Lecture 19 - Cell Cycle Checkpoints Flashcards
Why might there be a delay or arrest in cell cycle progression?
In response to problems completing a specific step in the cell cycle or in response to other cellular problems
What might checkpoints do?
- Pause the cell cycle and promote repair before continuing
- Induce permanent arrest or apoptosis
When are many checkpoints not essential?
Under normal conditions
What are checkpoints essential for?
Protecting against cancer
How is SAC recruited to unattached kinetochores?
Mad2 component of SAC is recruited to the kinetochore, activated, and released, so it binds and inhibits APC/C-Cdc20
What happens at the spindle assembly checkpoint?
APC/C-Cdc20 is inhibited in response to incomplete kinetochore/microtubule attachments and allows the spindle to set up properly before anaphase is initiated
What happens at the G1 DNA damage checkpoint?
- Activated if DNA damage is detected in G1 –> G1 or G0 arrest
- Allows cell cycle to pause while DNA is repaired
*A similar checkpoint operates in G2 and prevents entry into mitosis
What is replicative senescence?
Telomere shortening –> G0 arrest (senescence)
What does replicative senescence (telomere shortening) protect against?
Chromosome fusion
What is oncogene-induced senescence?
Hyperproliferation due to oncogene activation –> G0 arrest (senescence)
What do some other checkpoints detect?
- Hypoxia
- Loss of cell adhesion
- Other abnormal cellular conditions
What gets recruited to sites of DNA damage?
ATM and ATR kinases
What preference does ATM have?
Double-stranded breaks
What preference does ATR have?
Exposed single-stranded DNA and other DNA damage
What functions do ATM and ATR have?
1) Help recruit repair machinery
2) Activate a cell cycle machinery
What do ATM and ATR phosphorylate?
Chk1 and Chk2 kinases to promote cell arrest (or apoptosis)
*They also phosphorylate to activate proteins involved in DNA repair
What else regulates Cdk2-cyclin A to control S-phase?
Wee1 and Cdc25
What do Chk1 and Chk2 phosphorylate?
Cdc25 phosphatase, targeting it for ubiquitination by SCF ubiquitin ligase and destruction by the proteosome
What keeps Cdk2 phosphorylated?
Wee1
How is the G2 checkpoint activated?
- Cdk1 remains phosphorylated by Wee1 so it is inactive
- Cells arrest in G2/M
In the case of more severe damage, what do ATM, ATR, Chk1, and Chk2 phosphorylate and activate?
Transcription factor, p53
What does p53 do?
- Induces a permanent cell cycle arrest by activating p21 expression so it can bind and inhibit Cdk2/cyclin A and Cdk2/cyclin E complexes so cells arrest in G1
- Can also induce apoptosis by activating the expression of pro-apoptotic genes
When is p53 inactive?
In undamaged cells due to Mdm2 binding
What does Mdm2 do?
Inhibits p53 activity and ubiquitinates p53 –> proteasomal degradation
What happens to phosphorylated p53?
It is released from Mdm2
What does the cell arrest in if telomeres are too short?
G0
What are telomeres?
Chromosome ends that contain multiple copies of a telomere repeat
What is telomerase activity required for?
Telomere replication
What are telomeres protected by?
Telomere binding proteins - sheltrin complex binds to telomere repeats and protects them from DNA repair machinery that repairs double-stranded breaks
What happens in most cell types in humans that don’t express telomerase?
Telomeres shorten each cell generation (every cell division) and if they shorten too much, there will be no more repeat sequences and sheltrin can no longer bind
How is replicative senescence carried out?
1) ATM binds to exposed double-stranded DNA at chromosome ends
2) p53 stabilization
3) p21 expression
4) Inactivation of G1/S and S-Cdks
*Induces permanent cell cycle arrest
What is oncogene-induced senescence?
When normal cells are induced to express high levels of oncogenes like myc or ras (genes that promote cell division), it induces a checkpoint that results in senescence
What does myc overexpression lead to?
The production of Arf, an inhibitor of Mdm2 so it can bind and cause the ubiquitination of p53 so it is able to mediate cell cycle arrest or apoptosis
What transcription factor is the cellular gatekeeper?
p53
