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

Question 1

3 events of the Cell Cycle?

Answer 1

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

Question 2

Four phases of the eukaryotic cell cylce?

Answer 2

Question 3

Timing of a typical cell cycle

Answer 3

• 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

Question 4

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

Answer 4

gap phases

Question 5

Gap phases

Answer 5

• 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.

Question 6

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

Answer 6

Question 7

Biochemical steps of mitosis

Answer 7

• 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

Question 8

Interphase

Answer 8

Question 9

prophase

Answer 9

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

Question 10

prometaphase

Answer 10

Question 11

Metaphase

Answer 11

alignment of the microtubule attached chromosomes at the metaphase plate

Question 12

Anaphase

Answer 12

sister chromatids synchronously separate

Question 13

Telophase

Answer 13

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

Question 14

Cytokinesis

Answer 14

cytoplasmic division

Question 15

The mitotic spindle is a microtubule-based machine

Answer 15

• 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

Question 16

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

Answer 16

S-phase

G1/S-Cdk complex

Question 17

Some cells, specifically higher plants and some vertebrate oocytes do not have centrisomes. In these cells, ______ are involved in forming the bipolar spindle.

Answer 17

motor proteins

Question 18

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

Answer 18

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

Question 19

Astral microtubules

Answer 19

– Radiate in all directions

– Contribute to forces that separate the poles

– Help orient and position the spindle

Question 20

Kinetochore microtubules

Answer 20

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

Question 21

Overlap mictrotubules

Answer 21

– Interdigitate at the equator

– Responsible for symmetry and bipolar shape of the spindle

Question 22

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

Answer 22

Kinesin-5

Question 23

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

Answer 23

kinesin-14

Question 24

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

Answer 24

Kinesins 4 and 10

Question 25

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

Answer 25

Dyneins

Question 26

Influence of opposing motor proteins on spindle length

Answer 26

- 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

Question 27

Microtubule stability dramatically changes in mitosis.

Answer 27

-The normally stable cytoplasmic MT become shorter and more unstable ## Footnote -This is a necessary change for the activities of mitosis

Question 28

\_\_\_\_ stabilize microtubules

Answer 28

MAPs

Question 29

\_\_\_\_ destabilize microtubules

Answer 29

catastrophins

Question 30

\_\_\_\_ microtubules attach sister chromatids to MTs

Answer 30

kinetochore

Question 31

The ___ attaches the kinetochore to the plus end of microtubules

Answer 31

Ndc80 complex

Question 32

Microtubules capture kinetochores

Answer 32

Question 33

\_\_\_\_ may help regulate bi-polar spindle attachment

Answer 33

Tension

Question 34

Tension may stabilize MT attachment

Answer 34

Question 35

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

Answer 35

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

Question 36

Kinetochore generated poleward force

Answer 36

– Plus-end depolymerization at the kinetochore – As the freed protofilaments curl outward, the push against the collar.

Question 37

Poleward microtubule flux

Answer 37

– 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

Question 38

Polar ejection force

Answer 38

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

Question 39

During metaphase (as an example of polward MT force) microtubules are said to ____ while the chromosomes are essentially stationary.

Answer 39

treadmill-growing at the plus end and shrinking at the minus end

Question 40

When does MT plux stop?

Answer 40

at the metaphase/anaphase transition

Question 41

chromatids seperate at \_\_\_

Answer 41

anaphase

Question 42

Major forces that separate daughter chromosomes at anaphase

Answer 42

Question 43

4 steps of cytokinesis?

Answer 43

• Initiation ## Footnote * Contraction * Membrane insertion * Completion

Question 44

The contractile ring is made of \_\_\_\_

Answer 44

actin and myosin filaments

Question 45

During cytokinesis...

Answer 45

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

Question 46

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

Answer 46

local activation of RhoA

Question 47

\_\_\_\_ is required for cytokinesis

Answer 47

ESCRT III