Control Of Cell Division Flashcards
What is the primary function of the cell cycle?
To control when cells divide
The cell cycle is essential for growth, development, and tissue repair.
What are the two main phases of the cell cycle?
S phase and M phase
S phase is where DNA is replicated, and M phase is where mitosis occurs.
What drives the cell cycle?
Cyclins and Cdk complexes
Cyclins activate cyclin-dependent kinases (Cdks) which regulate cell cycle progression.
What is the role of checkpoints in the cell cycle?
To ensure proper division of chromosomes and prevent damage
Checkpoints assess conditions before allowing progression to the next phase.
What happens during the G1 phase of the cell cycle?
Cell growth and preparation for DNA synthesis
G1 phase is the first gap phase before the S phase.
Fill in the blank: The human genome is divided into ______ chromosomes.
46
What occurs during the G2 phase of the cell cycle?
Preparation for mitosis and further cell growth
G2 phase follows S phase where DNA has been replicated.
True or False: All cell types in the human body have the same set of genes.
True
With a few exceptions, all cells are genetically identical.
What is Cdk1’s role in the cell cycle?
To activate mitosis
Cdk1 must be activated for the cell to transition from G2 to M phase.
What triggers the destruction of Cyclin B?
Activation of the APC
The anaphase-promoting complex (APC) regulates the transition out of mitosis.
What does the spindle assembly checkpoint (SAC) monitor?
Chromosome alignment on the spindle
The SAC ensures that all kinetochores are attached before mitosis proceeds.
List the three key decision points in the cell cycle.
- Entry into S phase
- Entry into M phase
- Exit from M phase
What is the significance of the T-loop in Cdk activation?
It occludes the active site until cyclin binds
Binding of cyclin causes a conformational change that activates Cdk.
What can lead to the activation of Cdk inhibitors?
DNA damage signals
Cdk inhibitors prevent cell cycle progression in response to damage.
What is the role of Cdc25 phosphatase?
To remove inhibitory phosphates from Cdk1
This activation is crucial for entering mitosis.
Fill in the blank: During ______, chromosomes condense and the nuclear envelope breaks down.
Mitosis
What are the two steps required for cell division?
- Nuclear division (mitosis)
- Cytoplasmic division (cytokinesis)
What are the consequences of entering S phase without sufficient building blocks?
Genome replication errors
This can lead to genetic instability and cancer.
True or False: Embryonic cell cycles have complex phases.
False
Embryonic cell cycles often consist of simple S➜M oscillators.
What is ‘quiescence’ in the context of the cell cycle?
G0 phase
A state where cells are not actively dividing
Quiescent cells can re-enter the cell cycle when conditions are favorable.
What are the main roles of the ubiquitin-proteasome system in the cell cycle?
To regulate protein degradation
This system controls the levels of cyclins and other regulatory proteins.
What is the function of the anaphase-promoting complex (APC)?
To trigger the transition from metaphase to anaphase
APC degrades cyclins and other proteins to facilitate mitotic exit.
How does the cell ensure checkpoints are met before proceeding?
Through signaling pathways that assess cell conditions
These pathways check for DNA damage and sufficient resources.
Why do cells divide?
- to sustain life
- propagate heritable traits
- generate mass
- generate diversity
What cyclins & Cdks are involved in different phases of the cell cycle?
- G1 —> Cyclin D and Cdk4 & Cdk6
- G1/S —> Cyclin E and Cdk2
- S —> Cyclin A and Cdk2
- M —> Cyclin B and Cdk1
How does cyclin binding activate Cdks ?
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
Regulation of cyclin-Cdk complexes
- 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
Where are the 3 checkpoints?
- At the end of G1, before S phase = is the environment favourable?
- At the end of G2 = is all DNA replicated? Is all DNA damage repaired?
- Towards the end of M phase = Are all chromosomes properly attached to the mitotic spindle?
What are the steps for getting in and out of mitosis?
- Switch off the DNA damage checkpoints —> DNA damage activates Wee1 to put inhibitory phosphates on Cdk1
- Activate Cdk1 to get into mitosis —> Cdc25 removes inhibitory phosphates leading to Cdk1 activation
- Silence the spindle assembly checkpoint —> when all kinetochores are attached to the spindle, APC is activated
- Inactivate Cdk1 to get out of mitosis —> Cyclin B is degraded and ubiquitinated, inactivating Cdk1
Activation of cyclin B-Cdk1
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
How does Cdk1 activation trigger mitosis?
- 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
How is cyclin B degraded?
- anaphase promoting complex (APC) = E3
- E3 ubiquitin ligase
- 13 different proteins
- activated by co-factors e.g. Cdc20
Silencing the SAC
- 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
