1-12 Cell Cycle

Question 1

Overview of control of cell cycle requirements for success

Answer 1

1. timer to regulate duration of each event
2. mech to ensure correct order
3. mech to ensure each event happens once
4. mech that triggers complete and irreversible events
5. redundancies in case of malfunction
6. adaptable system for different cell types/conditions

Question 2

Cyclin / Cyclin-dependent kinase

Answer 2

Cyclins: oscillate their availability during cell cycle (expression/degredation)

CDKs: present throughout cell cycle, phosphorylates

Come together to make active and now can phosphorylate downstream targets

Activity regulated by 1-Cyclin kinase inhibitors (CKI), 2-inhibitor and activating phosphrys of cdk/cyclin

Question 3

Cycle/CDKs partners and function

Answer 3

CDK1/Cyclin B: G2/M
CDK2/Cyclin E: G1/S transition
CDK2/Cyclin A: S phase, G2 phase
CDK4/Cyclin D: G1
CDK6:Cyclin D: G1 phase

Question 4

Cyclic proteolysis of cyclins

Answer 4

1. inactive APC activating by binding of activating subunit
2. activated APC + ubiquitin +E1 +D2 enzymes ubiquitinate cyclin
3. ubiquitinated cyclin degraded by proteasome

Question 5

Regulation of Cdk activity (4)

Answer 5

1. Association with cyclins
2. activating phosphorylation of cdk
3. inhibitory phosphorylation of cdk
4. association with cdk inhibitors (CKIs)

Question 6

Cyclin proteolysis of CKIs

Answer 6

1. kinase phosphorylates the CKI
2. SCF has special f-box protein that recognizes phospho-cki and binds with ubiquitylation enzymes and UBs
3. degreated by proteasome

Question 7

2 families of CKIs

Answer 7

1. INK4 family, mainly G1 phase

2. CIP/KIP family- broad spectrum inhibition

Question 8

3 checkpoints of cell cycle

Answer 8

G2 checkpoint:pass if cell size adequate, chromosome replication successfully completed
Metaphase checkpoint: pass if all chrom attached to mitotic spindle
G1 checkpoint: pass if cell size adequate, nutrient availability sufficient growth factors/signals from other cells present (most major checkpoint)

Question 9

2 roles of cdks

Answer 9

1. indirect: cell cycle specific transcription
2. Direct: control of cell cycle specific processes (kinetochore assembly, replication origin firing, growth factor response)

Question 10

mitogen stimulation of cell division

Answer 10

1. binds mitogen receptor on outside of cell
2. activates ras-> MAPkinase
3. activates Myc-> increased cycle D synth, increased CKI degredation, Increased E2F synth

Progresses to S phase

promotes assembly of cyclinD+CDK4orCDK6, and a CIP/KIP protein into a complex

Question 11

Rb regulation

Answer 11

Rb: binds E2F on the DNA during low mitogen conditions, expression of Sphase genes in INHIBITED

mitogen conditions increase, cyclinD/CDK4 increase, cyclinE/CDK2 increase, Rb is phosphorylated by cyclinE/cdk2 leading to inactivation

inactivated Rb releases E2F, E2F active and expression of Sphase genes activated

positive feedback with: E2F->CDK2/cyclinE->phosphoRb->E2F active

also positive feedback: cyclinE and cyclinA, inhibit Rb, increase E2F, increase Sphase transcription, increase cyclin E/A—>DNA synth

Question 12

Licensing

Answer 12

limits activation of origins of replication to once per cell cycle to avoid re-replicating DNA

G1 1. ORC (origin recognition complex) bound by helicase and proteins but kep inactive
G1/S 2. S-cdk triggers S phase by degrading proteins that keep ORC inactive
S 3. pre-initiation complex phosophrylated ORC “fired” and DNA replicated
G2/M 4. M-CDK maintains phosphorylation of ORC so wont fire again

Question 13

G1/S dna damage checkpoint (p53/p21)

Answer 13

1. DNA damaged
2. p53 phosphorylated (activated) by cascade activated by DNA damage
3. active p53 binds to regulatory region of p21 gene to transcribe
4. p21 translated, CK1 that inactivates G1/S and S cdks

primary targets: CyclinD/cdk4/6, cyclinE/cdk2

Question 14

DNA replication supresses mitotic progression

Answer 14

Replication fork suppress mitotis until DNA replication is completed-

Question 15

Senescence

Quiescence

Answer 15

permament cell cycle arrest, fxns to remove potentially damaged cells from cell population, induced by DNA damage, mito dysfunction, aneuploidy, telomere attrition, oxidative stress

temporary removal of normal cells from the cell cycle, “reversible non proliferative state”, differentiated cell tyeps/stemcells/induced by low mitogen/nutrients and reinformed by CKIs- INK4 binds cyclinD-CDK4/6 relasing KIP1 which inhibits cyclinE/CDK2, cyclin D levels are low, Rb remains hypo-pohosprylated and bound to E2F

Question 16

hayflicks limit

Answer 16

replicative senescence, hayflicks limit improsed by shortened telomeres, defense mech against cells that want to overproliferate

as telomere length shortenes drives into senescence

germine cells have high telomerase activity/telomrere length throughout many divisions

Crisis early: cells must overcome telomere induced cell cycle arrest/death (on method is to inhibit p53)

Crisis late: cells overcome limit of shortened telomeres by re-expressin telomerase or Alt pathway

Question 17

paploma virus example of overcoming hayflicks limit

Answer 17

knows that cells need to proliferate to spread virus, therefore mutates p53, binds/mutates Rb