L5: Cell cycle Flashcards
(10 cards)
what controls the cell cycles
Kinases
Cyclin dependent Kinases (CDKs) and their partner Cyclin proteins
Specialised mitotic kinases, such as PLK1 and Aurora B or Wee1
Checkpoint kinases that arrest the cell cycle if DNA damage is sensed
Enzymes that apply phosphate groups to target proteins to change their function and become activated and phosphorylated.
Phosphatases
PP1 and PP2A – reverse the effect of CDK activity on cell cycle substrates
CDC25, acts on Cyclin:CDK complexes to remove inhibitory phosphorylation
Remove phosphatases
Proteolysis
SCF1 and APC/C are two major cell cycle ubiquitin ligases that allow the proteolysis of cell cycle substrates and ensure unidirectionality
Regulated destruction of proteins. In cell cycle: use regulated proteolysis to make sure cell cycle runs forward and does not move back.
yeast
Commitment to cell cycle entry: yeast
Yeast cells cn grow or mate but cant do both. Mate when secrete mating factor. In the presence of these the yeast stop growing, move closer to eachother, change shape and meet. Fuse to mix their cytoplasm.
Matting factor addition induces cell cycle arrest
Addition to newly budded cells arrested them in g1
Addition to older cells arrested them in the next g1
Suggests existence of commitment point in g1 and that once cell starts the cell cycle they go all the way
metazoa
Withdrawal and replacement of serum (full of mitogens and growth factors) cause cell cycle arrest but arrest is not uniform.
In g1- cell will only continue when mitogens and growth factors. Once past restriction point can continue cell ycle and gfs and mitogens? No longer required
G1- decision to exist g1 and enter g0. Recieve differentiation cue: differentiate, g0.
When stem cell recieves they exit cell cycle and enter g0.
Withdrawal of serum: quiscence, g0
Accumulation of dna damage: sesescence not g0.
Cells can stay in g0 for hours to the life of organism
reentering cell cycle from G0
Serum return stimulates transcription of 3 waves of genes
- immediate early: tfs, for growth and division, tissue repair factor, migration, secretion
Delayed early: cyclins for later stages
Late: chromatin associated and cyclines
E.g: fibroblast wound healing response upon exposure to serum in blood
points of control for cell cycle
Points of control in the cell cycle
Morphological events during division
Mitosis entry
Nuclear envelope breakdown
Cell rounds up
Mitotic spindle assembly
Chromosomes condense and captured by spindle
Mitosis exit
Duplicated chromatids separate on the spindle
Cleavage furrow ingresses and cleaves cell during cytokinesis
Spindle condenses to form a midbody
Nuclear envelope regenerated
Cdk- attached to cyclin. Enable biochemical changes for mitosis? And morphological changes so cells can undergo mitosis.
Budding (s. cerevisiae) and fission (s.pombe) yeasts used
- grow fast, amenable to genetic and microscopal manipulation
Conditional mutants identified
Wee 1- divides too quickly. Lacking protein wee1
Cdc25 grows but doesnt divide
Cdc2- grows but doesnt divide
Yeast start out small, grow, build division plane? In the middle.
Wee1 - very small. So dividing too quickly,
Cdc25 mutants look very long so failiure in cell division. Growing and not dividing. Lacking cdc25.
Cdc2: grow but doesnt divide
Tells use wee1- negative reg od division
Cdc25 is a positive regulator of division (and cdc2)
(rewatch)
Wee1 adds a inhibitory phosphorylation to cdk. Cdc25 is a phosphatase that removes this inhibitory phosphorylation. To reg cell division, reg post-translation modifications of cdk. So can make innactive form and remove inhibitory phosphate and so allow cells to explosivley phosphorylate during mitosis? By timing, when cdc25 is activated, the cell can build up inactive cyclin/cdk and then rapdily activate it by desphosphorylating it.
fine-tuning MPF activity and proteolysis
Cdc25 phosphatase ensures tat mitotic cylcin/cdk complex accumulates in an inactive form and activated at mitotitic entry.
Proteolosysis important in cell cycle
Cyclin-cdk complex ubiquitination and destrucion = inactive cdk **
E3
SCF complex (Skp1/Cullin/F-box protein): constant through cell cycle, degrons in target proteins often phosphorylated
Anaphase Promoting Complex/Cyclosome (APC/C): active only in M and G1 due to association with regulatory subunit
Slide 25
1. Temporal ordering by biochemical specificity
Different cyclins are expressed in different phases of the cell cycle. Cyclin d-g1 until mitosis where it decreases
Cyclin e increases as cell moves to s phase
Cyclin a iexpressed as cells enter pc2
Cyclin b peaks as cells enter mitosis
Logical- diffeent subrates are phosphorylated in each phase of the cell cycle to perform specific functions
Expression of individual cyclins known to vary throughout cell cycle
Bt many cyclin/cdks can be genetically removed without impacting cell cycle order
A mitotic cyclin can initiate s-phase in xenopus egg extracts. A single cyclin/cdk chimaera can substitute for all cyclin/cdks in yeast
Most cdks are dispensable in mammals
Delete cdk1- no cell division (two-cell embryo)
Have cdk1 .. Decreased haematopoetic precuroses and cardiomyocytes. **
Cdk1 and cdk4 sufficient for adult mouse to live.
