L14 - Checkpoint Control

Question 1

What is the role of checkpoint in the cell cycle?

Answer 1

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

Question 2

G1/S checkpoint

Answer 2

A cell will not enter S phase if genome is in need of repair

Question 3

S checkpoint

Answer 3

DNA replication paused in response to DNA damage

Question 4

G2/M checkpoint

Answer 4

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

Question 5

Spindle assembly checkpoint

Answer 5

A cell will not enter anaphase until all of its chromosomes are assembled on mitotic spindle

Question 6

What do cancer cells have that normal cells do not have?

Answer 6

Acquire activated oncogenes and inactivated TSGs

Inactivate one or more checkpoint controls

Question 7

What does pRb block?

Answer 7

Advancement through the cell cycle

Question 8

What is pRb?

Answer 8

A nuclear phosphoprotein absent or present in a defective form in many tumours

Question 9

What is Rb?

Answer 9

Molecular governor of the R point

Question 10

When does pRb undergo phosphorylation?

Answer 10

Alongside the advance of cells through the cell cycle

Question 11

What needs to happen before cells can go through the R point?

Answer 11

pRb needs to be hyperphosphorylated

Question 12

How is pRb phosphorylation controlled?

Answer 12

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

Question 13

What do the E2F transcription factors do?

Answer 13

Enable pRb to implement growth vs quiescence decisions

Question 14

Unphosphorylated or hypophosphorylated pRb binds?

Answer 14

E2F

Question 15

Hyperphosphorylated pRB dissociates from?

Answer 15

E2F

Question 16

What happens to E2F early on during G1?

Answer 16

pRb binds E2F preventing transcription of E2F-dependent genes

Question 17

What happens to E2F at the R point?

Answer 17

pRb hyperphosphorylation –> E2F released allowing transcription of genes mediating G1/S transition

Question 18

How long does active transcription promotion by E2F last?

Answer 18

Begins at R point
As cells undergo G1/S transition - cyclin A/CDK2 inhibits transcriptional activity of E2F
- Targeted for degradation by ubiquitination

Question 19

What positive feedback loops are involved in cell cycle regulation?

Answer 19

Cyclin E transcription promoted by E2F - E2F drives its own expression
E-CDK2 induces p27Kip1 degradation  more E-CDK2 released from inhibition

Question 20

What happens at the spindle checkpoint?

Answer 20

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

Question 21

What are the molecular mechanisms for progression past the spindle checkpoint?

Answer 21

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

Question 22

What are the molecular mechanisms for blocking progression past the spindle checkpoint?

Answer 22

APC/C ubiquitinate securing –> degradation and activation of separase
Separase breaks link between 2 sister chromatids by degrading cohesion
- Leads to initiation of anaphase

Question 23

What does the G2/M checkpoint check for?

Answer 23

DNA integrity

DNA replication

Question 24

What happens if there are problems in the G2/M checkpoint?

Answer 24

Cell cycle halted and cell attempts to complete DNA replication or repair damaged DNA
If damage irreparable the cell may undergo apoptosis

Question 25

What phases of the cell cycle are DNA damage checkpoints found?

Answer 25

Active in G1, S and G2

Question 26

What 2 proteins is cell cycle arrest mediated by?

Answer 26

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

Question 27

What is the role of p53?

Answer 27

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