Figure 4.

Model of the magnetic-field effect on the MA. (Upper) Representation of cells and the MA. The small fusiform shape represents the spindle. (Lower) Free-body diagrams of the spindle. The arrows represent the forces, exerted on the spindle poles by the astral microtubules. At maximum, these forces are equivalent to the force at which each of the microtubules buckle. (a) Cell in which the mitotic spindle has formed in zero magnetic field. The sum of the torques on the spindle caused by the astral microtubules (τ_AS) is zero for the spindle in the center of the cell as shown. (b) In a large magnetic field, the shapes of the asters change as the microtubules bend to align with the magnetic-field vector B. This bending changes the distribution of forces acting on the spindle poles and makes τ_AS such that the spindle tends to be turned perpendicularly to the applied magnetic-field vector (B). If τ_AS exceeds the torque exerted directly on the spindle by B then the spindle will tend to rotate into the configuration shown in c, which will be stable.