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Large scale motion of cytoplasm called cytoplasmic streaming occurs in somelarge eukaryotic cells to stir the cell's constituents. In Drosophila oocytes,microtubules have been observed to undergo undulating motion, curving to formtravelling waves during cytoplasmic streaming. Here we show that this wave-likemotion can be understood physically as due to the hydrodynamic drag ofstreaming impellers attached to kinesin motors moving toward the plus-ends ofmicrotubules whose minus ends are anchored to the cell cortex. The tangentialforces applied to such microtubules by kinesin give rise to bending and leadsto chiral symmetry breaking causing the microtubules to propagate longtravelling waves. The waves are reminiscent of those seen in flagellar motionbut of a much longer time scale and by a different physical mechanism. We showhow kinesin movement can produce a bulk flow of cytoplasm surrounding amicrotubule with the range of flow greatly enhanced by the effect ofhydrodynamic coupling between impellers. That is, a relatively small number ofmotors can move a large amount of fluid. The chaotic nature of the fluid motionof cytoplasm caused by kinesin movement along constantly changing microtubuletrajectories is important as it greatly enhances the efficiency of mixing.Existing data on in vitro microtubule gliding assays also show this chiralinstability in two dimensions and an analysis of this gives quantitativeestimates for the forces exerted by motors and the drag coefficient.

The mechanics of a microscopic mixer: microtubules and cytoplasmic streaming in Drosophila oocytes