Week 5 - Cell Division - Holy

Question 1

What proteins are responsible for driving the cell cycle? How are they regulated, and what are their targets? What are the two primary ways Cdk activity is regulated? What are Ckis?

Ya thats a lot!

Answer 1

Cyclin dependent Kinases (CDK) and cyclins basically run the whole show.

CDK’s must be bound to cyclin and CAK (cdk activating kinase) must phosphorylate CDK’s activating site for CDK be active.
Wee1 can inhibit by phosphorylating an inactivation site, but this can be reversed by Cdc25 phosphatase to activate the Cdk-cyclin complex once more! Hoorah!

Watch out though! Cki (Cyclin-dependent kinase inhibitor) can stop the party AGAIN by binding and inhibiting the whole complex. Usually its for a good cause because they have to repair some DNA damage or something.

Question 2

How are positive feedback loops used in cell cycle regulation?

Answer 2

Positive feedback loops are used in the mitogen receptor – RAS – MAP kinase – E2F pathway!

Basically what happens is that after G1-CDK is activated through c-myc its going to activate E2F by phophorylating Rb.
E2F is then going to go on to help transcribe G1/S cyclins and S cyclins. These activated cyclins and CDKs are actually going to positively enhance the activation of E2F again and again! BAM! Positive feedback!

Question 3

How is S-phase triggered? What are ORCs and pre-RCs? What features of this system control the block to re-replication? Why is overall Cdk activity very low in early G1?

Answer 3

ORC’s are origins of replication in eukaryotic DNA. Proteins are there to mark the spot. Pre-replication complexes (pre-RC’s) form at the ORC’s at the beginning of G1 because there is very low CDK activity. S-CDK arrives and triggers S-phase as it phosphorylates in order to degrade and inactivate Cdc6 and Cdt1. These proteins will stay phosphorylated throughout the rest of the cell-cycle, keeping S-phase from reoccurring because they can’t form the pre-RC whole phosphorylation keeps them inactive.

Question 4

What are the cell cycle checkpoints????

Answer 4

G1 Checkpoint

• Is environment favorable?
• Enter S phase!

G2 Checkpoint

• Is all DNA replicated?
• Is environment favorable?
• Enter Mitosis!

Metaphase Checkpoint

• Are all chromosomes attached to the spindle?
• Exit Mitosis!

Question 5

What are the main molecules involved in sensing and transducing DNA damage into cell cycle arrest? What are p53, ATM, Chk, and Bub proteins, and how do they function

Answer 5

DNA damage aactivates ATM kinase, which activates Chk1/Chk2.
Chk1/Chk2 activate p53 by phosphorylation.
P53 then binds to the regulatory region of the p21 gene and transcribes/translates p21
P21 is a CDK inhibitor, so this inactivates the CDK and arrests cell cycle until the damage is taken care of.
Bub- activated when one side of the kinetochore is not attached to the spindle. Causes cell cycle halt.

Question 6

What are the 3 arms of the MAPkinase pathway?

Answer 6

MAPKKK - MAPKK - MAPK

p38

JNK

(p38 and JNK are both Rho proteins that can be involved with other stuff as well)