cyclin and a M-cyclin which bind to the Cdk at their respective times. Once the cyclin is bound there is a downstream affect in that other proteins involved in that part of the cell cycle are phosphorylated. Once their time is finished, they are degraded.
4 Cdk’s that we need to know
G1 Cdk - promotes passage through the restriction point
G1/S Cdk - commits cells to replication
S Cdk - initiates replication
M Cdk - promotes mitosis
- each has a cyclin that is responsible for binding to it for it’s activation
S-Cdk
- ORC + Cdc6 + Mcm
- Scdk triggers the phosphorylation and degradation of Cdc6
- role is to control the initiation of DNA replication. Once per cell at replication origins.
- a large multi-protein complex binds to the origins of replication called Origin Recognition Complex (ORC)
- The ORC binds to origins throughout the cell and acts as landing pads for other regulatory proteins to initiate replication at different positions throughout the genome so that the entire genome is duplicated
S-Cdk pre-replicative complex
Basically when in G1, the ORC is bound to the DNA. This recruits Cdc6 to bind to the ORC, which recruits the Mcm proteins, which make up the helicase. Once the S-Cdk is activated, the Cdc6 is phosphorylated and degraded. S-Cdk then phosphorylates the ORC. The DNA pol is assembled as well as other replication machinery. The Mcm proteins then slide along the DNA and act as helicases.
How does S-Cdk know when to start?
It is the S-cyclin
How does the cell prevent re-replication?
S-Cdk will remain active even in G2 so that the Cdc6 remains phosphorylated and degraded so as not to make another pre-RC. M-Cdk also phosphorylates Cdc6 and Mcm.
- In order to get ready for the next cycle, we reduce all Cdk activity to zero so that there will bee non-phosphorylated Cdc6 and Mcm so that we can make another pre-RC complex.
M-Cdk
- End of G2/M. Similar to S-Cdk, the system is generally poised and ready to go and is just waiting for the proper signal.
- You have an inactive Cdk1 that binds M-cyclin in the proper time.
- Then two kinases come - CAK and Wee1
- CAK phosphorylates the M-Cdk with the activating phosphate. Wee1 phosphorylates the M-Cdk with an inhibitory phosphate.
- When M-Cdk is ready to be activated, Cdc25 (a phosphatase) removes the inhibitory phosphate (Cdc25 is the switch).
- M-Cdk then phosphorylates proteins that are responsible for assembly of the spindle for chromosome segregation, chromosome condensation, and breakdown of the nuclear envelope.
M-Cdk feedback loops
1) When active, M-Cdk will inhibit Wee1 by phosphorylation
2) When active, M-Cdk will phosphorylate more Cdc25, which activates even more Cdc25, which leads to increased activation of M-Cdk
How do we inactivate M-Cdk?
A further downstream target is a proteasome so that it will force it’s own degradation. Controlled by APC
How do we ensure absence of Cdk activity in G1?
1) ubiquitin mediated degradation (as we saw in M-Cdk)
2) Cyclin Kinase Inhibitor (CKI) accumulation (p27)
3) Decreased cyclin transcription (Rb protein)
p27
Is a Cyclin Dependent Kinase Inhibitor (CKI) that binds to the active cyclin-Cdk complex and inactivates it.
Retinoblastoma Protein (Rb protein)
S-cyclin expression only occurs in late G1, not early G1. It is dependent on the transcription factor, E2F. Rb binds to E2F and doesn’t allow for E2F to function. When the cell is ready to enter into the S phase, it will receive a signal and activate G1-Cdk, which phosphorylates Rb, which then causes Rb to dissociate from E2F. E2F will then activate replication of S-Cyclin.
- Once activated, E2F then increases its own expression. E2F expression leads to production of G1/S-Cdk and S-Cdk, which in turn phosphorylates more Rb and releases more E2F. The increase in G1/S-Cdk and S-Cdk enhances the phosphorylation of ubiquitin ligases and CKIs, leading to their destruction in the proteasome and as a consequence activate G1/S-Cdk and S-Cdk.
Retinoblastoma
If you have a mutuation in both copies of the Rb gene then you will have retinoblastoma. This is because you will have constitutively on E2f so cells will always be going into the S phase.
When the DNA is ddamaged in latet G2, what happens that doesn’t allow it to enter into M?
The damaged DNA sends a signal that blocks Cdc25 activity so that the inhibitory phosphate stays on the M-Cdk. (Hint to remember Cdc25 is for M: Cdc25 for G2. Both have the number 2)
