Lecture 19 - Cell Cycle Checkpoints

Question 1

Why might there be a delay or arrest in cell cycle progression?

Answer 1

In response to problems completing a specific step in the cell cycle or in response to other cellular problems

Question 2

What might checkpoints do?

Answer 2

• Pause the cell cycle and promote repair before continuing
• Induce permanent arrest or apoptosis

Question 3

When are many checkpoints not essential?

Answer 3

Under normal conditions

Question 4

What are checkpoints essential for?

Answer 4

Protecting against cancer

Question 5

How is SAC recruited to unattached kinetochores?

Answer 5

Mad2 component of SAC is recruited to the kinetochore, activated, and released, so it binds and inhibits APC/C-Cdc20

Question 6

What happens at the spindle assembly checkpoint?

Answer 6

APC/C-Cdc20 is inhibited in response to incomplete kinetochore/microtubule attachments and allows the spindle to set up properly before anaphase is initiated

Question 7

What happens at the G1 DNA damage checkpoint?

Answer 7

• 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

Question 8

What is replicative senescence?

Answer 8

Telomere shortening –> G0 arrest (senescence)

Question 9

What does replicative senescence (telomere shortening) protect against?

Answer 9

Chromosome fusion

Question 10

What is oncogene-induced senescence?

Answer 10

Hyperproliferation due to oncogene activation –> G0 arrest (senescence)

Question 11

What do some other checkpoints detect?

Answer 11

• Hypoxia
• Loss of cell adhesion
• Other abnormal cellular conditions

Question 12

What gets recruited to sites of DNA damage?

Answer 12

ATM and ATR kinases

Question 13

What preference does ATM have?

Answer 13

Double-stranded breaks

Question 14

What preference does ATR have?

Answer 14

Exposed single-stranded DNA and other DNA damage

Question 15

What functions do ATM and ATR have?

Answer 15

1) Help recruit repair machinery
2) Activate a cell cycle machinery

Question 16

What do ATM and ATR phosphorylate?

Answer 16

Chk1 and Chk2 kinases to promote cell arrest (or apoptosis)
*They also phosphorylate to activate proteins involved in DNA repair

Question 17

What else regulates Cdk2-cyclin A to control S-phase?

Answer 17

Wee1 and Cdc25

Question 18

What do Chk1 and Chk2 phosphorylate?

Answer 18

Cdc25 phosphatase, targeting it for ubiquitination by SCF ubiquitin ligase and destruction by the proteosome

Question 19

What keeps Cdk2 phosphorylated?

Answer 19

Wee1

Question 20

How is the G2 checkpoint activated?

Answer 20

• Cdk1 remains phosphorylated by Wee1 so it is inactive
• Cells arrest in G2/M

Question 21

In the case of more severe damage, what do ATM, ATR, Chk1, and Chk2 phosphorylate and activate?

Answer 21

Transcription factor, p53

Question 22

What does p53 do?

Answer 22

• 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

Question 23

When is p53 inactive?

Answer 23

In undamaged cells due to Mdm2 binding

Question 24

What does Mdm2 do?

Answer 24

Inhibits p53 activity and ubiquitinates p53 –> proteasomal degradation

Question 25

What happens to phosphorylated p53?

Answer 25

It is released from Mdm2

Question 26

What does the cell arrest in if telomeres are too short?

Answer 26

G0

Question 27

What are telomeres?

Answer 27

Chromosome ends that contain multiple copies of a telomere repeat

Question 28

What is telomerase activity required for?

Answer 28

Telomere replication

Question 29

What are telomeres protected by?

Answer 29

Telomere binding proteins - sheltrin complex binds to telomere repeats and protects them from DNA repair machinery that repairs double-stranded breaks

Question 30

What happens in most cell types in humans that don't express telomerase?

Answer 30

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

Question 31

How is replicative senescence carried out?

Answer 31

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

Question 32

What is oncogene-induced senescence?

Answer 32

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

Question 33

What does myc overexpression lead to?

Answer 33

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

Question 34

What transcription factor is the cellular gatekeeper?

Answer 34

p53