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CELL DIVISION
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Stages of mitosis (animal cell)
prophase:- chromosomes condense
(replicated in S phase)- centrosomes separate
(duplicated in S phase)
prometaphase:- nuclear envelope breaks down- MTs contact chromosomes,
spindle forms
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metaphase:- chromosomes align at spindle
equator (metaphase plate)
anaphase:- sister chromatids separate- chromosomes move to poles- poles move apart
telophase:- nuclear envelope reforms- chromosomes decondense- interphase array of MTs reforms
cytokinesis:- contractile ring pinches cell in
two
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Centrosome cycle (animal cell):
- centrosome (centriole) duplication begins at the start of S phase- remains as one complex until M phase
In early embryonic cells, the centrosome cycle can operate without a nucleus - egg cell extracts
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Mediators of mitotic chromosome structure:- cohesins: deposited along the length of sister chromatids as the DNA is replicated - hold sisters together- condensins: coil DNA - mediate chromosome condensation
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Structure of a spindle:3 classes of MTs
(polar MTs)
How does a spindle form, and how does it work to separate chromosomes?
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Prophase: changes in MT dynamics- more MTs nucleated from centrosome- shorter, more dynamic MTs
Quantifying MT dynamics:- inject fluorescent tubulin- bleach with laser- measure recovery (newly formed MTs)- t1/2 = time to 50% recovery
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MT dynamics: regulated by MAPs vs. catastrophins
higher catastrophe rates = shorter MTs
centrosomes incubated in Xenopus egg extracts
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Spindle formation in vitro:mitotic extracts + DNA + centrosomes
- abnormal spindles form when ratio of MAPs:catastrophins is perturbed
no MAP (MTs are too short)
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Centrosome separation in prophase is driven by plus-end motors (KLPs) - balanced by minus-end motors
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Yeast mutants: identification and characterization of spindle motors
(-) (+)
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Prometaphase: kinetochores capture MTs (mechanism of attachment??)
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Forces that drive chromosomes to metaphase plate:- kinetochores pull chromosomes to poles: (-)end directed motors?- astral ejection force: (+)end directed motors on chromosome arms
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Metaphase: - chromosomes continue to oscillate at metaphase plate(vertebrate cells)- MTs undergo poleward flux (function?)
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- poleward flux of metaphase MTs can be measured with caged fluorescein
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Dynamics of individual MTs can be measured with fluorescence speckle microscopy:- poleward flux of metaphase MTs occurs in kinetochore and overlap MTs but not in astral MTs
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Anaphase A: - kinetochore MTs shorten- chromosomes move to poles
Fluorescent tubulin injections show locations of MT growth, depolymerization
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Two models for kinetochore movement along MTs:
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Anaphase B: - poles separate- overlap MTs lengthen
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Model for motor protein activity in anaphase B
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Bipolar spindles can assemble without centrosomes or chromosomes
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Cytokinesis (animal cell)
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What determines the position of the cleavage furrow?- signal from asters to cortex- signal from central spindle- chosen before mitosis (position of spindle from previous mitosis)
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Contractile ring of actin and myosin:red = actin, green = myosin II
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Cytokinesis in plants
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