It is well known that multivalent cations can cause DNA to condense from solution to form high-density nanometer scale particles. However, several fundamental questions concerning the phenomenon of DNA condensation remain unanswered. DNA condensation in vitro has been of interest for many years as a model of naturally occurring DNA packaging (e.g. chromatin, sperm head and virus capsid packing). More recently, DNA condensation has been of interest in optimizing artificial gene delivery, where packaging genes to an optimal size is essential to developing efficient uptake and delivery systems. The research presented in this dissertation provides an in depth biophysical study of the factors that control DNA condensate size and morphology. Millimolar changes in the ionic strength of the solution were found to alter the size of toroidal condensates. Variations in the order of addition of the counterions also significantly changed the size and morphology of the condensates. Studies were also performed to investigate the effects of static curvature and increased DNA flexibility on DNA condensation. These include the addition of static bending by sequence directed curvature, dynamic bending through protein-DNA interactions and reducing DNA persistence length by condensing single-stranded DNA. Several new models of DNA condensation are proposed based on the experimental data presented in this thesis.

<p> A three dimensional photoelastic method of analyzing the stresses in toroids, such as Bourdon Tubes, is proposed in this thesis, The experimental techniques and procedures developed are documented in detail, The experimental method is used to study a thick walled toroidal shell under a uniform internal pressure. </p> <p> The principal stresses on the inside and outside surfaces of the shell were determined and from these the maximum shear stresses calculated. The experimental results are compared to Lame's solution of a thick walled ring under iniform internal pressure, a thin walled toroidal shell subjected to uniform internal pressure, and a long, straight thin walled cylinder with closed ends loaded by internal pressure. </p> / Thesis / Master of Engineering (ME)

We studied regenerating bilayered tissue toroids dissected from Hydra vulgaris polyps and relate our macroscopic observations to the dynamics of force-generating mesoscopic cytoskeletal structures. Tissue fragments undergo a specific toroid–spheroid folding process leading to complete regeneration towards a new organism. The time scale of folding is too fast for biochemical signalling or morphogenetic gradients, which forced us to assume purely mechanical self-organization. The initial pattern selection dynamics was studied by embedding toroids into hydro-gels, allowing us to observe the deformation modes over longer periods of time.We found increasing mechanical fluctuations which break the toroidal symmetry, and discuss the evolution of their power spectra for various gel stiffnesses. Our observations are related to singlecell studies which explain the mechanical feasibility of the folding process. In addition, we observed switching of cells from a tissue bound to a migrating state after folding failure as well as in tissue injury. We found a supra-cellularactin ring assembled along the toroid’s inner edge. Its contraction can lead to the observed folding dynamics as we could confirm by finite element simulations. This actin ring in the inner cell layer is assembled by myosin-driven length fluctuations of supra-cellular F-actin bundles (myonemes) in the outer cell layer.