Cell Division due to protein phosphorylation, dephosphorylation 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: NEBD mt 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 Dynamically unstable + end + end overlap MAP (motors) stabilizes

Separation of two spindle poles in prophase

Kinetochore MT attaches in metaphase 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 **mechanism of Ca2+ rise during anaphase is a mystery 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

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

Motor proteins in anaphase B

A model for how motor proteins may act in anaphase B

Telophase Kinetochore MT disappears Polar MT elongate still more 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