The Cell Cycle Control System

What’s the cell cycle?

a set of biochemical events driven by a control system that tells cells whether they can enter the next phase of the cell cycle

What controls the cell’s decision to enter the cell cycle?

multiple checkpoints in the cell cycle

What’s the cell cycle?

a set of biochemical events driven by a control system that tells cells whether they can enter the next phase of the cell cycle

What controls the cell’s decision to enter the cell cycle?

multiple checkpoints in the cell cycle

What is the function of the mitosis checkpoint?

Checks if all of the chromosomes are properly attached to the mitotic spindle to successfully pull the duplicated chromosomes apart.

What is the function of the G2 checkpoint?

Checks if all the DNA is replicated. Checks if all the DNA damage has been repaired.

How does the cell-ccle progress through difference phases?

precise coordination of multiple gene expressed during each phase

What are the two molecular switches that control the cell cycle?

Protein kinases and protein phosphatases

What does a protein kinase do?

covalently adds a phosphate group to a protein

What does a protein phosphatase do?

removes a phosphate group from a protein

What are the two irreversible points in the cell cycle?

1) S phase/the replication of genetic material

2) M-phase, the separation of sister chromatids

What happens at a cell-cycle checkpint?

The cell cycle process is assessed and possibly halted

Describe the experimental procedure and result for the discovery of MPF activity?

Procedure: Take the cytoplasm of an oocyte undergoing the M-Phase and inject it into an oocyte that is arrested is M-Phase

Result: the cytoplasm drives the arrested oocyte into M-phase

What does MPF stand for?

Mature/Mitosis Promotion Factor

How does MPF activity progress through the cell cycle?

MPF oscillates through the cell cycle. MPF activity rises only for mitosis and drops sharply for interphase

What protein kinases are found in MPF actvity?

cell-cycle dependent kinases (M-cdk)

What are cyclins?

A group of distinct proteins sharing 100 amino acids in binding and activation domains. Controls MPF activity

Describe the cycles of MPF and Cyclin over the cell cycle.

Cyclin and MPF levels peak at M-phase. Cyclin levels start to rise starting at S-phase

What makes up MPF’s physically?

MPF = cdk + M-Cyclin. The cyclin is a regulatory subunit while the cdk is a catalytic subunit

What does cdk stand for?

Cyclin-dependent kinase

When is MPF-activity active?

When cyclin levels are high enough to trigger passage through G2 to M-phase

What are the 4 steps of activating the MPF (cdk-cyclin complex)?

1) cdk and M-cyclin combine to form inactive M-cdk

2) the inhibitory kinase (Wee1) and the activating kinase (Cak) phosphorylates both the inhibitory and activating sites on the cdk subunit

3) Phosphatase cdc-25, dephosphorylates the inhibitory site

5) the M-cdk complex is finally activated

What are the two inhibitory and activating kinases? What’s the activating phosphatases?

wee1 (inhibitory kinase) and Cak (activating kinase). Cdc-25 is the activating phosphotases

What is there an extra step in the process of activating the M-cdk complex?

it helps regulate cyclin levels so the cell doesn’t enter M-phase pre-maturely.

What happens if you mutate wee1 or cdc-25?

For wee1, the cell doesn’t know when to stop replicating because there is no inhibitory kinase activity/no activation for the inhibition site/stays in M-phase.

For cdc-25, the cell continues to keep growing because there is no activating phosphatases/no removal of the phosphate on the activation site it stays in G2 to grow/no M-phase.

Why does cyclin concentrations accumulate slowly yet MPF activity rises so sharply.

Because of the positive feedback loop. Activated M-cdk complex is a part of a positive feedback loop to activate more activating phosphatase (cdc-25)

How is the exit out of mitosis regulated?

through the rapid degradation of cyclins by the Uniquitin-protesome pathway

What is the ubiquitin-protesome pathway?

Lots of ubiquitin tag the cyclin. As a result, the cyclin gets degraded and the cdk becomes inactive.

How many cdk-cyclin protein complexes are there?

Theres a complex and a cyclin for each cell cycle stage.

Describe the G1/S cell cycle checkpoint. 3 things…

1) aka the restriction point. It’s the primary decision point and the most critical

2) If a cell receives the Go signal based on internal and external signals, it divides

3) if the cell does not receive the signals, it exits the cycle and switches to the G0 phase

What happens to cell that do not pass the G1 checkpoint?

They exit the cell cycle and go into G0.

Describe the G0 phase? 4 things…

1) non-dividing, differentiated state

2) most human cells are in the G0 phase

3) liver cells are in G0 most of the time but they can re-enter the cell cycle

4) nerve and muscle cells are highly specialized and stay arrested in G0

What are the 3 functions of G1 checkpoint?

1) Check if DNA replication was completed correctly, that there are no errors

2) detects unreplicated DNA, holds at cell in G0 or arrests at G1

3) detects damaged DNA, arrests the cell in G1 until the damage was repaired

What happens when DNA is damaged?

the activating phosphatases, cdc-25, is inhibited. In general, the Mitotic cyclin-cdk complex is inhibited

What are cdk inhibitors?

important at the G1-s checkpoint to arrest a cell with DNA damage. The inhibitor protein p27 binds to an activated cyclin cdk-complex. This protects it from activating the target proteins required to progress through G1 into S phase

What are the steps of using p21 to inhibit cdk? 5 steps…

1) DNA damage is detected by p53

2) p53 is phosphorylated and activated to binds to the p21 transcriptional region

3) p21 is then transcribed into RNA then translated into a protein

4) p21 cdk inhibitor protein binds to G1/S1 cdk complex and inactivates it

5) p53 is degraded via ubiquitin-proteosome when not needed

Describe p53? 6 things…

1) aka the Guardian of the Genome

2) it’s a transcription factor

3) p53 levels increase in the presence of DNA damage

4) p53 activated gene coding for p21

5) if DNA is not repaired successfully, p53 induced apoptosis

6) 50% of all cancers have mutations in p53

What happens if a mouse is deficient for p53?

It’s predisposed to getting multiple types of tumors

What is the consequence of the loss of cell cycle control at the g1/S checkpoint?

abnormal cells continue to go through the cell cycle