Cell Cycle Checkpoints

Question 1

Coordination between different events of the cell cycle is achieved by …

Answer 1

A system of checkpoints

Question 2

Function of cell-cycle checkpoints

Answer 2

Prevents entry into the next phase of cell cycle until the events of the preceding phase have been completed

Question 3

What are the 3 main checkpoints in the cell cycle?

Answer 3

G1 checkpoint (G1-S)
G2 checkpoint (G2-M)
M-phase checkpoint (metaphase-anaphase)

Question 4

G1 checkpoint

Answer 4

Monitors the environment before allowing the cell cycle to progress

Question 5

G2 checkpoint

Answer 5

Monitors the DNA for proper replication & the environment

Question 6

M-phase checkpoint

Answer 6

Spindle assembly checkpoint (SAC)

Monitors that chromosomes for proper attachment to MTs & for proper alignment at the metaphase plate

Question 7

What is needed for a cell to grow?

Answer 7

Growth factors

Question 8

Growth factor, GEF, activates which pathway?

Answer 8

Ras pathway

Question 9

How is Ras pathway involved in cell growth?

Answer 9

Activated Erk from the Ras pathway targets Cdk4/6

Question 10

What is the ‘restriction point’ in cell growth?

Answer 10

The point at which cells are committed to entering the S phase and no longer need growth factors

Question 11

Before restriction point …

Answer 11

growth factors needed

Question 12

After restriction point …

Answer 12

growth factors not needed as cell is committed to S phase

Question 13

What is the role of E2F in cell growth?

Answer 13

E2F is a transcription factor in the nucleus that transcribes the genes needed for cells to commit to S phase

Question 14

What is the role of Rb in cell growth?

Answer 14

A protein that binds to E2F and inhibits it from transcribing genes needed for S phase

Question 15

What is the role of Cdk4/6 in cell growth?

Answer 15

Once activated by Erk, they phosphorylate Rb which causes it to unbind from E2F

Question 16

What is the effect of E2F not being blocked in cell growth?

Answer 16

Key S phase protein are expressed and the restriction point reached

Question 17

What happens if the cell does not commit to S phase?

Answer 17

It enters a specialised state - G0 state

Question 18

What is G0 state?

Answer 18

Cells can enter a non-dividing state in which they can remain in for a long period of time until signalled to emerge

Question 19

What is the effect of DNA damage on the cell cycle?

Answer 19

DNA damage can arrest the cell cycle in G1 phase

Question 20

What effect does inducing p21 have on the cell cycle?

Answer 20

p21 binds to and inhibits complexes of cyclin-cdk. In G1, this prevents phosphorylation of Rb so the cell remains in G1

Generally, it can bind to main cellular Cdks and block cell cycle at multiple stages

Question 21

What effect does inducing Puma have on the cell cycle?

Answer 21

Puma inhibits Bcl2, a key protein that inhibits cell death. So, inducing Puma will allow cell death to happen

Question 22

What is the role of p53 in cancer?

Answer 22

p53 is inactivated therefore cell cycle arrest and apoptosis are inhibited so unrestrained replication of damaged DNA leads to high rate of mutations that tend to become cancerous

Question 23

How does DNA damage prevent cells from entering M phase?

Answer 23

DNA damage activates ATR which activates Chk1 which phosphorylates and inactivates Cdc25 therefore cyclin B-Cdk1 is not activated

Question 24

What are mitogens?

Answer 24

Signals that stimulate cells to divide

Question 25

What is the effect of activating ATM and ATR on the cell cycle?

Answer 25

They activate and phosphorylate downstream kinases Chk1 and Chk2

Question 26

What is the effect of activated Chk1 and Chk2 on the cell cycle?

Answer 26

They phosphorylate and stabilise p53

Question 27

What is the effect of activated p53 on the cell cycle?

Answer 27

It induces two proteins: p21 and Puma

Question 28

What is the role of MCM and Cdt1 on replication?

Answer 28

These proteins help form the pre-replication complex

Question 29

If there is no DNA damage, what happens to p53?

Answer 29

it is degraded by associating with Mdm2

Question 30

What targets DNA for degradation?

Answer 30

Ubiquitin

Question 31

What are pre-replication complexes?

Answer 31

Pre-replication complexes sit on the DNA (just like E2F) but are are being inhibited from carrying out their role (replicating DNA) by Cdt1

Question 32

What is the effect of Ctd1 on pre-replication complexes?

Answer 32

Ctd1 is needed for the formation of pre-replication complexes, however, when Ctd1 is bound to the complexes they are silent and cannot start replicating

Question 33

What is the role of Geminin in S phase?

Answer 33

Geminin binds to Ctd1 and removes it from pre-replication complex so that replication can start

Question 34

What happens to Geminin in anaphase?

Answer 34

It is degraded therefore allowing Ctd1 to form pre-replication complex

Question 35

What is the ultimate goal of Geminin and Ctd1?

Answer 35

To ensure that the DNA replicates ONCE per cell cycle

Question 36

What complex is needed for the cell to enter anaphase?

Answer 36

APC/C (anaphase promoting complex/cyclosome)

Question 37

How is APC/C activated?

Answer 37

APC/C is activated when it associates with Cdc20 and they form a ubiquitin ligase

Question 38

What is the role of ubiquitin ligase?

Answer 38

It adds a protein of 76 amino acids called ubiquitin to proteins and targets them for proteolysis (degradation)

Question 39

What are the 2 targets of active APC/C?

Answer 39

Cyclin B and Securin

Question 40

What is the effect of active APC/C on cyclin B?

Answer 40

Leads to degradation of cyclin B so M-phase Cdk is no longer active

Question 41

What is the effect of active APC/C on securin?

Answer 41

Seperase binds to degrades cohesion proteins so that sister chromatids can be seperated Securin keeps seperase in its inactive form

Question 42

What does Mad1 do?

Answer 42

Mad1 is attached to kinetochore and can change the conformation of Mad2 from open to closed when they bind

Question 43

What does Mad2 do?

Answer 43

When it is in a closed form, it binds to and inhibits Cdc20

Question 44

When there are no Mts attached to kinetochore:

Answer 44

cMad2 binds and inhibits Cdc20 so that cell doesn't proceed to anaphase (because there are no Mts)

Question 45

When Mts are attached in a stable way to kinetochore:

Answer 45

Mad1 dissociates from kinetochore and stops producing closed form of Mad2 p31comet displaces cMad2 from Cdc20 Free Cdc20 can now associate with APC/C and activates it to initiate anaphase

Question 46

What does "Mts bound stably mean"?

Answer 46

Mts bound to kinetochore in amphitelic fashion (both kinetochores are attached to Mts from the two spindle poles)

Question 47

What are the 3 unstable forms of attachment?

Answer 47

Merotelic (only 1 kinetochore is attached to Mts that come from both spindle poles) Monotelic (only 1 kinetochore is attached to MTs that comes from 1 spindle pole) Syntelic (both kinetochores attached to Mts that come from 1 spindle pole)

Question 48

How does cell know that Mts are attached in stable manner?

Answer 48

When they are attached in amphitelic way, there is TENSION in chromatids and the amphitelic attachments move the kinetochore further away from Aurora B, preventing phosphorylation of key kinotchore proteins (this does not happen with 3 other attachments)

Question 49

What happens when 3 other attachments take place?

Answer 49

No tension, so aurora B phosphorylation of kinetochore proteins occurs - Ndc80 complex active which loosens MT attachments and allows them to reform correctly again