Investigation of Force, Kinetochores, and Tension in the Saccharomyces Cerevisiae Mitotic Spindle
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CitationNannas, Natalie Jo. 2013. Investigation of Force, Kinetochores, and Tension in the Saccharomyces Cerevisiae Mitotic Spindle. Doctoral dissertation, Harvard University.
AbstractCells must faithfully segregate their chromosomes at division; errors in this process causes cells to inherit an incorrect number of chromosomes, a hallmark of birth defects and cancer. The machinery required to segregate chromosomes is called the spindle, a bipolar array of microtubules that attach to chromosomes through the kinetochore. Replicated chromosomes contain two sister chromatids whose kinetochores must attach to microtubules from opposite poles to ensure correct inheritance of chromosomes. The spindle checkpoint monitors the attachment to the spindle and prevents cell division until all chromatids are attached to opposite poles. Both the spindle and the checkpoint are critical for correct segregation, and we sought to understand the regulation of these two components. The spindle is assembled to a characteristic metaphase length, but it is unknown what determines this length. It has been proposed that spindle length could be regulated a balance of two forces: one generated by interaction between microtubules that elongates the spindle and a second due to interactions between kinetochores and microtubules that shortens the spindle. We tested this force-balance model which predicts that altering the number of kinetochores will alter spindle length. We manipulated the number of kinetochores and found that spindle length scales with the number of kinetochores; introducing extra kinetochores produces shorter spindles and inhibiting kinetochores produces longer spindles. Our results suggest that attachment of chromosomes to the spindle via kinetochores produces an inward force that opposes outward force. We also found that the number of microtubules in the spindle varied with the number of kinetochores. In addition to establishing a spindle, cells must also guarantee that chromosomes are correctly attached to it. Correct attachment generates tension as the chromatids are pulled toward opposite poles but held together by cohesin until anaphase. The spindle checkpoint monitors this tension which causes stretching of chromatin and kinetochores. Lack of tension on activates the checkpoint, but is unknown if the checkpoint measures stretch between kinetochores (inter-kinetochore stretch) or within kinetochores (intra-kinetochore). We tethered sister chromatids together to inhibit inter-kinetochore stretch and found that the checkpoint was not activated. Our results negate inter-kinetochore models and support intra-kinetochore models.
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