regulation of 1st cell cycle checkpoint

Question 1

cyclin/Cdk complexes regulate each other

Answer 1

favourable extracellular enviro

• > G1-cyclin synthesis
• > G1-Cdk complexes

-> G1/S-cyclin + S-cyclin synthesis

after S-phase
-> M-Cdk drives G2/M transition
+ inhibits DNA re-replication

high APC/C level drives through 3rd checkpoint

Question 2

cyclin/Cdk activity inhibited at checkpoints

Answer 2

before S-phase:
DNA damage inhibits G1/S-Cdk + S-Cdk

unreplicated DNA + DNA damage inhibits M-Cdk

chromosome unattached to spindle
inhibits APC/C

Question 3

upstream and downstream regulation of cyclin/Cdks

Answer 3

up
- regulation of cyclin/Cdk activity

down
- regulation of target proteins by cyclin/Cdk

Question 4

mitogens

Answer 4

extracellular signal molecule that stimulates proliferation

Question 5

mitogens regulating progression through Start checkpoint
(upstream regulation)
- features
- example

Answer 5

stimulate G1/S-Cdk and S-Cdk activity

MAPK/ERK pathway involved

signalling via enzyme-linked receptors

EGF

Question 6

mitogens regulating progression through Start checkpoint
- 1st process

(producing the transcription regulatory protein)

Answer 6

1. mitogen activates mitogens receptor
2. activates Ras protein
3. activates MAP kinase cascade
4. ERK protein translocates into nucleus via pore
5. activates Elk1 or Myc

Question 7

mitogens regulating progression through Start checkpoint

- 2nd process

Answer 7

1. Myc upregulates expression of delayed-response genes
   = G1-cyclin
2. G1-cyclin levels increase + interact with Cdk
3. active G1-Cdk phosphorylates Rb
   - > releases active E2F
4. active E2F activates transcription of G1/S-cyclins + S-cyclins

Question 8

mitogens regulating progression through Start checkpoint

- 3rd process

Answer 8

1. G1/S-Cdk + S-Cdk phosphorylate Rb
   - > + feedback loop
2. E2F proteins cause further E2F transcription
   - > + feedback loop

Question 9

effect of:

• constitutively active Ras
• abnormally high Myc levels
• lost Rb function

Answer 9

= deregulated cell cycle

-> cancer

Question 10

regulators of G1/S-Cdk and S-Cdk at Start checkpoint (upstream regulation)

• promoters of entry into cell cycle passed 1st checkpoint:

Answer 10

Ras = GTPase

Raf/Mek/Erk = kinases

Myc = TF for delayed-response genes

Cdks = kinases

E2F = TF for S-phase genes

Question 11

regulators of G1/S-Cdk and S-Cdk at Start checkpoint

• block entry into cell cycle when active

Answer 11

Rb = transcriptional repressor

- inhibits E2F

Question 12

Cdk activity regulated by..?

Answer 12

Cdk inhibitor proteins binding

e.g. p21

Question 13

how does DNA damage prevent G1/S-Cdk and S-Cdk activation?

Answer 13

1. DNA damage activates ATM
2. ATM phosphorylates Chk2 kinase
3. Chk2 kinase phosphorylates p53
4. p53 unbinds from Mdm2 (ubiquitin ligase)
5. p53 activates transcription of downstream genes e.g p21
6. p21 encodes Cdk inhibitor
7. CKI binds to G1/S-Cdk and S-Cdk inactivating them

Question 14

p53

Answer 14

most frequently mutated protein (TF) in cancer

Question 15

S-Cdk activation triggers S-phase
(downstream regulation)

Pre-Rc

Answer 15

Pre-RC contains:
 ORC, 
Cdc6, 
Cdt1 
DNA helicases

Question 16

S-Cdk activation triggers S-phase

• in G1 phase
• in S phase

Answer 16

G1:
Pre-RCs form

S-phase:

1. origins of replication on DNA open
2. S-Cdk inhibits cdc6 + assembly of new Pre-RCs
3. prevents further DNA replication

Question 17

geminin

Answer 17

involved in S-phase

- inhibits Cdt1

Question 18

M phase

Answer 18

(chromatids held together by cohesin)

M-Cdk

• > cohesin broken down
• > chromatids pulled apart

Question 19

downstream regulation of G1 phase again

Answer 19

APC/C activated
-> degrades geminin

+ S-cdk inhibited

= allows pre-RCs to form

Question 20

how does S-Cdk inhibit Cdc6?

Answer 20

phosphorylation