| Summary: | Self-replication of cells involves nuclear separation and cytokinesis, both of which depend on the mechanical nature of cytoplasm. The cytoplasm is a multicomponent and active gel, solid enough to integrate forces over the cell, but liquid enough to permit dynamic organization. This thesis addresses force generation and cytoplasmic organization during cytokinesis at two levels: how do the emergent mechanics of cells facilitate cytokinesis, and how do these mechanics emerge from their molecular composition and interactions? These broad questions were addressed in two systems: eggs of the frog Xenopus laevis, and JCVI-syn3.0, a genomically minimized bacterium derived from Mycoplasma mycoides. The X. laevis system enables essentially undiluted cytoplasmic extracts and is arguably the most physiological non-living system, and JCVI-syn3.0 is arguably the least complex living system, so they bridge the interface between non-living molecules and living cells. In each system, this thesis characterizes mechanically relevant molecules then develops a continuum model for the mechanics. In the X. laevis system, the most abundant cytoplasmic components – microtubules, F-actin, intermediate filaments, endoplasmic reticulum, mitochondria, and cytosol – are imaged, in order to characterize flows involved in nuclear positioning and cytoplasmic organization. While the system had been conceptualized as a solid network of microtubules immersed in a liquid cytosol, progress reported in Chapters 2 and 3 shows the system behaves as a continuum and develops a new model of nuclear positioning based on surface forces and internal stresses. In the M. mycoides system, cell morphology dynamics are imaged using a microfluidic chemostat, and the genetic basis for normal morphology is determined using an approach agnostic to known gene function. While previous models had considered the cytoskeletal protein FtsZ as the primary driver for cell division, progress reported in Chapter 4 shows cell morphology is more complex and depends on five additional genes of unknown function, which likely affect membrane mechanics. In both systems, this thesis shows a mechanically integrated view of the cell is necessary and enabling in order to understand cytokinesis and physiology.
|
|---|