13.) The Cell Cycle

Question 0

How can you determine the birthdate of a cell during development?

Answer 0

With radiolabeled nucleotides: Only cells that are replicating their DNA will take up these nucleotides.

Question 1

What is the general sequence of events that comprise the cell cycle?

Answer 1

G1 phase (RNA and Protein Synthesis)
S-phase (DNA Replication)
G2-Phase (RNA and Protein Synthesis)
M-phase (mitosis and cytokinesis)
G0 phase: At this point, terminally differentiated cells will withdraw from the cycle indefinitely. Cells re-enter in early G1 phase.

Question 2

What parts of the cell cell cycle are regulated? Ie what are the specific cell cycle checkpoints?

Answer 2

Important restriction point is between G1 and S phase. Once a cell passes this point, it is committed to the S-phase.

Question 3

What does it mean to say that the cell cycle control system is a protein kinase-based machine?

Answer 3

The entire regulatory control of the cell cycle is mediated by cyclin dependent protein kinases. Its activity is modulated by the presense of different types of cyclins, which change based on the different phases of the cell cycle (eg there are G1, S, and M cyclins).

Question 4

What are Cdks? What role do they play in the cell cycle?

Answer 4

Cdk = cyclin dependent protein kinase. This is a serine/threonine kinase enzyme that will create a wide range of phosphorylation events in the cell that correlate with the cell cycle. The activity of cyclin dependent protein kinase is regulated by cyclin molecules. The same CDK will be chaperoned differently based on the type of cyclin that binds to it. The different types of cyclin correspond to the different phases of the cell cycle.

Question 5

What are cyclins? What role do they play in the cell cycle?

Answer 5

Cyclins play the role of activating and chaperoning CDK to specific substrates. They are constantly formed and degraded during the cell cycle. There are different types of cyclins that will chaperone CDK to different, specific substrates depending on what time of the cell cycle the cell is in.

Question 6

Compare/contrast the differences between a mutation in cdc and wee.

Answer 6

• cdc:* Mutations in these proteins will arrest the cell cycle at a specific point. Typically due to a mutation in a protein that is required to get past a cell cycle checkpoint.
• wee:* Mutations in these proteins will cause cells to divide at a smaller size than normal. Typically this is due to a mutation in a protein that inhibits passage through a size checkpoint (G1 and G2?)

Question 7

How is the cell cycle connected to the environement?

Answer 7

A favorable extracellular environment (eg sufficient nutrients) will stimulate G1-Cdk formation, and in turn G1/S-cyclin synthesis.

• DNA damage* will inhibit G1/S-Cdk, M-Cdk, and S-Cdk formation. (eg. double strand breaks)
• Unreplicated DNA* will halt M-Cdk synthesis.

A chromosome that is unattached to its spindle will block the activity of APC/C.

Question 8

How does the cell cycle contribute to the understanding of cancer?

Answer 8

Cancer = uncontrolled replication.
This could be due to many of reasons, one of which is an activating mutation of a proliferation gene, (or possibly the continuous inactivation of a tumor suppressor gene?).

Question 9

Define proliferation genes.

Answer 9

These are genes that promote cell growth by promoting the assembly of the cell cycle control system, specifically at the G1 choke point.

Question 10

Define anti-proliferation genes.

Answer 10

These are genes that halt the cell cycle control system. These code for tumor suppressors.

Question 11

Define tumor suppressors.

Answer 11

These are proteins that bind to certain transcription factors to prevent the formation of proteins that drive the cell cycle forward.

Question 12

How does a flow cytometer determine which part of the cell cycle a cell is in?

Answer 12

It measures the relative amount of that is in a cell. The most being in G2 and Mphase, the second most in S phase, and the least is in G1phase.

Question 13

What is the APC/C enzyme? What is its function? How is it regulated?

Answer 13

APC/C = Anaphase promoting complex or cyclosome.

It is a ubiquitin ligase enzyme that is activated at the beginning of M-phase (mitosis) so that it can polyubiquinate cyclins. This signals degradation of cyclins by proteosomes.

APC/C needs to be activated by a Cdc subunit.

Question 14

True or False: The concentration of CDK’s in the cell oscillate depending on the phase of the cell cycle.

Answer 14

False: It is the concentration of cyclins that oscilalte. CDK concentration remains the same, often their concentration exceeds that of cyclins.

Question 15

What are securins? What would happen if these proteins were deactivated/destroyed?

Answer 15

Securins protect proteins that hold sister chromatids together. If they are destroyed, this causes activation of a protease (APC/C) that separates the chromatids and triggers anaphase.

Question 16

How do wee and cdc proteins affect the activity of M-Cdk? How is this process regulated?

Answer 16

M-cyclin protein binds to Cdk, exposing a T-loop in the Cdk protein, but the protein still needs to be phosphorylated to be active. It is then simultaneously phosphorylated by Wee1 kinase (Cdk-inactivating kinase) and by CAK (Cdk-activating kinase). This gives the complex an inhibitory phosphate and an activating phosphate. Cdc25 (inactive phosphatase) will then remove the inactivating phosphate to yield the active M-Cdk.

M-Cdk will modulate the activity of other M-Cdk’s by inhibiting Wee1 and stimulating Cdc25.

Question 17

What is an oncogene?

Answer 17

This is an activating mutation in a proliferating gene.

Question 18

What is the major cell cycle control point in eukaryotes?

Answer 18

G1 phase to S phase.

Question 19

How do growth factors contribute to the cell cycle?

Answer 19

Binding of growth factors will stimulate the MAPK cascade, which will activate TF such as Jun and Fos. These will promote the synthesis of cyclins, CDKs, and other transcription factors such as E2F, providing the necessary enzymes and proteins for DNA synthesis and allowing the passage from G1 phase into S phase.

Question 20

Retinoblastoma protein is an example of a…

Answer 20

tumor suppressor protein. Phosphorlyation inactivates this protein.