  Microtubules (MTs) nucleated by centrosomes form star-shaped structures
referred to as asters. Aster motility and dynamics is vital for genome
stability, cell division, polarization and differentiation. Asters move either
towards the cell center or away from it. Here, we focus on the centering
mechanism in a membrane independent system of Xenopus cytoplasmic egg extracts.
Using live microscopy and single particle tracking, we find that asters move
towards chromatinized DNA structures. The velocity and directionality profiles
suggest a random walk with drift directed towards DNA. We have developed a
theoretical model that can explain this movement as a result of a gradient of
MT length dynamics and MT gliding on immobilized dynein motors. In simulations,
the antagonistic action of the motor species on the radial array of MTs leads
to a tug-of-war purely due to geometric considerations and aster motility
resembles a directed random-walk. Additionally our model predicts that aster
velocities do not change greatly with varying initial distance from DNA. The
movement of asymmetric asters becomes increasingly super-diffusive with
increasing motor density, but for symmetric asters it becomes less
super-diffusive. The transition of symmetric asters from superdiffusive to
diffusive mobility is the result of number fluctuations in bound motors in the
tug-of-war. Overall, our model is in good agreement with experimental data in
Xenopus cytoplasmic extracts and predicts novel features of the collective
effects of motor-MT interactions.
