17 Mitosis II - BME 418, Quantitative Cell Biology Lecture...

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BME 418, Quantitative Cell Biology Alan J. Hunt Lecture #17: Mitosis II. Although progress has been made identifying the molecules involved in mitosis, many aspects of the process remain opaque. The complexity of the mitotic apparatus and the necessary interplay of biochemical and mechanical events have confounded mechanistic description. So there are many questions…. How do chromosomes condense? What signals control the transitions between various phases? How is the spindle organized, what are its mechanical properties, and how do chromosomes move? Although much remains to be determined, much progress has been made through careful observations and some mechanical and molecular genetics manipulations. Fluorescence micrograph of prometaphase mitotic spindle. Microtubules are stained green and chromosomes are blue. How do chromosomes become attached to spindle microtubules ? This is difficult to answer because: - Very high density of microtubules (MTs) in the spindle. - Kinetochores normally become attached to microtubules very quickly after the nuclear envelope breaks down upon entry into prometaphase.
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BME 418, Quantitative Cell Biology Alan J. Hunt Hermit the newt ( Taricha ). Right: Differential interference contrast photomicrograph of a living and dividing newt lung cell. (http://www.wadsworth.org) Two theories persisted for some time: 1) MTs grow out from kinetochores to spindle poles, 2) MTs extending from the spindle poles are captured by kinetochores. A great aid for resolving this was the use of cultured newt pneumocyte (lung) cells: - Spindle is in an optically clear area. - Cells are very flat. - Frequently chromosomes are well separated from the poles, and so experience delayed attachment (up to 5 hrs) in areas of low MT density. In 1990 Rieder et al. (J. Cell Biol. 110) used differential interference contrast and immunofluorescence microscopy to observe chromosomes in newt lung cells as they attached laterally to single microtubules before rapidly transporting poleward. During these studies they observed that: - Spindle pole MTs attach to kinetochores. Attachment can occur laterally as evidenced by MT extending beyond chromosome, and movement of particles distal to the attachment point along the microtubule’s trajectory. -
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This note was uploaded on 09/06/2008 for the course BIOMEDE 418 taught by Professor Hunt during the Winter '08 term at University of Michigan.

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17 Mitosis II - BME 418, Quantitative Cell Biology Lecture...

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