EXAM2_L22_Regulation_of_Cell_Division

Question 1

CDK-
What required for activity?
cdk levels in the cell?
how many complexes active?

Cyclin-
levels? when made? when degraded?
What function?
What inhibition?

What is limitation of Cyclin-CDK?

Answer 1

CDKs:

• REQUIRE CYCLIN
• Phosphorylate Threonine/ Serine
• CDK levels remain constant/Cyclin levels vary
• one cdk-cyclin dominates at any given time

CYCLINS:

• synthesized only during phases when needed
• Determines which proteins phosphorylated by CDK
• cdk-c inhibited by CDKIs even if cyclin still bound to cdk

Cyclin-CDK limited to the phase where complex forms

Question 2

What 3 checkpoints of focus?

What is each point dependent on?

Answer 2

1. Restriction point (g1)– MITOGEN signals
2. g2-M Checkpoint–DNA proofreading
3. M-phase checkpoint (spindle assembly checkpoint)
   - - Proper alignment of chromosomes in metaphase

Question 3

How does cell cycle begin?
What signal? Where bind?
What activity activated? What does it make?
What TF activated?
What does it activate? What phase? why special?

Answer 3

MITOGEN (Growth factor- extracellular ligand binds)

• Ligand binds receptor tyrosine kinases
• phosphorylation cascade initiated (MAP kinase cascade)
• MAP kinase makes/activates transcription factor MYC
• MYC activates CYCLIN D (cyclin for g1 phase)
• Cyclin D-CDK4/6 is first active complex in cell cycle

Question 4

G1 phase activation

Answer 4

mitogen>ras>map kinase cascade> MYC> CyclinD > CyclinD-CDK 4/6

Question 5

When will the cell enter quiescent state G0?

How do cells re-enter the cycle?

Answer 5

No Mitogens
Low Cyclin D
No CDK4/6 activity

Activate G0 to G1 cycle by:
–MITOGEN SIGNAL to make CYCLIN D

MITOGEN-> RAS>MKC>MYC>CyclinD>CyclinD-CDK4/6

Question 6

What 2 things required to pass g1 restriction point and proceed to S phase?
Why is this necessary?/what does it activate?
What does this activate? why important?

What is Rb? What named after? When will it present?

Answer 6

1st- CyclinD-CDK4/6
2nd- CyclinE-CDK2
Both required to phosphorylate Rb
-Rb dissociates from E2F (TF) & E2F NOW ACTIVE

Rb- Tumor supressor protein

• named by “retinoblastoma”- cancer resulting from bad Rb gene
• Retinoblastoma AGE 0-4

Question 7

Stepwise INACTIVATION of Rb:

5 Steps

Answer 7

1. Mitogens> Cyclin D > CyclinD-CDK4/6
2. CDK4/6 hypophosphorylates few THR/SER on Rb
3. gives low activity of E2F > Cyclin E> CyclinE-CDK2
4. CyclinE-CDK2 hyperphosphorylates Rb
5. E2F FREE to promote Transcription of S-phase

Question 8

What regulates the completion of G1 and entry into S Phase?

Answer 8

phosphorylation state of Rb

cyclinD-CDK4/6 & cyclinE-CDK2 >phosphorylate Rb off of E2F (becomes active/uninhibited)

Question 9

What regulates G2-M and M-phase checkpoints?
What required to pass G2-M checkpoint?
What required to pass M-phase checkpoint?

Answer 9

CyclinB-CDK1

Passing G2->M checkpoint requires CyclinB-CDK1 ACTIVATION

Passing M-phase (spindle assembly) requires CyclinB-CDK1 DEACTIVATION

Question 10

What is MPF? what is old name?

Answer 10

Maturation Promoting Factor (sometimes cdc2)
-Cyclin B-CDK1 Complex

Regulates G2-m and M checkpoints

Question 11

Crossing G2-M checkpoint
What keeps CyclinBCDK1 inactivated during G2?
what activates it? How?

Answer 11

1. CyclinB-CDK1 (MPF) accumulated at end of G2
2. Phosphorylations of MPF keep it INACTIVE until ready
3. Dcd25 dephosphorylates MPF (now active)
4. M-phase Transition

Question 12

Passing Spindle Assembly Checkpoint (M-phase)

APC= anaphase promoting complex

Why does CyclinB-CDK1 need to be DEACTIVATED?

Answer 12

APC causes CyclinB and Cohesin DEGRADADATION

1. CyclinB degraded> MPF DEACTIVATED
2. APC Degrades Separase inhibitor> Separase Activated > Separase degrades Cohesin> chromatids separate

Mitosis requires CDK1 to be dephosphorylated ending mitosis. - possible by cyclinB degrading & phosphatases

Question 13

7 CDKI's (Cyclin-Dependent Kinase Inhibitors)
2 Groups (what are they called?)

What are the 3 CDKI’s?
What do each bind/inhibit?
What phase does each inhibit?

In environmental stress which is stimulated?
In DNA damage which is stimulated?
what promotes this CDKI?
What phase/point does this CDKI inhibit? How?

Answer 13

INK4 Group (inhibitors of CDK4/6)- G1 phase only
- p16 (environmental stress/sickness, starvation)

WAF1/CIP1 Group- (Stops Any phase)

• p21 (DNA damage)
• p51 promotes p21 mRNA>p21 protein>binds Cyclin & inhibits
• p21 inhibits cyclinE-CDK2 (stops Restriction point) by preventing Rb hyperphosphorylation

Question 14

What prevents Rb hyperphosphorylation?

What point is restricted? why?

Answer 14

p21 inhibits cyclinE-CDK2

• prevents hyperphosphorylation of Rb
• stops at the restriction point of G1 b/c E2F can’t be activated