Box 1.

Accuracy: Conformity of a measure to a true or expected value. For example, how close is the average spindle position to the cell center.

Anterior-posterior (AP) axis: In prolate ellipsoidal cells, such as the one-cell C. elegans embryo, the AP axis corresponds to the long axis.

Astral microtubules: microtubules that nucleate from the centrosomes and grow out towards the cell cortex.

Buckling: When a slender rod is subject to compressive forces acting at their ends, there is a critical force, termed the Euler force, below which the rod remains straight and above which the rod bends into a sinusoidal or hemi-sinusoidal shape (Howard, 2001).

Catastrophe: The conversion of a growing microtubule end to a shrinking one.

Centering force: The force that drives towards and maintains the spindle in the cell center.

Centering stiffness: The centering force divided by the distance between the spindle and the cell center.

Cell cortex: The inner surface of the plasma membrane, usually reinforced by actin, myosin and other actin-binding proteins (Salbreux et al., 2012).

Cortical force generators: A protein complex, including the motor protein cytoplasmic dynein, that binds to the depolymerizing ends of microtubule and generates tensile forces.

Drag coefficient: When multiplied by the velocity equals the force that resists motion.

Euler force: See buckling.

Maintenance phase: In the one-cell C. elegans embryo, the metaphase plate is established between nuclear envelope breakdown and the onset of anaphase. The maintenance phase is a time interval of about 1 to 2 minutes during this period when the centrosomes remained stably centered on the A-P axis, and there is little drift in the transverse direction.

Microtubule dynamics: The polymerization and depolymerization of microtubules, which switch between growing and shrinking phases under the control of tubulin's GTPase cycle(Howard and Hyman, 2003). See catastrophe.

Microtubule organizing center (MTOC): the centrosome of interphase cells from which an aster of microtubules nucleates. In yeast it is the spindle pole body.

Mitotic spindle: A bipolar structure comprising two radial arrays of microtubules emanating from two centrosomes, which form the poles.

Motor protein: An enzyme that uses chemical energy derived from the hydrolysis of ATP to generate directed motion. Dynein and kinesin are two large families of proteins that move along microtubules.

Precision: The repeatability of process; for example, how much spindle position varies from cell to cell. A process can be precise, but not accurate. Accuracy requires precision.

Spindle microtubules: Microtubules that nucleate at the centrosome and grow towards the chromosomes; the K-fibers bind end-on to the kinetochores while the polar/interpolar microtubules are cross-linked to microtubules from the other pole.

Stability: Magnitude of fluctuations around a mean value, measured as the standard deviation. For example, the spindle position fluctuates over time during the maintenance phase.

Thermal fluctuations: Positional variability of particles, such as organelles or the mitotic spindle, driven by impacts from randomly moving solvent molecules.