Cell Cycle Flashcards
Replicated chromosomes are then segregated to two
daughter cells
Phases of the cell cycle
Four phases of the cell cycle: G1, S, G2, and M phase. M phase includes mitosis and cytokinesis
G1 - Interval of cell growth before
DNA replication (chromosomes unduplicated)
S- Interval of cell growth when the DNA is replicated (all chromosomes duplicated)
G2- Interval after DNA replication; the cell prepares to divide
Interphase ends for parent cell
Prophase
Metaphase
Anaphase
Telophase
cytoplasmic division; each descendant cell enters interphase
M-phase
Prophase -Chromosome condensation, Assembly of Mitotic Spindle
Prometaphase – Breakdown of nuclear envelope. Chromosome attachment to spindle microtubules
Metaphase - Chromosome alignment at equator of spindle, midway between spindle poles
Anaphase - Sister chromatids separate
Telophase – Two sets of daughter chromosomes arrive at poles of spindle and decondense. Nuclear envelope reassembles around each set (completion of two nuclei)
Cytokinesis: division into two cells (begins in Anaphase and ends shortly after telophase)
Cell cycle control system Operates like a
timer that triggers events in a set sequence
- Switches are binary (on/off). Events launched in a irreversible fashion
- Robust due to backup mechanisms
- Highly adaptable-can be modified to suit specific cell types or respond to intracellular and extracellular signals
Cyclins are proteins that control
cyclical changes in Cdk activity. Form a complex w/ Cdks and Act as “Activating Subunit.”
Cyclical changes in cyclin protein levels result in
the cyclic assembly and activation of cyclin-Cdk complexes at specific stages in the cell cycle
Cyclin-dependent complexes include
G1/S Cdk, S-Cdk, M-Cdk
Cyclin protein does not simply “activate” Cdk, but
directs it to specific target proteins
Levels of G1/S Cdk, S-Cdk, M-Cdk throughout the cell cycle
Cdk Activation
The T loop blocks the active site, when bound to cyclin, the loop is weakened, CAK phosphorylates the T loop, causing a conformational change and an increases ability to phosphorylate
Wee1
(kinase): Inhibits Cdk activity by phosphorylation at amino acid pair (kinase active site), at 2 sites
Cdc25
(phosphatase): Increases Cdk activity via dephosphorylation. (removes the inhibitory phosphate)
CKIs
(Cdk inhibitor proteins): inactivate cyclin-Cdk complexes, rearrangement of Cdk active site, inactivation. (wraps around CDK) (Cdk site is active during G1 and S, so this does not play then)
Prophase M cdk
induce assembly of mitotic spindle, triggers chromosome condensation (chromosomes are visible, compact), breakdown of nuclear envelope, rearrangements of actin cytoskeleton and Golgi apparatus
Prophase M cyclin
M cyclin accumulates (high stability in interphase), corresponds with increase in M-Cdk (Cdk1 and M-cyclin) as cell approaches mitosis
M cyclin and M Cdk in Prophase
(cdc25 phospate is activated by s-cdk)
Why activate M-Cdk with CAK, and then inactivate M-Cdk with Wee 1 kinase?
cell is at the G2-M cell cycle checkpoint. The “pause” is necessary to check -> cell growth, DNA (If damage, time to repair)
Explain how progression of the cell into mitosis can be described as a “biochemical switch”
M-cdks are primed and ready to act (inhibited by wee1 kinase) If cell is ready for division than pos feedback loops activate m-cdks (quickly promote activation of all m-cdk complexes in cell) promotes and abrupt and complete transition into mitosis
Centrosome duplicates in ___
During ___ they migrate to oposet ends of the cell
S phase (semiconservative replication)
Most animal cells contain a SINGLE centrosome
During prophase they migrate
Condensin is a
5 subunit protein complex, forms a ringlike strucure that uses energy (ATP hydrolysis) to promote compaction and resolution of sister chromatids
this is during prophase
Changes coiling of DNA-important for
condensation - the DNA is less likely to be damaged
This is energy intensive, only ocours when the cell is ready for division
Prometaphase is when
Nuclear Envelope Breakdown
mitotic spindle occurs
microtubule instability
kinetochore attachment.
