Lecture 14: Mitosis and Cytokenesis

Question 1

Define the cell cycle

Answer 1

• a mechanism by which a cell grows and duplicates its contents then divides by two.
• each parental cell gives rise to two identical daughter cells.

Question 2

Name four phases in the eukaryotic cell cycle

Answer 2

1) M phase (mitosis): mitosis (nuclear division) and cytokinesis (cytoplasmic division).
2) S phase (synthesis): DNA replication, duplication of uncondensed chromatin.
3) G1 phase (Gap 1): interval between M phase and S phase.
4) G2 phase (Gap 2): interval between S phase and M phase.

Question 3

Cell cycle time in a typical human cell

Answer 3

• cell cycle (M phase + G1 + S + G2) lasts 24 hrs.
• M phase: 1 hr.
• Interphase: 23 hrs (95% of the time spent here= G1+S+G2).

Question 4

Events occurring during interphase

Answer 4

cell grows at a steady rate and continues to:
- transcribe its genes, synthesize its protein, grow and duplicate organelles except for the nucleus, organelles and cell grow in mass.
- during G1 and G2 cell grows but does not duplicate DNA. chromatin uncondensed.
- during S phase cell duplicates DNA, chromatin uncondensed.
Centrioles and centrosomes duplicated but remain on one side of the nucleus, nuclear envelop remains in tact .

Question 5

Describe the first phase of mitosis, Prophase

Answer 5

• replicated chromosomes condense, each chromosome consisting of two sister chromatids are held together at their centromere where kinetochores (protein complexes) are assembled on the centromere DNA sequences.
• centrosomes separate and move to opposite poles of cell (spindle poles).
• some microtubules overlap where microtubule associated proteins cross-link these microtubules together, stabilizing their (+) ends by decreasing probability of depolymerization.
• these interacting microtubules form the mitotic spindle
• nuclear envelop still in tact.

Question 6

Metaphase

Answer 6

• starts with the disassembly of the nuclear envelop into small membrane vesicles.
• this occurs by the phosphorylation of lamins.
• two sister chromatids attach to the (+) end of kinetochore microtubules via kinetochores, so that each replicated chromosome becomes linked to both spindle poles.
• chromosomes align at the equator of the spindle forming the metaphase plate.

Question 7

Name and define three types of microtubules present in a mitotic spindle during metaphase

Answer 7

1- kinetochore: are attached to mitotic (condensed) chromosomes at their centromeres, via kinetochores; play critical role in separating chromosomes.
2- polar: overlap each other, are stabilized by spindle kinesins (microtubule associated protein).
3- astral: extend outward from the centrosomes to cell periphery and have freely exposed (+) ends.

Question 8

Describe Anaphase

Answer 8

1- paired chromatids separate to form two daughter chromosome which are gradually pulled to the spindle pole which it is attached.
2- this movement is due to three independent processes mediated by three types of the mitotic microtubules: kinetochore, astral and polar.

Question 9

How do microtubules work together to pull apart sister chromatids?

Answer 9

a) disassembly and shortening of the kinetochore microtubules at the kinetochore pull the daughter chromosomes toward opposite poles.
b) The plus (+) end -directed spindle kinesins generate a sliding force between polar microtubules from opposite poles to push them apart
c) A minus (-) end -directed force exerted by dynein pulls astral microtubules toward the cell periphery and away from the other pole (centrosome).

Question 10

How do kinesins generate a sliding force between polar microtubules?

Answer 10

• Kinesin moves from the (-) to the (+) end of the polar microtubule #1 using ATP hydrolysis which in turn:
• Polar microtubule #1 moves towards the centrosome.
• Polar microtubule #2 moves towards other centrosome.

Question 11

How does dyenin pull astral microtubules toward cell periphery?

Answer 11

• Dynein moves from the + to the – end of the astral microtubule while astral microtubule moves
  toward the cell periphery (plasma membrane).

Question 12

Describe Telophase

Answer 12

• two sets of daughter chromosomes arrive at the poles of the spindle.
• new nuclear envelope re-assembles around each set of daughter chromosomes, forming of two nuclei.
• condensed mitotic chromosomes decondense into their interphase state.
• contractile ring is starting to form at the equator of the parent cell.

Question 13

Explain how new nuclear envelope re-assembles around each set of daughter chromosomes

Answer 13

The dephosphorylation of lamins triggers the re-assembly of the nuclear lamina, which in turn causes a new nuclear envelope to assemble from the vesicles.

Question 14

Steps in cytokenisis

Answer 14

Step 1: The cytoplasm (cytosol + organelles) is divided in two by a contractile ring composed of actin filaments and myosin II.

Step 2: The contractile ring pinches in the parental cell to create two daughter cells, each with one nucleus and equally distributed membranes, cytoskeleton, organelles and soluble cytosolic proteins.

Question 15

How does myosin II help in cytokinesis and where does this process occur?

Answer 15

• Small filaments composed of myosin II slide actin filaments over each other (+ to -), thus shortening of actin filament bundle.
• This process occurs in the contractile bundles of actin filaments and small myosin II filaments that assemble briefly in the contractile ring that divides cells in two following mitosis (cell division)