The Mitotic Spindle

The mitotic spindle is composed of hollow protein cylinders called microtubules arranged in two sets at opposite poles. Each microtubule has a minus end meeting at the spindle pole center and a plus end extending outward. Three types of microtubules—kinetochore, interpolar, and astral—work together to organize and move chromosomes during cell division.

Kinetochore microtubules bind chromosomes by attaching their plus ends to kinetochores, which are large protein complexes assembled at the centromere of each chromatid. This attachment links sister chromatids to opposite spindle poles, positioning them for separation during anaphase of mitosis and ensuring proper chromosome segregation.

Two families of motor proteins—kinesin-related proteins and dynein—are integral to spindle construction. Kinesin-related proteins typically move toward plus ends of microtubules, while dynein moves toward minus ends. These motors facilitate spindle assembly, chromosome segregation, and the coordinated movement necessary for proper cell division.

Interpolar microtubules extend from opposite spindle poles with their plus ends overlapping in the middle, resembling clasped hands. Motor proteins associate with these microtubules to direct spindle assembly and maintain the bipolar structure essential for separating sister chromatids to opposite cell ends during anaphase.

Astral microtubules anchor the spindle apparatus within the cell, projecting outward from each spindle pole to the cell cortex in a starburst pattern. These microtubules help position the spindle and ensure proper chromosome segregation by stabilizing spindle pole locations during mitosis and anaphase.

Most animal cells contain two centrosomes, one at each spindle pole. Each centrosome consists of two centrioles surrounded by the pericentriolar matrix and produces, organizes, and anchors microtubules. This organization creates the bipolar spindle structure necessary for proper chromosome segregation and cell division.

The intrinsic polarity of microtubules—with distinct minus and plus ends—enables motor proteins to move directionally along them. This directional movement, combined with the bipolar spindle arrangement, ensures sister chromatids are pulled to opposite cell poles during anaphase, facilitating accurate chromosome segregation.