Cell Cycle 1

Question 1

What does this figure suggest?

Answer 1

That cyclin B is associated with M-phase regulation, b/c its protein levels peaked at Mphase and dropped immediately after

Question 2

In general, cyclins show kinase activity toward what structure?

Answer 2

Histones

Question 3

What is CDK?

Answer 3

Cyclin Dependent Kinase - binds to cyclin (specifically we learned about cyclin B) and phosphorylates targets

Question 4

What are the targets of CDK?

Answer 4

Nuclear lamins - phosphorylation promotes breakdown of nuclear envelope

Condensin - phosphorylataion stimulates condension, making DNA more compact

Microtubule associated proteins - promotes spindle assembly

Membrane associated proteins - promote dispersal of ER and golgi

Question 5

Explain what this figure is showing.

Answer 5

• MPF = clycin B/CDK –> the activity of this enzyme complex is needed for mitosis to occur
• This complex also needs to be stopped in order for mitosis to end
• Level of cyclins varies, but level of CDK remains constant –> MPF activity variation is due to changes in [cyclin B]

Question 6

At what stage of the cell cycle does cyclin B expression begin to rise?

Answer 6

G2 - it has to rise here b/c mediates transition into M phase. Also - if you look at the graph - the MPF activity begins to rise at the beginning of G2. If CDK is constant, then that must mean that cyclin B activity/expression increases at the start of G2.

Question 7

Describe how cyclin B is degraded.

How is this degradation pathway initiated?

Answer 7

• All cyclins contain an a series of amino acids that make up the “destruction box”. The sequence of amino acids in this part of the protein are targeted by Anaphase Promoting Complex (APC) which is an E3 ubiquitin ligase, so E3 polyubiquitinates the protein at this location and targets the protein for degradation in the proteasome.
• Degradation of cyclin B is triggered by the activity of cyclinB/CDK. When cyclinB/CDK are at their highest levels of activity (metaphase), they phosphorylate APC, which activates APC and leads to degradation of cyclin B.