Control Of Cell Division

Question 1

What is the primary function of the cell cycle?

Answer 1

To control when cells divide

The cell cycle is essential for growth, development, and tissue repair.

Question 2

What are the two main phases of the cell cycle?

Answer 2

S phase and M phase

S phase is where DNA is replicated, and M phase is where mitosis occurs.

Question 3

What drives the cell cycle?

Answer 3

Cyclins and Cdk complexes

Cyclins activate cyclin-dependent kinases (Cdks) which regulate cell cycle progression.

Question 4

What is the role of checkpoints in the cell cycle?

Answer 4

To ensure proper division of chromosomes and prevent damage

Checkpoints assess conditions before allowing progression to the next phase.

Question 5

What happens during the G1 phase of the cell cycle?

Answer 5

Cell growth and preparation for DNA synthesis

G1 phase is the first gap phase before the S phase.

Question 6

Fill in the blank: The human genome is divided into ______ chromosomes.

Answer 6

46

Question 7

What occurs during the G2 phase of the cell cycle?

Answer 7

Preparation for mitosis and further cell growth

G2 phase follows S phase where DNA has been replicated.

Question 8

True or False: All cell types in the human body have the same set of genes.

Answer 8

True

With a few exceptions, all cells are genetically identical.

Question 9

What is Cdk1’s role in the cell cycle?

Answer 9

To activate mitosis

Cdk1 must be activated for the cell to transition from G2 to M phase.

Question 10

What triggers the destruction of Cyclin B?

Answer 10

Activation of the APC

The anaphase-promoting complex (APC) regulates the transition out of mitosis.

Question 11

What does the spindle assembly checkpoint (SAC) monitor?

Answer 11

Chromosome alignment on the spindle

The SAC ensures that all kinetochores are attached before mitosis proceeds.

Question 12

List the three key decision points in the cell cycle.

Answer 12

• Entry into S phase
• Entry into M phase
• Exit from M phase

Question 13

What is the significance of the T-loop in Cdk activation?

Answer 13

It occludes the active site until cyclin binds

Binding of cyclin causes a conformational change that activates Cdk.

Question 14

What can lead to the activation of Cdk inhibitors?

Answer 14

DNA damage signals

Cdk inhibitors prevent cell cycle progression in response to damage.

Question 15

What is the role of Cdc25 phosphatase?

Answer 15

To remove inhibitory phosphates from Cdk1

This activation is crucial for entering mitosis.

Question 16

Fill in the blank: During ______, chromosomes condense and the nuclear envelope breaks down.

Answer 16

Mitosis

Question 17

What are the two steps required for cell division?

Answer 17

• Nuclear division (mitosis)
• Cytoplasmic division (cytokinesis)

Question 18

What are the consequences of entering S phase without sufficient building blocks?

Answer 18

Genome replication errors

This can lead to genetic instability and cancer.

Question 19

True or False: Embryonic cell cycles have complex phases.

Answer 19

False

Embryonic cell cycles often consist of simple S➜M oscillators.

Question 20

What is ‘quiescence’ in the context of the cell cycle?

Answer 20

G0 phase

A state where cells are not actively dividing

Quiescent cells can re-enter the cell cycle when conditions are favorable.

Question 21

What are the main roles of the ubiquitin-proteasome system in the cell cycle?

Answer 21

To regulate protein degradation

This system controls the levels of cyclins and other regulatory proteins.

Question 22

What is the function of the anaphase-promoting complex (APC)?

Answer 22

To trigger the transition from metaphase to anaphase

APC degrades cyclins and other proteins to facilitate mitotic exit.

Question 23

How does the cell ensure checkpoints are met before proceeding?

Answer 23

Through signaling pathways that assess cell conditions

These pathways check for DNA damage and sufficient resources.

Question 24

Why do cells divide?

Answer 24

• to sustain life
• propagate heritable traits
• generate mass
• generate diversity

Question 25

What cyclins & Cdks are involved in different phases of the cell cycle?

Answer 25

- G1 —> Cyclin D and Cdk4 & Cdk6 - G1/S —> Cyclin E and Cdk2 - S —> Cyclin A and Cdk2 - M —> Cyclin B and Cdk1

Question 26

How does cyclin binding activate Cdks ?

Answer 26

Without a bound cyclin, the Cdk is inactive because the T-loop occludes the active site and prevents substrates from binding When bound, a conformational change pulls the T-loop away from the active site and allows substrate proteins to bind

Question 27

Regulation of cyclin-Cdk complexes

Answer 27

- Phosphorylation = activating or inhibitory - Binding of CKI = Cdk inhibitors - Localisation = Cyclin B is imported into the nucleus just before mitotic entry - Ubiquitin mediated proteolysis = provides directionality

Question 28

Where are the 3 checkpoints?

Answer 28

1. At the end of G1, before S phase = is the environment favourable? 2. At the end of G2 = is all DNA replicated? Is all DNA damage repaired? 3. Towards the end of M phase = Are all chromosomes properly attached to the mitotic spindle?

Question 29

What are the steps for getting in and out of mitosis?

Answer 29

1. Switch off the DNA damage checkpoints —> DNA damage activates Wee1 to put inhibitory phosphates on Cdk1 2. Activate Cdk1 to get into mitosis —> Cdc25 removes inhibitory phosphates leading to Cdk1 activation 3. Silence the spindle assembly checkpoint —> when all kinetochores are attached to the spindle, APC is activated 4. Inactivate Cdk1 to get out of mitosis —> Cyclin B is degraded and ubiquitinated, inactivating Cdk1

Question 30

Activation of cyclin B-Cdk1

Answer 30

CAK (Cdk activating kinase) adds a phosphate to the complex Now active Wee1 (a kinase) adds inhibitory phosphates to the complex Now inactive Cdc25 (a phosphotase) removes the inhibitory phosphates Now active again and ready for mitotic entry

Question 31

How does Cdk1 activation trigger mitosis?

Answer 31

- activation of Cdk1 - phosphorylation of target proteins including other kinases - activates mitotic pathways and inactivates interphase pathways - brings about the morphological changes associated with mitosis e.g. chromosome condensation and spindle assembly

Question 32

How is cyclin B degraded?

Answer 32

- anaphase promoting complex (APC) = E3 - E3 ubiquitin ligase - 13 different proteins - activated by co-factors e.g. Cdc20

Question 33

Silencing the SAC

Answer 33

- APC becomes active in mitosis - but the SAC which is activated by the unattached kinetochores blocks full activation of APC until all the chromosomes are correctly aligned on the spindle - when all kinetochores are attached, the SAC is silenced and the APC is activated - Cyclin B is degraded by the APC, triggering sister chromatid separation - anaphase and mitotic exit are coupled