Cell Division

Morphological changes in M-phase

due to protein phosphorylation, dephosphorylation

• Chromosome condensation: histone

• NEBD: nuclear lamins

• Cytoskeletal rearrangement(spindle, contractile ring): caldesmon, c-src

Centrosome cycle

• Formation of mitotic spindle pole• Independent to nuclear cell cycle• S-phase: centriol replicate• Prophase: centrosome split & move apart• Prometaphase:

NEBDmt from each controsome to enter nucleus, &interact with chromosome spindle pole

Centriol replication

The centrosome cycle

Aster formation

Polar MT formation

Six steps in M-phase

prophase

prometaphase

metaphase

anaphase

telophase

cytokinesis

Mitosis in an animal cell

Time course for mitosis

Prophase

• Chromosome condensation: form 2 sister chromatids held together at centromere

• Centrosome split & move apart• Dynamic microtubules: Half life of MT decreased 20X

Prometaphase

• Centrosome segregate to the pole

• NEBD at early prometaphase– Enables mitotic spindle to interact with chromosome

• Formation of mitotic spindle

– Kinetochore MT: orientation and movement of chromosomes– Kinetochore act as cap that protect + end from depolymerizing– Centrosome at spindle pole protect – end from depolymerizing

Mitotic spindle

Formation of bipolar mitotic spindle

MAP (motors) stabilizes

Dynamically unstable + end

+ end overlap

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Kinetochore

• Developed from centromere

• MT attaches in metaphase

• Consist of a specific DNA sequence

• Large mutiprotein complex, platelike trilaminar structure

• Human; 20-40 MT, yease; 1 MT

• A puzzle: how MT & kinetochore connected to each other * hold on to a MT end, yet allow that end to add or loose subunits

Centromere in the yeast

Yeast kinetochore

Metaphase

• Kinetochore MT align chromosome in metaphase plate• MT are dynamic

Aster exclusion force

• The origin is not known• Aligning chromosomes at the spindle

Evidence for an astral ejection force

How to align the chromosomes in metaphase plate

-> Balanced bipolar force

A model for the centrosome-independent spindle assembly

How MT & kinetochore connected to each other

• Microinjection of labeled tubulin:

• metaphase; incorporate tubulin near kinetochore• anaphase; reverse action at same site

• Puzzle:

• Hold on to a MT end, yet allow that end to add or loose subunits

• Sliding collar based model

• Microinjection of labeled tubulin:- metaphase; incorporate tubulin near kinetochore- anaphase; reverse action at same site

Anaphase• Paired kinetochore separate –> separation & segregation of chromatid• Start abruptly by specific signal• Signal may be intracellular Ca2+ rise: 1) Rapid, transient 10X increase Ca2+ at anaphase in some cells 2) Injection of low level of Ca2+ into metaphase cell ->premature anaphase 3) Accumulation of Ca2+ containing membrane vesicle at spindle pole 4) Clamp Ca rise by EGTA, BAPTA -> arrest anaphase

**mechanism of Ca2+ rise during anaphase is a mystery

• Anaphase A shortening of kinetochore MT -> poleward movement of chromatids no energy required for shrinking of kinetochore

• Anaphase B elongation of polar MT -> two spindle poles move further apart ATP hydrolysis is required for elongation of polar MT; kinesin ATPase drug chloral hydrate inhibits Anaphase B not A

pulling aster MT -> -end moter binds cell cortex & aster MT-> pull spindle pole apart

Chromatid separation at anaphase

Motors as anchors

How kinetochore hold on to a MT end, yet allow that end to add or loose subunits

Motor proteins in anaphase B

A model for how motor proteins may act in anaphase B

Telophase

• Chromatids separate completely

• Kinetochore MT disappears

• Polar MT elongate still more

• Nuclear envelope reassemble

• Nucleoli reappear

Cytokinesis

• Begins at anaphase

• Cleavage furrow occurs in the plane of metaphase plate, right angle to the long axis of the mitotic spindle

• Aster is responsible for cleavage furrow position

• contractile ring: assembles in the early anaphase (actin & myosin II) myosin dephosphorylation triggers cytokinesis

• Midbody: bridge between two cells, contains polar MT

• organelles partitioned with no special mechanisms mitochondria, chloroplasts; grow, fission -> # doubles ER, Golgi; fragmentation, vesiculation -> even distribution

• unequal segregation of cell components C. elegans “p-granules” to posterior -> form germ line cells

(independent to MT, but dependent on actin filament) styela yellow cresent (myoplasm) to vegetal -> form muscle (microfilament first phase, MT second phase)

Asters signal to the cortex to initiate a cleavage furrow

An asymmetric cell division of the nematode egg

Spindle rotation Asymmetric cell division

The contractile ring

Mitosis without cytokinesis in the fly embryo