IT2 - Cell Cycle Flashcards
(46 cards)
What is the difference between embryonic and non-embryonic cell cycles?
Embryonic cell cycles are driven by an oscillator that periodically triggers S and M phase, and there’s no feedback to the oscillator. As such, the cycle is constant.
In non-embryonic cell cycles, cell cycles events negatively control the oscillator i.e., if a certain cell cycle event is not completed, the oscillatory is blocked.
How was tubulin discovered? Describe its structure.
Colchicine is a drug known to block mitosis, and it was shown using radioactive drug binding assays that it does so through blocking microtubule proteins.
Two subunits (alpha and beta) that form a heterodimer. These bind to GTP and polmerise into a filament with defined polarity.
How is cytokinesis regulated?
Greatwall/ENSA inhibits PP2A-B55 (sets phosphorylation environment).
PRC1 crosslinks spindle microtubules.
Aurora B recruits Kif4A to shape spindle structure.
PRC1 recruits PLK1 → activates Ect2 → RhoA activation.
MKLP1 (centralspindlin) supports vesicle transport to furrow.
How is the genome only replicated once in the cell cycle?
Cdk complexes phosphorylate and inhibit DNA replication proteins e.g., MCM2 and ORC
Inhibitor proteins e.g., Geminin which inhibits Cdc6.
What 4 things might stop progression through the cell cycle?
- High CDK1 from mitosis would block entry through the restriction point
- Lack of nutrients and growth factors should block entry through the restriction point
- Unreplicated chromosomes from S phase
- Unattached kinetochores from metaphase
Describe the bistable switch mechanism of CDK1-cyclin B complex activation.
When cyclin B is absent:
CDK1 activity is low
Wee1 activity is high
Cdc25 activity is low.
As cyclin B increases:
Wee1 phosphorylates CDK1 (T14/Y15), keeping it inactive.
Upon reaching a critical cyclin B threshold:
CDK1 inactivates Wee1 and activates Cdc25.
This leads to rapid CDK1 activation (requires T161 phosphorylation).
Cyclin B degradation by APC/C:
CDK1 activity decreases.
Cdc25 activity drops.
Wee1 activity increases.
How is the APC/C regulated?
Activation requires CDC20.
Inhibition is via the mitotic checkpoint complex (MCC), composed of MAD and BUB proteins.
How are sister chromatid cohesins protected?
- Shugoshin (Sgo1 recruits PP2A a phosphatase, to the centromere.
- Together, they protect centromeric cohesin by counteracting phosphorylation by mitotic kinases, thus inhibiting Wapl activity at the centromere.
- Shugoshin is removed from centromeres in a tension-dependent manner during metaphase, taking PP2A with it, allowing centromeric cohesin to be cleaved by separase at anaphase onset.
How have sea urchins been used to study the cell cycle?
Metabolic labeling of sea urchin eggs showed cyclical synthesis of proteins, coordinated with CDK activity.
Led to identification of cyclins and that their synthesis drives the cell cycle.
Further research showed CDK-cyclin complexes are conserved amongst many animals.
How is mitotic entry regulated at the level of CDK1-cyclin B?
- Cyclin B is synthesized in late S/G2 phases and accumulates in the nucleus.
- Cyclin B binds CDK1 in the inactive form.
- CDK1 is phosphorylated by the CDK-activating kinase (CAK).
- CDC25 removes the inhibitory phosphates on CDK1, placed there by Wee1.
- CDK1- cyclin B is now fully active and phosphorylates substrates for mitotic entry.
What is the difference between astral microtubules and kinetochore fibres?
These are the 2 main populations of microtubules.
Astral microtubules position the spindles along the midline of cells. Kinetochore fibers capture and pull chromosomes into position.
What are centrosomes? What are they composed of?
How are microtubules anchored to centrosomes?
Organelles that function as the microtubule organising centers.
They’re composed of a pair of centrioles that are embedded in a meshwork of proteins (PCM).
When cells enter mitosis, the PCM recruits the microtubule - ends and anchors them at the centrosome.
What is the retinoblastoma protein?
A tumor suppressor protein that suppresses E2F transcription factors. When is becomes hyperphosphorylated at the restriction point by CDK4-cyclin D, E2F is able to promote transcription of other cyclins and CDKs.
How does PLK1 act as a timer for abscission?
Microtubules target ESCRT III to the division site, but only after PLK1 is degraded.
What is the anaphase central spindle?
A bundle of microtubules caused by the overlap during chromosome separation that marks the cell cleavage site.
What is the function of FOXM1?
It’s a transcription factor that promotes mitotic entry.
APC/C-mediated degradation of FOXM1 is thus important for resetting the cell cycle.
What are the roles of Cdc2, Cdc25 and Wee1?
CDC2, also known as CDK1, is a protein kinase that forms a complex with cyclins, which leads to phosphorylation of target proteins that promote cell cycle progression.
Wee1 is a protein kinase that plays a critical role in the negative regulation of CDC2 activity. It phosphorylates a specific site on CDC2, which inhibits its activity and prevents the cell from entering mitosis.
CDC25 is a phosphatase that activates CDC2 by removing the inhibitory phosphorylation.
What are the two types of SMC protein used in mitosis?
- Cohesin
- Condensin
How have budding and fission yeast been used to study the cell cycle?
Budding:
- Genetic screens identified cell cycle mutants.
- Pheromones were used to synchronise cell cycle
- Identified different cell cycle proteins
Fission:
- Paul Nurse discovered the cdc2 gene from mutants and complementation experiments
- cdc2 gene encodes a CDK that was shown to require phosphorylation for activation
- Identified Wee1
How have human cell lines been used to study the cell cycle?
Genetic studies showed that CDK-cyclin activation requires phosphorylation:
- Wee1 found to inhibit CDK1
- CDC25 found to activate CDK1
What are the advantages and disadvantages of using a bistable system for CDK1-cyclin B activation?
Advantages:
- The complex can stabilize its own activity
- Process is rapid
- Kinase activity used is robust to temperature and oxidative stress fluctuations
- Allows further regulation by other stimuli
Disadvantages:
- The complexes’ active state is highly stable so requires a special mechanism to reverse this.
Describe the structure and function of cohesin molecules.
A ring-like multi-subunit ATPase that binds and links sister chromatid DNA following DNA replication.
What are microtubules? What are they composed of?
Dynamic polymers with defined polarity.
They’re composed of alpha and beta tubulin heterodimers that bind GTP to grow at the + end.
What is the role of separase, and how is it activated?
Separase cleaves cohesin subunits when it’s phosphorylated in mitosis. This protease is normally inhibited, but when the APC/C becomes activated, its inhibitor (securin) is degraded.
