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Using fluorescence recovery after photobleaching, we observed that MTs throughout the preanaphase B spindle are very dynamic and display complete recovery of fluorescence, but during anaphase B, MTs proximal to the poles stabilize and therefore display lower recovery than those elsewhere. None of these changes occurred in the presence of nondegradable cyclin B. Modeling suggests that they depend on the establishment of a spatial gradient of MT plus-end catastrophe frequencies, decreasing toward the equator.
The resulting redistribution of ipMT plus ends to the overlap zone, together with the suppression of minus-end depolymerization at the poles, could constitute a mechanical switch that initiates spindle elongation. Chromosome segregation during mitosis depends on the action of the spindle, a protein machine that uses ensembles of kinesin and dynein motors plus microtubule MT dynamics to move chromatids polewards anaphase A and to elongate the spindle anaphase B; Scholey et al.
In addition, prometaphase spindle MTs use dynamic instability to search for chromosomes and then capture and align them on the spindle equator Mitchison and Kirschner, ; Wollman et al.
However, in many spindles, MTs suddenly become stable at the onset of anaphase Zhai et al. In budding yeast, the suppression of MT dynamics is regulated by the cell cycle—regulated Cdc14 phosphatase and is essential for proper chromosome segregation, as loss of MT stabilization at anaphase onset leads to defects in both anaphase A and B Higuchi and Uhlmann, In preanaphase B metaphase and anaphase A spindles, it is proposed that a kinesin-5 KLP61F —driven interpolar MT ipMT sliding filament mechanism is balanced by kinesin KLP10A —dependent ipMT depolymerization at the poles to maintain the spindle at a steady-state length while simultaneously driving poleward flux within ipMTs.