Biochemistry L9 - Cell Cycle Control

Question 1

Give an example of a heterodimeric protein kinase involved in control of cell cycle

Answer 1

Cyclin-Cdk complex

Question 2

Which is the regulatory and catalytic subunit of the cyclin-cdk complex

Answer 2

Regulatory = cyclin
Catalytic = cdk (cyclin dependent protein kinase)

Question 3

How is kinase activity of cdk activated

Answer 3

When the right cyclin binds to cdk, kinase activity is activated

Question 4

What does cyclin do

Answer 4

Tethers target protein so cdk can phosphorylate it

Question 5

Why is cell cycle control crucial

Answer 5

Determines size and shape of organs/tissues,

Loss of control of the cell cycle can lead to cancer (uncontrolled cell division)

Question 6

What drives cell cycle progression

Answer 6

Sequential activation of different cdk-cyclin complexes

Question 7

Using G1 cdk-cyclin as an example, what are some of the actions of an activated cdk-cyclin complex

Answer 7

Activates transcription factors (turning on genes that encode DNA pol), activates genes for enzymes that produce deoxynucleosides, activates proteins involved in duplication of chromosomes, activates genes for subunits of next cdk cyclin complex

Question 8

What allows the rb-E2F complex to dissociate so E2F can translocate to the nucleus and promote transcription

Answer 8

Phosphorylation of Rb by cdk-cyclin

Question 9

What induces synthesis of G1 cyclins

Answer 9

Growth factors (mitogens)

Question 10

In the absence of growth factors can the cell cycle go past the restriction point

Answer 10

No, the cell enters G0 phase

Question 11

What protein does p53 activate

Answer 11

P21

Question 12

What does p21 do

Answer 12

Binds cdk-cyclin and inhibits kinase activity until DNA is repaired

Question 13

What does p53 recognize

Answer 13

DNA mismatches/damage

Question 14

What is ubiquitin

Answer 14

A small regulatory protein, binding of ubiquitin signals cyclin for degradation

Question 15

What do checkpoints ensure

Answer 15

That chromosomes are present and that critical stages of cell cycle are completed before next stage begins, DNA is not damaged, spindles are properly formed and chromosomes are properly attached, and that the cellular environment is favorable

Question 16

Concentration of which protein oscillates during a cell cycle?

Answer 16

Cyclin

Question 17

Describe how levels of cyclin influence the cell cycle

Answer 17

Cyclin binds to CDK at the end of G1 phase. Cyclin levels then begin to lower as it is degraded, reducing the activity of the dimer. Towards the next phase, this G1 CDK-cyclin complex will activate the transcription factors for the cyclin for the following phase. Levels of this cyclin will build, and once they reach the right level, they’ll bind to the appropriate Cdk, forming the dimer for the next phase.
AKA the Cyclin-Cdk complex of the previous phase triggers the production of the complex needed for the next phase.

Question 18

What are the three mechanisms of controlling Cdk-Cyclin activity?

Answer 18

1. Degrading cyclin concentrations
2. Hyper-phosphorylating Cdk (2 P’s are inhibitory)
3. P53 activates P21, which inhibits Cdk-cyclin complexes