Chromosome Segregation and Spindle Positioning


How does the spindle position itself?

Cell division is an essential biological process in which DNA in chromosomes are replicated, chromosomes are aligned in the equator of the cell in metaphase and separate in anaphase. The division of our genetic material occurs inside our body 10 million times per second. Failure to properly divide would cause a cell to die or survive with disordered properties. An actively adjusting molecular scaffold, called the spindle, plays an important role in mediating this process. 

We study the physics and biology of mechanisms responsible for spindle positioning during the first mitosis in C. elegans embryos. To do this we analyze the motion of the cellular fluid via NV diamond tracers, and track labelled microtubules. We can then perturb the system with a laser ablation and watch the resulting forces during recovery from both the tracked fluid, and microtubule bundles. 

Oscillations in Spindle Positioning during First Mitosis

 

During cell division the miotic spindle is responsible for the segregation of chromosomes and determines the plane of cleavage. Cell division is asymmetric, with the spindle positioned off-center towards the posterior. Oscillations of the poles of the spindle occur for a period of time about their final position, which provides a window to study the forces involved in spindle positioning.

    Proposed Molecular Forces

    The spindle is a bipolar structure composed of microtubules which are dynamic polymers that assemble from tubulin heterodimers. A microtubule is polar structure that has a plus end and a minus end. Usually the plus end is more dynamic in spindle undergoing transitions between polymerization and depolymerization. In the spindle, microtubules focus their minus ends at two spindle poles, with their plus ends radiating outwards. Microtubules with their plus end on a kinetochore are called kinetochore microtubules, while Microtubules with plus ends between two spindle poles are called interpolar microtubules and those with their plus ends at the cell cortex are called astral microtubules. 

    There are two types of forces that may be responsible for the positioning of the spindle. Forces due to microtubule polymerization/depolymerization and restoring forces of microtubule bending; and forces due to the motion of motor proteins

    Microtubule Forces: Pushing Forces and Transverse Forces

    Motor Protein Forces: Cortical Force and Cytoplasmic Force

    NV Diamonds for Cellular Fluid Tracing

    To analyze the motion of the cellular fluid we use Polyethylene Glycol coated diamonds containing Nitrogen-vacancy centers as fluorophores on the scale of 100 nm. We microinject these tracing particles into the gonads of GFP-tubulin expressing C. elegans.

    Laser Ablating the Oscillating System

    A Ti:Sapphire femtosecond pulsed laser is used to induce micro-cavitation in the fluid that in turn can ablate microtubules. By ablating posterior astral microtubules at different timings of oscillations we can study what sort of forces the microtubules induce.

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    Project Members:

    Hai-Yin Wu

    Graduate Student

    Physics Department

    Che-Hang Yu

    Graduate Student

    Applied Physics Department