18.5: The Mitotic Spindle
The mitotic spindle—or spindle apparatus—is a eukaryotic, cytoskeletal structure made up of long protein fibers called microtubules. Formed during cell division, the spindle separates sister chromatids and moves them to opposite ends of a parental cell, where the now individual chromosomes are distributed to two daughter cell nuclei.
The bipolar configuration of the mitotic spindle facilitates chromosomal segregation, preparing the cell for division. One mechanism that ensures bipolar mitotic spindle formation relies on centrosomes.
Some cells, such as vertebrate oocytes and higher plant cells, lack centrosomes; however, most animal cells have two centrosomes when they enter mitosis. Each centrosome associates with a circular arrangement of microtubules on opposite ends—or poles—of the mitotic spindle. In other words, centrosomes nucleate microtubules.
Motor proteins—notably kinesins and dynein—typically operate at or near the ends of microtubules and facilitate bipolar mitotic spindle formation and the separation of sister chromatids.
For example, kinesin-5 motors at the spindle midzone attach to and slide apart two microtubules extending from opposite spindle poles; this process promotes spindle bipolarity and elongation by pushing the spindle poles away from each other.
Kinesin-5 activity is thought to be counterbalanced by kinesin-14. Kinesin-14 motors pull on microtubules extending from opposite poles, effectively bringing the poles together. The coordinated activity of these motors allows the spindle to assemble correctly.
Kinesin-4 and kinesin-10 are chromokinesins, kinesins that can associate with mitotic chromosomes. Kinesin-4 and kinesin-10 associate with chromosome arms, pushing the chromosomes and the spindle pole apart.
Dyneins organize microtubules in various parts of the cell. For example, they link astral microtubules to the actin cytoskeleton, moving the spindle poles away from each other.
The structure, organization, and components of the mitotic spindle allow sister chromatids to separate, preparing the cell for proper division.