Lecture 15

Question 1

What regulates cell cycle transitions?

Answer 1

CDKs

Question 2

What cyclin and CDK present in G1?

Answer 2

Cyclin D
Cdk4 and 6

Question 3

What cyclin and CDK present in S?

Answer 3

Cyclin E
Cdk2

Question 4

What cyclin and CDK present in G2?

Answer 4

Cyclin A
Cdk2

Question 5

What cyclin and CDK present in M?

Answer 5

Cyclin B
Cdk1

Question 6

What do cyclins accumulate in response to?

Answer 6

Growth factors
Transcriptional activation

Question 7

What do cyclins do after completed functions?

Answer 7

Oscillate and degrade to prevent cell stages reoccurring

Question 8

Positive regulation of CDKs

Answer 8

• Mitogen activated signalling
• Transcriptional regulation of cyclin expression
• Binding of cyclin subunits
• Activation by phosphorylation of CDK by CDK activating kinase (CAK)
• Removal of inhibitory phosphorylation sites

Question 9

Mitogen dependent signalling

Answer 9

• Cells receive signals from adjacent cells or mitogens
• Extracellular signals mediated from membrane receptor to nucleus
• Activation of MAPK kinase signalling leads to Myc activation

Question 10

CDK regulation in G1 and S-phase

Answer 10

Myc promotes cyclin D and E2F expression

E2F is major transcriptional regulator of S-phase genes and cyclin E/A

Question 11

Cyclin binding + CAK

Answer 11

CDK inactive in absence of cognate cyclin binding partner

CDK phosphorylated on T161 by CAK leading to increased catalytic efficiency by conformational change

Question 12

Negative Regulation

Answer 12

CDK inhibitor proteins – bind to active sites and prevent phosphorylation of substrates.

Inhibitory phosphorylation (wee1) previous lecture.

Degradation of cyclin subunits by ubiquitin mediated proteasomal regulation

Question 13

CDKs drive cell cycle progession

Answer 13

• Cyclin levels accumulate and drop
• Degraded by proteasome
• Sequential cyclin expression provides increasing CDK profile through cell cycle
• Increase in CDK activity = Cell cycle regulated transitions
• DNA replication controlled by distinct CDK activity thresholds

Question 14

Regulation of replication licensing and activation

Answer 14

1. ORC=origin recognition complex which binds to specific regions within the genome termed DNA replication origins
2. Upon exit from mitosis when Cdk activity is low the MCM complex is loaded through interaction with loading factors: ORC, Cdt1 and Cdc6. Now termed the pre-replication complex (preRC).
3. S phase Cdk and DDK phosphorylate target proteins; ORC, Cdt1 and Cdc6 dissociate (therefore cannot re-initiate at the same origin). Irreversible.
4. Additional factors associate, MCM helicase activated and DNA helix unwound at the origin.
5. DNA polymerases loaded through interaction with accessory factors and DNA replication initiated. This complex that copies the DNA is referred to as the replisome
   r
6. Destruction of licensing factors prevents re-replication