How Nuclear Envelope Breakdown
Remove barrier: M-Cdk phosphorylates nuclear pore complexes, which initiates disassembly of complexes and dissociation from envelope
M-Cdk during the nuclear envelope breakdown
Phosphorylates components of nuclear lamina
Why Nuclear envelope breakdown
Nuclear envelope separates chromosomes from microtubules and centrosomes
Mitotic Spindle
Bipolar array of microtubules, pulls sister chromatids apart in anaphase, assembly is triggered by M-Cdk
Mitotic Spindle assembly is triggered by
M-Cdk
Types of microtubules in mitotic spindle
- Astral - connect with cell cortex, role in positioning mitotic spindle
- Kinetochore - assocate with kinicore of chromosome
- interpolar
Kinesins: Usually move toward
plus end of MT
(exception kinesin14)
Dyneins: Move toward
minus end of MT
Kinesin-5
Interact with plus ends of antiparallel MT (spindle midzone). Two motor domains move toward + end of MTs, they slide the two antiparallel MT past each other toward spindle poles
Kinesin-14
moves towards minus
shorten mitotic spindle
inc. in crossing/overlap
Kinesin- 4,10
makes direct contact with sister chromatids
away from poles twards center
Microtubule Instability in mitosis
abrupt change in cell’s MT.
Interphase: Few, long MT from single centrosome
Mitosis: Larger number of shorter, more dynamic MT, from 2 centrosomes
Half-life of MT decreases dramatically. Increased ability of centrosomes to nucleate MT (dense arrays of spindle MTs)
Regulatory proteins influence MT dynamics
Kinetochore Attachment
End of prophase, sister chromatid pairs are bombarded by plus end MTs (coming from 2 directions). Kinetochores do not instantly achieve the correct MT attachment to both spindle poles
Most initial attachments are lateral, kinetochore approaches the side of a passing MT (assisted by kinesin). Soon, plus ends capture correct orientation
lots of trial and error. seen in image- sliding past each other.
When sister chromatid pair is properly bi-oriented on spindle, two kinetochores are
pulled in opposite directions (strong forces).
Sister chromatid cohesion resists the poleward forces, which creates high levels of tension within kinetochores
Low vs High tension-kinetochore
Low tension-kinetochore sends out inhibitory signal, allows detachment of microtubules (Aurora B kinase generates signal at kinetochore - it phosphorylates key proteins resulting in the release of microtubules)
High tension: Aurora kinase unable to reach kinetochore proteins thus the attachment stays.
Metaphase
Multiple Forces Act on Chromosome in the Spindle
Kinetochore force: Produced by proteins at kinetochore. Depolymerization at plus end of microtubules (poleward)
Microtubule force: Poleward force, microtubules dismantled at minus ends
Polar ejection force: Pushes chromosomes away from poles. Motor protein diected (kensin 4,10)
Microtubule Flux: MT are pulled toward spindle poles and dismantled at minus ends
Kinetochore force:
Produced by proteins at kinetochore. Depolymerization at plus end of microtubules (poleward)
Microtubule force:
Poleward force, microtubules dismantled at minus ends
Polar ejection force:
Pushes chromosomes away from poles.
(polar wind): Motor proteins (Kinesin 4 and 10) interact with interpolar MT and transport chromosomes away from poles
Microtubule Flux:
MT are pulled toward spindle poles and dismantled at minus ends
In metaphase, the addition of new tubulin at the plus end of MT compensates for the loss of tubulin (minus end), MT length remains constant (despite movement of MT toward spindle pole)
Evidence for Polar ejection force
How opposing forces may drive chromosomes to metaphase plate?
Anaphase
triggers metaphase to anaphase transition
Protein destruction
APC/C (anaphase-promoting complex , or cyclosome):
Member of ubiquitin ligase enzyme family
Catalyzes the ubiquitylation and destruction of two protein types (securin and S and M cyclins)
remains active in G1 (stable period of Cdk inactivity), turned off when G1/S Cdk is activated in late G1
Securin:
Securin destruction activates protease, separates sisters in anaphase
S and M cyclins durring metapase to anaphase transition
Destruction inactivates most Cdks in the cell. Proteins phosphorylated by Cdks from S phase to early mitosis are dephosphorylated (anaphase).
Proteolysis by APC/C
discovery of spindle assembly checkpoint
Drugs that destabilize MT, arrest cells in mitosis for hours (days)-
Spindle Assembly Checkpoint
Ensures that chromosomes are correctly bi-oriented on spindle before anaphase
Sensor mechanism: monitors strength of MT attachment to kinetochore (sensing tension)
If not kinetochore is not properly attached to the spindle, a negative signal BLOCKS Cdc20-APC/C activation
Mad2 (recruited to unattached kinetochores): conformational change, so Mad2 can bind and inhibit Cdc20-APC/C
Anaphase A and B
A - microtubules shrink
B- lengthen spindle
More motor protein driven
Identify forces at each end of the microtubule
A - metahase - addtion at plus, loss at + (keticore dorce) loss balances with add
B - Anapase - no addtion
Telophase
Oposite of prophase
dissemble mitotic spindle
Nuclear envelope in Telophase
dephosporlation of lanins by phosptases or because mCdks are no longer active
dephosporlation promoste formation of nuclear envolope.
Cytokinesis
Division of Cytoplasm
Appearance of cleavage furrow on cell surface. Furrow deepens and spreads around cell-until cell is divided in two
