The Mitotic Spindle

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.