Lecture 20: Cell Cycle-overview and mechanics; mitosis; cytokinesis Flashcards Preview

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Flashcards in Lecture 20: Cell Cycle-overview and mechanics; mitosis; cytokinesis Deck (47):
1

3 events of the Cell Cycle?

  1. cell growth and chromosome replication
  2. chromosome segregation
  3. cell division

Purpose: 

• Precisely duplicate the genetic material 

• Accurately distribute the replicated chromosomes into the two daughter cells 

2

Four phases of the eukaryotic cell cylce?

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3

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 

4

___ are pauses that allow the cell to grow and respond to environmental cues. 

gap phases

5

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. 

6

what are the 6 parts of the M (mitosis) phase?

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7

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

8

Interphase

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9

prophase

chromosomes condense, spindle migration continues, microtubule dynamic instability increases greatly

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10

prometaphase

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11

Metaphase

alignment of the microtubule attached chromosomes at the metaphase plate

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12

Anaphase

sister chromatids synchronously separate

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13

Telophase

daughter chromosomes arrive at the poles, nuclear envelope reforms, assembly of the contractile ring begins

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14

Cytokinesis

cytoplasmic division

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15

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 

16

Centrisome duplication is triggered at the beginning of ____ by the activation of _____, which also triggers DNA replication

S-phase

G1/S-Cdk complex

17

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

18

What are the three classes of microtubules form the mitotic spindle?

  1. astral microtubules
  2. kinetochore microtubules
  3. overlap microtubules

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19

Astral microtubules

– Radiate in all directions 

– Contribute to forces that separate the poles 

– Help orient and position the spindle 

20

Kinetochore microtubules

– Attach end-on to the kinetochore (which form at the centromere) 

21

Overlap mictrotubules

– Interdigitate at the equator 

– Responsible for symmetry and bipolar shape of the spindle 

22

Bipolar plus-end directed motors, like ___, push the spindles apart 

Kinesin-5

23

Minus-end directed kinesin like ____ pull the spindles towards one another 

kinesin-14

24

Conventional plus-end directed kinesins like ____ attach chromosomes to MTs for positioning 

Kinesins 4 and 10

25

___ attach the astral MTs to the cell periphery and pull the spindles apart as the walk towards the minus end of MTs

Dyneins

26

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

27

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 

28

____ stabilize microtubules

MAPs

29

____ destabilize microtubules

catastrophins

30

____ microtubules attach sister chromatids to MTs

kinetochore

31

The ___  attaches the kinetochore to the plus end of microtubules

Ndc80 complex

32

Microtubules capture kinetochores

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33

____ may help regulate bi-polar spindle attachment 

Tension

34

Tension may stabilize MT attachment 

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35

What are the multiple forces that move chromosomes on the spindle?

  • Kinetochore generated poleward force
  • Polward microtubule flux
  • Polar ejection force

36

Kinetochore generated poleward force

– Plus-end depolymerization at the kinetochore 

– As the freed protofilaments curl outward, the push against the collar. 

37

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

38

Polar ejection force

– Plus end directed kinesins-4 and 10 attach to chromsomes at their ends and push away from the spindle pole. 

39

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

40

When does MT plux stop?

at the metaphase/anaphase transition

41

chromatids seperate at ___

anaphase

42

Major forces that separate daughter chromosomes at anaphase

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43

4 steps of cytokinesis?

• Initiation 

• Contraction 

• Membrane insertion 

• Completion 

44

The contractile ring is made of ____

actin and myosin filaments

45

During cytokinesis...

interpolar microtubules split between the dividing cells as well as the matrix 

46

Assembly and contraction of the contractile ring are triggered by...

local activation of RhoA

47

____ is required for cytokinesis

ESCRT III