Lecture 20: Cell Cycle-overview and mechanics; mitosis; cytokinesis Flashcards
3 events of the Cell Cycle?
- cell growth and chromosome replication
- chromosome segregation
- cell division
Purpose:
- Precisely duplicate the genetic material
- Accurately distribute the replicated chromosomes into the two daughter cells
Four phases of the eukaryotic cell cylce?
Timing of a typical cell cycle
- A typical human cell may have a cell cycle taking 24 hours.
- 23 of those 24 hours are spent in interphase with 1 hr for M-phase (mitosis)
- Within interphase, S-phase (DNA replication) may take 10-12 hours
- The remaining intervals in interphase are the gap phases, G1 prior to S-phase and G2 following S-phase
___ are pauses that allow the cell to grow and respond to environmental cues.
gap phases
Gap phases
- These are important decision making transitions. For example, the commitment to undergo another round of cell division occurs with a specific point in G1-called start or restriction point.
- Transitioning this boundary commits the cell to fully cycle as it enters into DNA replication.
- Cells may choose to enter a non-cycling, resting state called G0 (for day, week or years) until environmental condition become more favorable.
what are the 6 parts of the M (mitosis) phase?
Biochemical steps of mitosis
- Chromosome condensation
- Nuclear envelope breakdown
- Replicated chromosome attach to the microtubule of the mitotic spindle
- Chromosomes align on the metaphase plate
- Chromosomes move to opposite poles
- Chromosomes decondense
- Nuclear envelope reforms
- The cell is pinched in two by cytokinesis
Interphase
prophase
chromosomes condense, spindle migration continues, microtubule dynamic instability increases greatly
prometaphase
Metaphase
alignment of the microtubule attached chromosomes at the metaphase plate
Anaphase
sister chromatids synchronously separate
Telophase
daughter chromosomes arrive at the poles, nuclear envelope reforms, assembly of the contractile ring begins
Cytokinesis
cytoplasmic division
The mitotic spindle is a microtubule-based machine
- The microtubule (MT) cytoskeleton and molecular motors kinesin and dynein are actively used to do work during mitosis.
- MTs attach to chromosomes and to each other (through motors) to generate motion.
- Specialized MTs called centrioles must duplicate to form the bipolar mitotic spindle
Centrisome duplication is triggered at the beginning of ____ by the activation of _____, which also triggers DNA replication
S-phase
G1/S-Cdk complex
Some cells, specifically higher plants and some vertebrate oocytes do not have centrisomes. In these cells, ______ are involved in forming the bipolar spindle.
motor proteins
What are the three classes of microtubules form the mitotic spindle?
- astral microtubules
- kinetochore microtubules
- overlap microtubules
Astral microtubules
– Radiate in all directions
– Contribute to forces that separate the poles
– Help orient and position the spindle
Kinetochore microtubules
– Attach end-on to the kinetochore (which form at the centromere)
Overlap mictrotubules
– Interdigitate at the equator
– Responsible for symmetry and bipolar shape of the spindle
Bipolar plus-end directed motors, like ___, push the spindles apart
Kinesin-5
Minus-end directed kinesin like ____ pull the spindles towards one another
kinesin-14
Conventional plus-end directed kinesins like ____ attach chromosomes to MTs for positioning
Kinesins 4 and 10
___ attach the astral MTs to the cell periphery and pull the spindles apart as the walk towards the minus end of MTs
Dyneins
Influence of opposing motor proteins on spindle length
- Kar3p (a kinesin-14) is a minus end motor protein
- Cin8p (a kinesin-5) is a plus end motor protein
- Too much Kar3p makes short spindles
- Too much Cin8p makes long spindles
Microtubule stability dramatically changes in mitosis.
-The normally stable cytoplasmic MT become shorter and more unstable
-This is a necessary change for the activities of mitosis
____ stabilize microtubules
MAPs
____ destabilize microtubules
catastrophins
____ microtubules attach sister chromatids to MTs
kinetochore
The ___ attaches the kinetochore to the plus end of microtubules
Ndc80 complex
Microtubules capture kinetochores
____ may help regulate bi-polar spindle attachment
Tension
Tension may stabilize MT attachment
What are the multiple forces that move chromosomes on the spindle?
- Kinetochore generated poleward force
- Polward microtubule flux
- Polar ejection force
Kinetochore generated poleward force
– Plus-end depolymerization at the kinetochore
– As the freed protofilaments curl outward, the push against the collar.
Poleward microtubule flux
– While treadmilling dominates prior to anaphase (addition of tubulin at kinetochore matches tubulin removal at the spindle pole) a switch occurs and tubulin removal begins at the kinetochore
Polar ejection force
– Plus end directed kinesins-4 and 10 attach to chromsomes at their ends and push away from the spindle pole.
During metaphase (as an example of polward MT force) microtubules are said to ____ while the chromosomes are essentially stationary.
treadmill-growing at the plus end and shrinking at the minus end
When does MT plux stop?
at the metaphase/anaphase transition
chromatids seperate at ___
anaphase
Major forces that separate daughter chromosomes at anaphase
4 steps of cytokinesis?
• Initiation
- Contraction
- Membrane insertion
- Completion
The contractile ring is made of ____
actin and myosin filaments
During cytokinesis…
interpolar microtubules split between the dividing cells as well as the matrix
Assembly and contraction of the contractile ring are triggered by…
local activation of RhoA
____ is required for cytokinesis
ESCRT III
