Cells 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.

Thesis (Ph.D.). / Written for the Dept. of Biology. Title from title page of PDF (viewed 2008/01/12). Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 163-179).

Thesis (Ph.D. in Molecular Biology) -- University of Colorado, 2006. / Typescript. Includes bibliographical references (leaves 126-147). Free to UCDHSC affiliates. Online version available via ProQuest Digital Dissertations;

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The Sli15-Ipl1-Bir1 chromosomal passenger complex is essential for proper kinetochore-microtubule attachment and spindle stability in the budding yeast Saccharomyces cerevisiae. Subcellular localization of this complex during anaphase is regulated by the Cdc14 protein phosphatase, which is kept inactive in the nucleolus until anaphase onset. I show here that the predominantly nucleolar ribosome biogenesis protein Utp7 is also present at kinetochores and is required for normal organization of kinetochore proteins and proper chromosome segregation. Utp7 associates with and regulates the localization of Sli15 and Cdc14. It prevents the abnormal localization of Sli15 on cytoplasmic microtubules, the premature concentration of Sli15 on the pre-anaphase spindle, and the premature nucleolar release of Cdc14 before anaphase onset. Utp7 regulates Sli15 localization not entirely through its effect on Cdc14. Furthermore, the mitotic exit block caused by Cdc14 inactivation is relieved partially by the simultaneous inactivation of Utp7. Thus, Utp7 is a multifunctional protein that plays essential roles in the vital cellular processes of ribosome biogenesis, chromosome segregation and cell cycle control. Protein phosphatase 1, Glc7 opposes in vivo functions of the Ipl1-Sli15-Bir1 kinase complex in budding yeast. I show here Scd5- a targeting subunit of Glc7 that regulates endocytosis/cortical actin organization and undergoes nuclear-cytoplasmic shuttling- is present at kinetochores. Ipl1 associates with both Glc7 and Scd5. The scd5-PP1[Delta]2 mutation, which disrupts the association between Glc7 and Scd5, also disrupts the association between Ipl1 and Scd5-Glc7 without affecting the kinetochore localization of these proteins. Genetic studies suggest that Scd5 may positively regulate both Glc7 phosphatase and the Ipl1 kinase complex. In accordance, Scd5 stimulates in vitro kinase activity of Ipl1. scd5-PP1[Delta]2 cells missegregate chromosomes severely due to several defects: i) at least one of sister kinetochores appears not attached to microtubule. ii) sister chromatids are persistently cohesed through anaphase. iii) Sli15 is hyperphosphorylated and less abundant on the anaphase spindle resulting in unstable mitotic spindle. These results together suggest that Scd5 functions in diverse processes that are essential for faithful chromosome segregation. How Scd5 coordinately regulates two apparently antagonistic enzymatic activities of Ipl1 and Glc7 remains to be determined. / text

Dissertation (Ph.D.)--University of South Florida, 2009. / Title from PDF of title page. Document formatted into pages; contains 155 pages. Includes vita. Includes bibliographical references.