Lecture12-EndCellCycle

Lecture12-EndCellCycle - Mitosis & Cytokinesis Recommended...

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Mitosis & Cytokinesis Recommended Reading: MBOC 5e, pages 1069 - 1090 1 Bio 103-Fall 2009 © Steven Gross
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Three microtubule-based structures are required for mitosis Three distinct types of microtubule (MT) structures form during mitosis - astral, interpolar and kinetochore MTs . Astral MTs - project from the poles and orient the spindle via interactions with the cell cortex. Interpolar MTs - can be free at either end , can penetrate between and through chromosomes , minus ends are close to centrosomes , overlap in zones of inter-digitation . Kinetochore plus ends are embedded in kinetochore , minus ends are at or near spindle pole (aka centrosome ), each human kinetochore captures around 20 independent MTs. 2 Bio 103-Fall 2009 © Steven Gross
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Capture of chromosomes by kinetochore MTs Small target! Mechanism 1 : 'search ' and 'capture' . * Microtubules extend from centrosomes towards chromosomes. * Initial binding of a MT to a chromosome often occurs laterally. * The chromokinesins kinesin 4 and Kinesin 10 can help move chromosome to MT end * When chromosome binds end-on to kinetochore, MT becomes stable * Process is repeated on opposite side of kinetochore and chromosomes become stabilized between kinetochore microtubules. Alternative forms of chromosome attachment are unstable and favor detachment of one MT. Key event: aquisition of tension across MT binding sites, which increases affinity of MT for kinetochores. 3 Bio 103-Fall 2009 © Steven Gross Only somewhat stable interaction without tension Mechansm 2: Chromosome- mediated nucleation of MTs •Chromos. nucleate MT, •Slide minus-ends away from MT, via +-end chromo-kinesins attached to chromosomes +-ends end up at chromosomes, some at kinetochores
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Kinetochores attach sister chromatids to the spindle Inner layer of kinetochore binds to repetitive centromeric DNA . Each kinetochore has collar structure (yellow) that surrounds the MT end and which allows kinetochore to remain attached to MT during polymerization or depolymerization of MT. 4 Bio 103-Fall 2009 © Steven Gross
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Metaphase Each chromosome has been successfully captured by MT from both spindles . Chromosomes are tugged back and forth until they settle at metaphase plate , a position equidistant from spindle poles . Treadmilling of MTs occurs at metaphase , with addition of tubulin to + end and removal at - end. Chromosomes oscillate gently for a brief (c. 20 min) and reproducible period before commencement of anaphase . 5 Bio 103-Fall 2009 © Steven Gross
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Anaphase - Telophase Anaphase commences with abrupt separation of the two sister chromatids . Metaphase - anaphase transition requires " unzipping " o f sister chromatids , via proteolysis of molecules ( cohesins ) that regulate pairing of sister chromatids. Separated sister chromatids first move to the two spindle poles (anaphase A) .
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This note was uploaded on 12/13/2011 for the course BIOSCI d103 taught by Professor Staff during the Fall '11 term at UC Irvine.

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Lecture12-EndCellCycle - Mitosis & Cytokinesis Recommended...

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