Cytokinesis separates replicated chromosomes and cytoplasm into two daughter cells. In animal cells, this is achieved by the formation of a cleavage furrow that bisects the mitotic (or meiotic) spindle. It is known that the mitotic apparatus defines the cell cleavage plane. However, it is not clear how the mitotic apparatus initiates the cleavage furrow. Each part of the mitotic apparatus; namely asters, central spindle (microtubule arrays and the spindle midzone), and chromosomes, has been found capable of inducing a cleavage furrow in certain cell types. Yet it is uncertain which part is the essential source of the signal and whether all parts act in concert. This thesis systematically examines in grasshopper spermatocytes 1) which spindle constituent is the essential source of furrow signal; 2) the impact of microtubules on distribution of actin filaments and positioning of cell cleavage relative to spindle reorganization; 3) the independent role of the spindle midzone relative to microtubules in furrow initiation and ingression. These examinations combine micromanipulation with digital-enhanced polarization microscopy and epifluorescence microscopy, in which mitotic spindles in living cells are mechanically dissected and rearranged as desired as well as microfixed to evaluate and propose models for cell cleavage. This thesis has come to the conclusion that none of structural constituents of the spindle apparatus is essential for cell cleavage induction except microtubules. First, furrow induction occurs regardless of a particular spindle constituent, so long as sufficient microtubules are present to form bipolar arrays. Second, microtubules continuously dictate distribution of actin filaments and positioning of cell cleavage. Asymmetric alterations of spindle microtubules dynamically affect the location of the spindle midzone, distribution of actin filaments, and ultimately position of the cleavage furrow in cells containing a bipolar spindle, monopolar spindle, or half-spindle. Third, actin filaments are distributed to the furrow region by microtubule-mediated transport, but organized by the midzone, which is essential for furrow ingression, but not initiation. These results suggest that during post-anaphase spindle assembly, actin filaments are excluded by bipolar microtubule arrays to the equatorial cell cortex where they bundle into a contractile ring with cytokinetic factors. / Graduation date: 2004

Thesis (Ph. D.)--University of Hong Kong, 2007. / Also available online.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

The Saccharomyces cerevisiae plasmid, 2 micron circle, resides in the yeast nucleus at a high copy number. It provides no apparent growth advantage to its host, nor imposes any significant growth disadvantage. The plasmid is an excellent paradigm for studying mechanisms utilized in the persistence of a eukaryotic selfish DNA element that is selectively neutral. The plasmid achieves stable propagation and copy number maintenance by combining a partitioning system and an amplification system. The partitioning proteins Rep1p and Rep2p promote the recruitment of the histone H3 variant Cse4p and the yeast cohesin complex to the partitioning locus STB during S phase, leading to the formation of a functional partitioning complex which segregates the plasmid equally during mitosis. The integrity of the mitotic spindle is a pre-requisite for the specific nuclear localization of the plasmid as well as for plasmid association with a subset of the partitioning proteins such as Cse4p and the cohesin complex. The work presented in this thesis reveals, using tools of molecular genetics and cell biology, the involvement and possible functions of a microtubule associated nuclear motor protein, Kip1p, in the 2 micron circle partitioning pathway. The plasmid missegregates in kip1[Delta] cells, but not in cells harboring deletions of genes coding for the other nuclear motors. Kip1p interacts with the plasmid partitioning system and promotes the association of Cse4p and the cohesin complex with STB. Lack of Kip1p function delocalizes the plasmid from its characteristic nuclear locale in close proximity to the spindle pole body. The distance between a reporter plasmid and the spindle pole body is nearly doubled in a kip1[Delta] host strain. We propose that, unlike the conventional roles played by nuclear motors in spindle function and chromosome segregation, the Kip1p motor assists the 2 micron circle in associating with the mitotic spindle and translocating to its ‘partitioning center’. / text

No description available.

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 Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (Ph.D.)--University of California, San Francisco, 2004. / Includes bibliographical references. Also available online.

Thesis (M.S.)--University of North Texas, May, 2009. / Title from title page display. Includes bibliographical references.

Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 90-110). Also available on the World Wide Web.