The Cell Division Cycle

Front 1

What are the steps of the cell cycle?

Back 1

1. G1-phase: cell growth and prep for S-phase.

2. S-phase: replication of chromosomes

3. G2-phase: cell growth and synthesis of proteins used in mitosis

4. M-phase: mitosis. Cells divide. Cytokinesis occurs.

Front 2

What is the cell-cycle control system?

Back 2

A complex network of regulatory proteins in eukaryotes that ensures that cells replicate all their DNA and organelles and divide in an orderly manner. Plays a central part in the regulation of cell numbers in tissues of the body.

Front 3

What does the cell-cycle control system ensure near the end of G1, G2, and Mitosis?

Back 3

1. Ensures the environment is favorable for replication in S phase.

2. Ensures all DNA is replicated and all damaged DNA is repaired before mitosis.

3. Ensures all chromosomes are properly attached to the mitotic spindle.

Front 4

What is the G0 phase?

Back 4

A period in the cell cycle in which cells exist in a quiescent (dormant/inactive) state. It's an extension of the G1 phase in which the cells rest until the conditions of cell division are favorable. (The stop/go point before S phase).

Front 5

What does the cell-cycle control system rely on?

Back 5

Cyclically activated protein kinases called Cdks.

Front 6

How is regulation largely carried out?

Back 6

Through phosphorylation (kinases) and dephosphorylation (phosphatases) of proteins involved in central processes of cell cycle.

Front 7

True or false: protein kinases are present in proliferating cells throughout the cell cycle, and they are always active.

Back 7

False. While they are always present, they are only activated at the appropriate times during the cycle, after which they are quickly inactivated.

Front 8

What are cyclins?

Back 8

A set of proteins in the control system partially responsible for for switching cell cycle kinases on and off. They have no enzymatic activity themselves, but they need to bind cell-cycle kinases before the kinases can become enzymatically active. Over the course of the cell cycle, concentration of each type of cyclin rises gradually and then falls abruptly.

Front 9

What are the kinases of the cell-cycle control system called?

Back 9

Cyclin-dependent protein kinases (Cdks).

Front 10

True or false: different cyclin/Cdk complexes trigger different steps of the cell cycle.

Back 10

True.

Front 11

What is M cyclin? What is M-Cdk?

Back 11

1. The cyclin that acts in the G2 phase to trigger entry into M phase.

2. The complex M cyclin forms when it binds with its Cdk.

Front 12

What do S cyclins and G1/S cyclins do?

Back 12

Bind to the appropriate Cdk late in the G1 phase, making S-Cdk and G1/S-Cdk complexes. These complexes help to launch S phase.

Front 13

What are G1 cyclins?

Back 13

They act earlier in G1 and bind specific Cdks to make the G1-Cdk complex. Helps drive the cell through G1 toward S phase. In animals cells, their formation usually depends on extracellular signal molecules that stimulate cells to divide.

Front 14

What are the 7 principles of cell cycle regulation?

Back 14

1. Different cyclin/kinases activate different phases and events

2. Specific Cdk/cyclins are active at only one stage of the cell cycle.

3. Cdk/cyclin activity (cycle progression) is controlled by 3 mechanisms.

4. Feedback amplifies and sharpens transitions.

5. Checkpoints use various mechanisms to pause the cell cycle until ready.

6. Growth factors regulate Cdk/Cyclin activity.

7. Eack Cdk/cyclin activity has its own set if substrates.

Front 15

What precipitates the rapid fall in cyclin concentration?

Back 15

A full-scale targeted destruction of the cyclin protein through ubiquitylation. Marks proteins with a chain of ubiquitin, which directs them to proteasomes for degredation. Cyclin degradation inactivates the Cdks.

Front 16

What is the Anaphase-promoting complex (APC)?

Back 16

A protein complex that triggers the separation of sister chromatids and orchestrates the carefully timed destruction of proteins that control progress through the cell cycle; the complex catalyzes ubiquitylation of M and S cyclins part way through M phase.

Front 17

How do cells exit mitosis?

Back 17

M cyclins are ubiquitylated by the ubiquitin ligase called Anaphase-promoting complex.

Front 18

What are Cdk inhibitor proteins?

Back 18

A way other than phosphorylation and dephosphorylation to regulate the activity of Cdks. When bound to cyclin/Cdks, they can maintain cyclin/Cdk complex in an inactive state. This pauses the cell cycle, allowing the cell to wait for favorable division conditions.

Front 19

What is Wee1, and what does it do?

Back 19

It is an inhibitory kinase. It phosphorylates M-Cdk complex at two adjacent sites so the M-Cdk complex is inactive.

Front 20

What is Cdc25, and what does it do?

Back 20

It is an activating phosphatase. It dephosphorylates the phosphate from Wee1 on the M-Cdk complex so it is active. It leaves the phosphate added by CAK, which also helps activate the complex. If it is blocked, the cell can't enter mitosis. This checkpoint is to avoid incomplete DNA replication or moving forward with damaged DNA.

Front 21

What is a Cdi? Give an example and explain what it does.

Back 21

A Cdk inhibitor. An example is p27. It binds to and activated cyclin/Cdk complex and prevents it from phosphorylating target proteins required for progress through G1 into S phase. Affects G1- and G1/S-Cdk complexes.

