cell bio: mitosis and cytokinesis

explain how microtubules change in the following stages of animal cell division:

a) interphase

b) early prophase

c) metaphase

d) early anaphase

a) they form a dynamic network radiating from the centrosome, maintaining cell shape and serving as tracks for intracellular transport.

b) they reorganize to form the mitotic spindle as centrosomes begin moving apart.

c) they attach to kinetochores and align chromosomes at the metaphase plate, maintaining tension for proper segregation.

d) they shorten at the kinetochore and / or spindle poles, pulling sister chromatids toward opposite poles.

explain how microtubules change in the following stages of animal cell division:

a) late prophase

b) prometaphase

c) late anaphase

d) late telophase

a) nuclear envelope breaks down, and spindle microtubules begin interacting with chromosomes.

b) they attach to kinetochores on chromosomes and search-and-capture positions to align them at the metaphase plate.

c) kinetochore microtubules continue to shorten, and interpolar microtubules elongate, pushing spindle poles further apart.

d) they disassemble, and the spindle collapses as the nuclear envelope reforms around the separated chromosomes

describe the differences in mitotic spindle in animal and plant cells

Animal cells:

- Spindle microtubules extend from focused centrosomal poles toward the metaphase plate, with kinetochore microtubules attaching to kinetochores to pull sister chromatids apart all at once.

Plant cells:

- Without centrosomes, spindle microtubules are organized by dispersed microtubule organizing centers, forming slightly more diffused spindles where poles are established at opposite ends and chromosomes are pulled apart in groups.

describe the three types of mitotic microtubules in animal cells:

a) astral microtubules

b) kinetochore microtubules

c) overlap microtubules

a) they radiate from the centrosomes toward the cell cortex and help position and orient the spindle.

b) they extend from spindle poles to kinetochores on chromosomes, attaching and pulling sister chromatids toward opposite poles.

c) Overlap microtubules from opposite spindle poles interdigitate at the spindle midzone and help push poles apart during anaphase.

explain how motor proteins work in mitosis

- Dyneins pull on astral microtubules from the cell edges to help position the spindle and separate centrosomes.

- Kinesins push overlapping microtubules apart to elongate the spindle.

- Motors at kinetochores pull sister chromatids toward opposite spindle poles during anaphase.

explain the model for spindle separation

- plus end-directed motors help separate the spindle poels

- outgrowth from centrosomes is in random direction

- when two microtubules from opposing poles overlap, sliding moves poles outward

explain how microtubule flux occurs during metaphase

- As spindle poles separate, microtubules are continuously removed from their minus ends at the poles.

- New tubulin subunits are added at the kinetochore (plus) ends, maintaining attachment to chromosomes and helping move the chromosomes poleward.

- coordinated addition and removal of tubulin causes microtubules to "flux," with speckles or subunits appearing to move toward the poles during metaphase.

explain how chromosomes separate during the following types of anaphase:

a) anaphase A

b) anaphase B

a) shortening of kinetochore microtubules; movement of daughter chromosomes to poles (outward)

---------> forces generated mainly at kinetochores

b) either:

- 1) a sliding force is generated between overlap microtubules from opposite poles to push the poles apart

- 2) a pulling force acts directly on the poles to move them apart

explain how plus end motor proteins contribute to the outward pulling of spindle poles during mitosis

- Plus-end-directed motors on overlap microtubules push overlapping microtubules from opposite poles apart, elongating the spindle.

- Plus-end-directed motors on astral microtubules interact with the cell cortex to help pull spindle poles outward and properly position the spindle.

within animal cytokinetic structures, what is the role of actin and myosin II in cytokinesis?

- Remaining overlap microtubules from the central spindle help specify the location of the cleavage furrow and guide contractile ring assembly.

- Actin and myosin II filaments form a contractile ring in the cleavage furrow that tightens to pinch the cell into two daughter cells.

- Leftover overlap microtubules remain in the center as the midbody, forming a dense structure of cytoplasm and microtubules that coordinates the final separation of the daughter cells.

the cleavage position can be determined via 3 different types of models. explain those 3 types of models:

a) astral stimulation model

b) central spindle stimulation model

c) astral relaxation model

a) signals from astral microtubules stimulate the cortex at the cell equator to specify the cleavage furrow position.

b) overlapping microtubules in the central spindle send signals that induce contractile ring assembly at the cell center.

c) astral microtubules inhibit contractility near the poles, so the cleavage furrow forms where cortical tension is lowest, at the equator.

describe the differences between plant and animal cytokinesis

Plants:

- no contractile ring / cleavage furrow

- vesicles carrying membrane and cell wall materials are delivered to the center via the phragmoplast, building the new cell wall and plasma membrane outward toward the existing cell edges WITHOUT a midbody structure

Animals:

- The contractile ring pinches the cell inward, forming a cleavage furrow that divides the cell from the outside in.

how would you know which motors are involved in which aspect of mitosis. can you predict what would happen in a mutant for each one of them?

- Dynein mutant: spindle poles are unfocused and mispositioned; chromosome movement toward poles is slower or defective.

- Kinesin-5 (Eg5) mutant: bipolar spindle formation fails, often resulting in a monopolar spindle.

- Kinesin-13 / chromokinesin mutants: microtubule depolymerization or polar ejection forces are disrupted, causing misaligned chromosomes or overly long microtubules.