Cell Cycle Regulation

Question 1

Describe G1-CDK

Answer 1

Promotes the passage through the restriction point.

Question 2

Describe G1/S CDK

Answer 2

Commits cells to replication.

Question 3

Describe S-CDK

Answer 3

Triggers DNA replication, acting as a switch to turn on (pre-RC) replication complex (= ORC + McM proteins + Cdc6).

Question 4

Describe M-CDK

Answer 4

Phosphorylates necessary proteins in mitosis, such as those that condense chromosomes, break down the nuclear envelope and assemble spindle fibers and centrosomes.

Question 5

How does the cell prevent re-replication (S-phase regulation)?

Answer 5

High levels of CDK leads to lower levels of Cdc6 that is needed for Pre-RC.

Question 6

How is M-phase regulated?

Answer 6

CAK partially activates M-CDK, BUT…
Wee-1 adds an inhibitory phosphate to prevent full activation.
So Cdc25 removes inhibitory phosphate to fully activate M-CDK for mitosis.

Question 7

Describe the M-CDK + Feedback loop.

Answer 7

More M-CDK inhibits Wee1, so that newly made M-CDK never sees inhibitory phosphate. M-Cdk’s also activate Cdc25 to remove phosphates at a FAST RATE.

Question 8

What deactivates M-CDK?

Answer 8

CDC20 and APC (a ubiquitin ligand) that phosphorylates M-CDK for proteasome degradation. In other words, M-CDK self-inactivates by ubiquination.

Question 9

What regulates G1 reentry?

Answer 9

An absence of Cdk’s by…

1. Ubiquitin-assisted degradation
2. Increased levels of (CKIs) Cyclin Kinase inhibitors;
   * they bind cyclin-Cdk’s to inhibit function
3. (Rb) Retinoblastoma proteins that repress S-cyclin in G1

Question 10

Describe the Regulation of Rb protein.

Answer 10

When an outside, mitogenic signal occurs, Rb can be phosphorylated to allow E2F to function as a cyclin-assisting transcription factor in G1/S and S-cyclins