Suggests cdk a
threshold model for cell cycle transitions
Threshold model for cell cycle transitions
Suggests any cdk can phosphorylate any substrate but rate that they phosphorylate changes?
Suggests substrate specificity isnt that important
A rising cdk activity throughout cell cycle phosphorylates any and all substrates.
The densitivy of substrates to cdk activity determines when in cell cycle they are phosphorylated.
What determins specificity? (change affinity of substrate for a cyclin)
Change localisation
Change affinity of subrates cyclin docking motifs for cclins
Local sequence around phhosphorylationsite
Accessibility of a serine or threoine residue for phosphorylation
Accessibility of a phosphorylated serine or threonine residue for dephosphorylation
3 major control points
Cell Cycle Regulation: Cyclin D, Cyclin E, and the Role of Rb
G1 Phase Progression and Cyclin D:
Growth factors stimulate the expression of Cyclin D.
Cyclin D binds to CDK4 or CDK6, forming an active complex (Cyclin D–CDK4/6).
This complex partially phosphorylates the Retinoblastoma (Rb) protein.
Role of Rb and E2F:
Unphosphorylated Rb binds to E2F, a transcription factor, preventing it from activating genes needed for S phase.
Phosphorylation of Rb causes it to release E2F, which then activates transcription of:
Cyclin E
S-phase genes
Other G1/S cyclins
Acquisition of Growth Factor Independence (Restriction Point):
Cyclin E is transcribed by E2F.
Cyclin E binds to CDK2 forming Cyclin E–CDK2, which fully phosphorylates Rb.
This complete phosphorylation irreversibly releases E2F.
Cyclin E–CDK2 activity is no longer dependent on growth factors, allowing the cell to pass the G1 restriction point and commit to DNA replication.
This creates a positive feedback loop: more E2F → more Cyclin E → more CDK2 activity.
Checkpoint Control – Control Point 2 (DNA Damage Checkpoint):
Key Players:
ATM and ATR: Kinases activated by DNA damage
ATM responds mainly to double-stranded breaks
ATR responds to single-stranded DNA, often from replication stress
CHK1 and CHK2: Checkpoint kinases activated by ATM/ATR
Mechanism of Cell Cycle Arrest in Response to DNA Damage:
DNA damage (e.g., replication stress, strand breaks) is sensed.
ATM/ATR are activated and then activate CHK1/CHK2.
These kinases phosphorylate and degrade CDC25A, a phosphatase needed to activate Cyclin–CDK complexes.
CDC25A normally removes inhibitory phosphates from CDKs.
Its removal prevents CDK activation, blocking cell cycle progression.
Lower Response: Activation of p53 Pathway
ATM/ATR also lead to phosphorylation and stabilisation of p53.
Stabilised p53 activates transcription of p21, a CDK inhibitor (CKI).
p21 inhibits Cyclin–CDK complexes, reinforcing cell cycle arrest and giving the cell time to repair DNA.
metaphase to anaphase transition
Here’s a clearer and logically structured version of your explanation about the metaphase to anaphase transition, including the role of cohesin, separase, securin, and the spindle assembly checkpoint (SAC)—all formatted for easy understanding:
Metaphase to Anaphase Transition
Role of Cohesin:
During S phase, cohesin assembles as a ring-like complex that encircles sister chromatids, keeping them tightly tethered.
This cohesion ensures that sister chromatids do not separate prematurely.
Triggering Chromatid Separation:
To initiate anaphase, cohesin must be degraded.
The enzyme separase cleaves cohesin, allowing chromatids to separate when pulled by spindle fibers.
However, separase is normally kept inactive by a protein called securin.
Role of APC/C (Anaphase Promoting Complex/Cyclosome):
APC/C is a ubiquitin ligase complex that targets proteins for degradation.
It becomes activated at the metaphase–anaphase transition.
APC/C ubiquitinates securin, marking it for degradation by the proteasome.
Once securin is degraded, separase becomes active, cleaving cohesin and enabling chromatid separation.
Spindle Assembly Checkpoint (SAC): Ensuring Accuracy
Purpose:
Prevents anaphase onset until all chromosomes are properly attached to the spindle (bi-oriented on the metaphase plate).
Ensures chromosome segregation is accurate to prevent aneuploidy.
Mechanism:
Unattached kinetochores recruit checkpoint proteins: BubR1, Bub3, and Mad2.
These proteins bind and inhibit CDC20, an activator of APC/C.
This prevents APC/C from degrading securin and cyclin B, delaying anaphase entry.
Once all kinetochores are correctly attached, microtubules displace SAC proteins, freeing CDC20.
APC/C is now active, securin is degraded, separase is released, and anaphase begins.
SAC and Cancer:
Defects in SAC lead to chromosome missegregation, causing:
Lagging chromosomes
Micronuclei
Catenated (interlinked) DNA
Ultimately, aneuploidy, a hallmark of many cancers
Therapeutic Targeting:
Chemotherapeutic agents like vinca alkaloids and taxanes exploit SAC by:
Disrupting microtubule dynamics
Causing prolonged SAC activation
Leading to mitotic arrest and apoptosis in cancer cells
