Spindle Assembly

During eukaryotic cell division, microtubules assemble around the duplicated chromosomes in a bipolar array, forming the spindle apparatus. Spindle assembly is aided by multiple collaborating mechanisms.

Spindle assembly begins after two centrosomes are positioned at opposite ends of the cell. In animal cells, spindle poles are located at the centrosomes.

The centrosomes mature and nucleate bipolar microtubules, where the minus-end is anchored to the spindle pole, and the plus-end radiates outwards.

Simultaneously, multifunctional enzyme complexes, called M-Cdks, phosphorylate several nuclear envelope components, triggering the nuclear envelope to breakdown and expose the condensed chromosomes to the cytoplasm.

Microtubule nucleation at the centrosomes generates three types of microtubules. Interpolar microtubules come from opposite poles whose plus ends overlap, creating an antiparallel array at the spindle midzone. Kinetochore microtubules have their plus-ends connect with the kinetochores of exposed chromosomes. Astral microtubules have their plus-ends contact and interact with the cell cortex, positioning the spindle pole.

In the absence of centrosomes, mitotic chromosomes aid in acentrosomal spindle assembly. Mitotic chromosomes activate Ran-GTP, a nuclear protein. Activated Ran-GTP induces the release of microtubule-stabilizing proteins from protein complexes in the cytosol. The local activation of these factors promotes localized microtubule nucleation and stabilization.

Several microtubule-dependent motor proteins also contribute to spindle assembly and stabilization.

Dynein links astral microtubule plus-ends with cell-cortex components and pulls the spindle poles toward the cell-cortex. In the spindle midzone, kinesin-5 associates with interpolar microtubule plus-ends, to slide them past each other and generate a force that pushes the poles apart.

Kinesin-14 cross-links the interpolar microtubules at the spindle midzone and generates tension that pulls the poles together. Kinesin-4 and kinesin-10, the chromokinesins,  connect microtubules with chromosomal arms to push chromosomes away from the poles.

The balance of these opposing forces generated by the motor proteins determines the final length and position of the assembled spindle.