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.

The mitotic spindle separates sister chromatids and moves them to opposite sides of the cell during anaphase of mitosis.

Fundamental structures of the mitotic spindle are hollow cylinders called microtubules. Two sets of microtubules are arranged on opposite ends, or poles, of the mitotic spindle.

Each microtubule has a minus end and a plus end. The minus ends of the microtubules meet at the center of the spindle poles. The plus ends extend outward from the poles.

There are different types of microtubules with distinct positions and roles in the mitotic spindle.

Kinetochore microtubules bind chromosomes to the spindle pole by attaching at their plus ends to kinetochores. Kinetochores are large protein complexes assembled at the chromatid centromere, a specialized DNA sequence that links sister chromatids.

The arrangement of interpolar microtubules resembles a pair of clasped hands. The plus end of an interpolar microtubule overlaps with the plus end of another interpolar microtubule extending from the opposite pole. Motor proteins associate with interpolar microtubules to direct spindle assembly.

Astral microtubules anchor the spindle in the cell. These microtubules collectively resemble a starburst, with each positive end projecting outward from the spindle pole to the cell cortex.

In most animal cells, the microtubules are organized around an organelle called a centrosome. One centrosome is present at each spindle pole.

A centrosome consists of two centrioles surrounded by a shapeless mass of proteins called the pericentriolar matrix. The centrosome produces, organizes, and anchors microtubules in the cell.

Two families of motor proteins are integral to the construction and operation of the mitotic spindle: kinesin-related proteins and dynein. Typically, kinesin-related proteins move toward the plus ends of microtubules and dynein moves towards the minus ends.

The intrinsic polarity of the mitotic spindle and its microtubules facilitates the mitotic segregation of chromosomes, preparing the cell for division.