Week 8(Cell Cycle Activities of CDKs and the Ubiquitin Ligases) Flashcards
When do G1/S cycling accumulate?
During G1
What is Retinoblastoma (Rb)?
A pocket protein that sequesters (holds out of action) members of the E2F family of transcription factors.
What do G1/S CDK do?
phosphorylates Retinoblastoma (Rb)
What does E2F stimulate?
stimulates the transcription of genes that promote entry into S-phase such as G1-cyclins, S-cyclins and DHFR (dihydrofolate reductase).
What is Phosphorylating Rb part of?
a Feedback Loop which Promotes Entry into S phase
Why might we want to prevent entry into S-phase?
The arrest of the cell cycle in G1 or G2
following DNA damage reduces the risk that
a neoplastic change will be passed onto the
daughter cells.
What can cause DNA damage?
DNA damage includes damage done by
ultraviolet and x-irradiation, and
chemotherapeutic drugs.
How does the DNA damage checkpoint work?
Severe DNA damage in G1 increases the activity
of p53.
p53 is a transcription factor.
In undamaged cells, p53 is unstable and is present
at low concentrations since it is bound by Mdm2,
a ubiquitin ligase that targets p53 for degradation.
What does DNA damage activate?
DNA damage activates kinases (ATM/ATR and
Chk1/Chk2) that phosphorylate p53,
releasing it from Mdm2.
p53 transcribes CKIs
Once stable and active, p53 turns on the expression of genes that encode CKIs (inhibitors of Cdk-cyclin complexes) such as p21 (p21 is similar to p27, a CKI normally present in early G1).
When p21 or p27 is bound to G1/S-Cdk or S-Cdk the complex is inactive.
What is Mdm2?
A ubiquitin ligase
describe the pathway from DNA Damage to Cell Cycle Arrest
DNA Damage
Stable p53
Transcription of p21 mRNA
p21 protein inhibits CDK-cyclin
Rb is unphosphorylated
No Free E2F
No entry into S phas
How is M-CDK activated?
-M-cyclins accumulate during G2.
-By the end of G2, the concentration of M-Cdk is high,
but the complex is inactive due to inhibitory phosphates
added by Wee1.
-Cdc25 removes the inhibitory phosphates.
-Cdc25 is activated by Polo kinase and by M-Cdk itself,
generating a positive feedback loop (M-Cdk also inhibits
Wee1).
Is M-CDK Is Activated by Positive Feedback?
Yes
What does M-Cdk do?
-Assembly of the mitotic spindle—
phosphorylates proteins that regulate
microtubule behaviour
-Chromosome condensation—phosphorylates the
condensin complex
-Nuclear envelope breakdown—phosphorylates
lamins
-Rearrangement of actin cytoskeleton and Golgi
-(Other kinases are also important).
DNA Replication Checkpoint Prior to Mitosis
Unreplicated DNA or unfinished replication forks send
a negative signal to block M-Cdk activation to
prevent mitosis.
The final target of the negative signal is Cdc25. As long
as cdc25 is inhibited, M-CDK will be inactive.