Front 22

What are the cell- cycle control system checkpoints?

Back 22

The cell's internal state and conditions in the environment.

1. Only allows entry to S phase if environmental contidions are appropriate. (Stop/Go). End of G1.

2. Triggers mitosis only after DNA has been completely replicated and is undamaged. End of G2.

3. Initiates chromosome segregation only after duplicated chromosomes are correctly aligned on the mitotic spindle. Near end of Mitosis.

Front 23

What is CAK? What does it do?

Back 23

It is a Cdk-activating kinase. It phosphorylates the M-Cdk complex to activate it. Competes with Wee1, which inactivates the complex.

Front 24

How many positive feedback loops ensure a quick switch from inactive M-Cdk in G2 to active M-Cdk in M-phase? What are they?

Back 24

Two.

1. Once activated, M-Cdk phosphorylates and activates more Cdc25, allowing the phosphatase to activate even more M-Cdk by removing inhibitory phosphate from Cdk subunit added by Wee1.

2. Once activated, it can also inactivate the inhibitory kinase Wee1.

Front 25

Why is G1 an important decision point for the cell?

Back 25

Depending on intracellular signals about cell size and extracellular signals about the environment, the cell-cycle control machinery can hold a cell transiently until conditions are right (G0 phase), allow it to prepare for entry into the S phase, or withdraw from the cell cycle altogether in terminally differentiated cells.

Front 26

What does the inhibition of APC do?

Back 26

Occurs when at least 1 chromosome fails to attach to a mitotic spindle. Prevents exit from mitosis.

Front 27

When DNA is damaged, say by an X-ray, how is the cell cycle arrested in the G1 phase? (Ex of a checkpoint)

Back 27

Specific protein kinases respond by activating/phosphorylating the p53 protein and stabilizing it. Activated p53 accumulates and stimulates transcription of the p21 gene, which encodes Cdk inhibitor protein p21, which then binds to G1/S-Cdk and S-Cdk to inactivate them, so cell cycle arrests in G1. Allows time for DNA repair.

Front 28

What is another example of a checkpoint?

Back 28

The spindle assembly checkpoint. Occurs when a mitotic spindle is damaged or unattached to a kinetochore, preventing exit from mitosis

Front 29

In quiescent cells, are Cdks active?

Back 29

No

Front 30

True or false: Cdk is active always in proliferating cells.

Back 30

False. During most of the G1 phase, Cdk is inactive. G1-Cdk complex activated at end of G1.

Front 31

What affects whether a cell will proliferate or not?

Back 31

Growth factors like PDGF (platelet derived growth factor) and mitogens (substances that induce or stimulate mitosis).

Ex: PDGF activates proliferation of macrophages.

Front 32

What is E2F?

Back 32

A transcription factor bound to a lot of genes that you want to activate in S phase.

It is inactive when bound by an Rb inhibitor protein until something like PDGF activates the G1- and G1/S-Cdk complexes, which phosphorylates Rb and inactivates it, releasing E2F.

(This example comes from how macrophages are told to proliferate).

Front 33

Cdk/cyclin activity has its own set of substrates. Give an example for S-Cdk complex.

Back 33

S-Cdk fires DNA replication and prevents re-replication.

1. Phosphorylates and activates helicase, triggering opening of replication fork. Also phosphorylates ORC, leading to recruitment of DNA polymerase to fork.

2. Cdc6 necessary to activate the ORC, allowing helicase binding.

3. S-Cdk complex phosphorylates Cdc6, leading to degradation so the ORC isn't fired again before the next cell division.

Front 34

Cdk/cyclin activity has its own set of substrates. Give an example for M-Cdk complex.

Back 34

M-Cdk complex disassembles the nucleus.

1. Near start of M phase, M-Cdks phosphorylates lamin proteins, causing breakdown of nuclear lamina (IF cytoskeleton).

2. Phosphatase de-phosphorylates lamin proteins at end of mitosis to allow nuclear lamins to reassemble.

Front 35

Which Cdk/cyclin complex activates condensins? Which activates E2F transcription factor? Which activates transcription of G1/S-Cdks?

Back 35

1. M-Cdk complex.

2. G1-Cdk complex.

3. G1-Cdk complex. The resulting complexes then further activate their own transcription. The G1-CDK complex also phosphorylates Rb proteins.

Front 36

What are two classic experiments that helped scientists figure out how the cell cycle was regulated?

Back 36

1. Injecting a frog oocyte with m- phase cell cytoplasm to induce mitosis. Injecting interphase cytoplasm didn't have this effect. Called whatever triggered it MPF (maturation promoting factor).

2. S. pomb yeast cdc2 mutants that prevented cells from dividing. Instead they continued to grow.

(Cdc2 now called Cdk was the cause).

Front 37

What are some of the negative regulators in the cell cycle? What happens when they're inactived?

Back 37

Cyclin/Cdks like p53, p21, and Rb. Often inactivated in tumor cells.

Front 38

What are some of the positive regulators of Cdk activity?

Back 38

Cyclin D (a G1 cyclin), and Cdc25. Often activated in tumor cells.

Front 39

What are some FDA-approved drugs for treatment of breast cancer?

Back 39

Ribociclib, palbociclib, and abemaciclib. They are G1 cyclin inhibitors. They inhibit Cdk4/6.