L14 - Checkpoint Control Flashcards
What is the role of checkpoint in the cell cycle?
Surveillance mechanisms to monitor each step of the cell cycle progression
Cells allowed to proceed with cell cycle if the pre-requisite step has been completed successfully
G1/S checkpoint
A cell will not enter S phase if genome is in need of repair
S checkpoint
DNA replication paused in response to DNA damage
G2/M checkpoint
A cell will not proceed until DNA replication of S phase has been completed
Entrance in M phase is blocked if the DNA is damaged
Spindle assembly checkpoint
A cell will not enter anaphase until all of its chromosomes are assembled on mitotic spindle
What do cancer cells have that normal cells do not have?
Acquire activated oncogenes and inactivated TSGs
Inactivate one or more checkpoint controls
What does pRb block?
Advancement through the cell cycle
What is pRb?
A nuclear phosphoprotein absent or present in a defective form in many tumours
What is Rb?
Molecular governor of the R point
When does pRb undergo phosphorylation?
Alongside the advance of cells through the cell cycle
What needs to happen before cells can go through the R point?
pRb needs to be hyperphosphorylated
How is pRb phosphorylation controlled?
Governed by components of cell cycle clock
Early G1 - D-type cyclin and CDK4/6 initiate pRb phosphorylation –> hypophosphorylation
- Hypophosphorylation necessary but not sufficient for pRb inactivation
R point - cyclin E/CDK2 mediate pRb hyperphosphorylation
What do the E2F transcription factors do?
Enable pRb to implement growth vs quiescence decisions
Unphosphorylated or hypophosphorylated pRb binds?
E2F
Hyperphosphorylated pRB dissociates from?
E2F
What happens to E2F early on during G1?
pRb binds E2F preventing transcription of E2F-dependent genes
What happens to E2F at the R point?
pRb hyperphosphorylation –> E2F released allowing transcription of genes mediating G1/S transition
How long does active transcription promotion by E2F last?
Begins at R point
As cells undergo G1/S transition - cyclin A/CDK2 inhibits transcriptional activity of E2F
- Targeted for degradation by ubiquitination
What positive feedback loops are involved in cell cycle regulation?
Cyclin E transcription promoted by E2F - E2F drives its own expression
E-CDK2 induces p27Kip1 degradation more E-CDK2 released from inhibition
What happens at the spindle checkpoint?
Separation of sister chromatids during anaphase is irreversible - cycle will not proceed until all chromosomes are attached to at least two spindle fibres from opposite poles of the cell
If chromosome is misplaced – mitosis paused allowing time for spindle to capture stray chromosome
What are the molecular mechanisms for progression past the spindle checkpoint?
Progression to anaphase controlled by anaphase promoting complex/cyclosome (APC/C)
All chromosome aligned –> APC/C activation
- Two sister chromatids attached to each other by cohesion
What are the molecular mechanisms for blocking progression past the spindle checkpoint?
APC/C ubiquitinate securing –> degradation and activation of separase
Separase breaks link between 2 sister chromatids by degrading cohesion
- Leads to initiation of anaphase
What does the G2/M checkpoint check for?
DNA integrity
DNA replication
What happens if there are problems in the G2/M checkpoint?
Cell cycle halted and cell attempts to complete DNA replication or repair damaged DNA
If damage irreparable the cell may undergo apoptosis
What phases of the cell cycle are DNA damage checkpoints found?
Active in G1, S and G2
What 2 proteins is cell cycle arrest mediated by?
ATM double stranded breaks
ATR single stranded breaks
They both lead to activation of signalling pathways leading to cell cycle arrest, activation of DNA repair and apoptosis
What is the role of p53?
Tumour suppressor lost or mutated in half of human tumours
It promotes expression of genes mediating growth-suppression, apoptosis and DNA repair
Target gene - p21Cip1 CKI
