Thesis (M.S.)--University of Toledo, 2005. / Typescript. "A thesis [submitted] as partial fulfillment of the requirements of the Master of Science degree in Biology." Bibliography: leaves 117-118.

Thesis (Ph. D. in Cancer Biology)--Vanderbilt University, Dec. 2005. / On t.p. "L̳" is subscript. Title from title screen. Includes bibliographical references.

Thesis (Ph.D.) - University of St Andrews, December 2009.

Thesis (Ph.D.)--University of Iowa, 2006. / Supervisor: Rebecca S. Hartley. Includes bibliographical references (leaves 177-190).

Chapter I. Pulse/chase labelling and DNA fibre autoradiography have been used to study the durations of the stages of the cell cycle, and the manner of chromosomal DNA replication, in Xenopus cells in tissue culture at 18°C, 23°C and 28°C. Cultures were grown in modified Eagle's basal medium, containing salts at concentrations appropriate to Amphibia, plus glutamine and foetal calf serum. The subculturing procedures were carried out every fortnight, with a medium change every week. For studying the durations of the stages of the cell cycle, cells were labelled with low specific activity tritiated thymidine (3H-TdR) for 30 min or 1 hour, then left to continue growth in non-radioactive medium, and fixed at regular intervals thereafter. Whole-cell autoradiographs, stained in Giemsa, were prepared from these fixations. For studying DNA replication, tissue cultures were treated with flurodeoxyuridine (FUdR) to arrest cells at the beginning of the S-phase, then labelled with high specific activity 3H-TdR for various times. In the case of pulse/stepdown labelling, the first period of labelling was followed by a further period in the presence of 3H-TdR at one quarter of the original specific activity. DNA fibre autoradiographs were prepared from such labelled tissue cultures. Chapter II. An analysis of the durations of the cell cycle stages obtained from pulse/chase labelling experiments gave the following results: (1) at 18°C G1 lasts for 31 hrs, S for 29.5 hrs, G2 for 8.5 hrs, M for 3 hrs and the total generation time is 72 hrs; (2) at 23°C G1 lasts for 14-3 hrs, S for 15-5 hrs, G2 for 5.7 hrs, M for 0.5 hrs and the total generation time is 36 hrs, and (3) at 28°C G1 lasts for 11.3 hrs, S for 13.5 hrs, G2 for 4.8 hrs, M for 0.4 hrs and the total generation time is 30 hrs. Pulse labelling followed immediately by fixation, and subsequent Giemsa staining, enables a quick and convenient assessment to be made of the relative durations of the cell cycle stages. In such preparations nuclei in S-phase are labelled, nuclei in G1 are small and unlabelled and nuclei in G2 are large and unlabelled. Chapter III. Pulse/stepdown labelling shows that DSTA replicates bidirectionally in the Xenopus cells. Origin to origin distances (initiation intervals) vary, but the range of and the mean initiation intervals at all three temperatures are much the same. The mean interval between initiation points is of the order of 60 to 66 mum. Staggering of initiations is evident at all three temperatures, and may be disproportionately greater at 28°C than at 23°C and 18°C. Evidence against the existence of replication termini is provided. Chapter IV. The rates of progress of DITA replication forks cannot be determined from pulse/stepdown preparations, so these rates had to be estimated from pulse labelled cultures. They are 5.5 mum/hr at 18°C, 10 mum/hr at 23°C and 16 mum/hr at 28°C.

No description available.

The controls over initiation of DNA synthesis, initiation of cell division, regulation of macronuclear DNA content, and the relationship between cell mass and growth rate were examined in cells growing under nutrient constraint, or in cells experiencing a change in growth conditions through nutritional enrichment (shift-up) or nutritional shift-down. Reduction in both cell mass and DNA content was achieved by growing Paramecium cells under nutritional limitation in the chemostat. Under the extreme condition in the chemostat, the normally balanced relationship between DNA content and cell mass (Berger, 1984 Kimball, 1967) is uncoupled. The DNA content in these cells is maintained at about 50 units, but cell mass can be as little as 24% of normal. The generation time in these slow growing cells was increased 4 to 5 times that of rapidly growing cells; the growth rate was also reduced by about the same proportion. Nutritional shift-up was done by transferring the chemostat cells to medium of excess food. Similarly, nutritional shift-down was performed by transferring cells either to the chemostat or to exhausted medium. The timing of DNA synthesis initiation is largely determined in the preceding cell cycle. Although growth rate (protein synthesis rate) responds quickly to the new conditions, the timing of DNA synthesis initiation is not readjusted immediately and reflects that of the parental cell cycle. The rate at which cells enter S phase however, is affected by a reduction in growth rate. The criteria for DNA synthesis initiation are not determined by cell mass per se. First, cell mass increases to about 180% of the initial G1 value at the time of DNA synthesis initiation following a nutritional shift-up. This value is much greater than that of well-fed controls (118%). However, the increase in cell mass up to the mean time of DNA synthesis initiation and cell division are not significantly different than that observed in well-fed cells. This suggests a mass-related control over initiation of DNA synthesis. Second, cells initiate DNA synthesis even when there is a net decrease in cell mass following nutritional shift-down. Thus, an increase in cell mass per se is not necessary for DNA synthesis initiation. Unlike initiation of DNA synthesis, the regulatory mechanisms determining the macronuclear DNA synthesized reflects solely the current nutrient conditions. Cells in chemostat culture normally maintain about half the normal amount of DNA (about 50 units). Following nutritional shift-up cells synthesize 100 units of DNA instead. Similarly cells synthesize only 50 units of DNA following nutritional shift-down. The amount of DNA synthesized, therefore, is related to the growth rate, and as discussed later, is also related to the commitment point to cell division. This study also reveals that the point of initiation of cell division is not time-dependent. It does not occur at a fixed duration following the previous fission or the initiation of DNA synthesis. The point of commitment to division occurs at about 95 minutes before fission regardless of growth rate. Analysis of the effects of macronuclear DNA synthesis inhibition in cc1 cells after the transition point for division indicate that cells synthesize 50 units of DNA before the point of commitment to division. This suggests that cells are committed to divide after synthesizing about 50 units of DNA. Following this point, rapidly growing cells will produce 50 units of DNA before fission; whereas slow growers will accumulate an amount proportional to their growth rate. There are reasons to believe that the threshold value of DNA for commitment to cell division may be 41 units instead of 50. / Science, Faculty of / Zoology, Department of / Graduate

No description available.

Glycogen Synthase Kinase-3αß is an enzyme that is involved in cell cycle regulation by promoting the degradation of cyclin D1 and cycling D3 in cells. Special emphasis is placed in its regulatory role in B cells, as there it is evidence that suggests that this protein is inhibited during germinal center formation, where B cells undergo proliferation, somatic hypermutation and class switch recombination. By inducing DNA recombination via the Cre/lLxP recombination system and utilizing tamoxifen as a Cre activity inducer, B cells were culture in 40LB cells to form induced germinal center in vitro. Flow cytometry analysis suggests that in the absence of GSK-3 αß B cells proliferate extensively in germinal centers and being the process of class switch recombination. Although the results of this study are in accord with current theory, more experiments and research need to be made to validate the conclusions set forth in this study.

No description available.