mitotic spindle
/ my-TOT-ik SPIN-dul /
Imagine two teams on opposite sides of a field, each throwing ropes to hook onto a row of boxes lined up in the middle, then reeling their share of boxes back to their own side so each side ends up with an equal pile. The mitotic spindle is the cell's version of that rope-and-pulley rig, built from microtubules, whose job is to share the duplicated chromosomes evenly when one cell divides into two.
Structurally the spindle is a football-shaped array of microtubules that stretches between two poles at opposite ends of a dividing cell (in animal cells the poles are the two centrosomes). Some microtubules reach in from each pole and attach to a special protein platform on each chromosome called the kinetochore; others overlap in the middle or push against the cell ends. Using a mix of motor proteins and the controlled growing and shrinking of microtubules, the spindle first lines all the chromosomes up at the cell's equator, then pulls the two copies of each chromosome to opposite poles, so each new cell receives one complete set.
We treat the spindle here as a cytoskeletal structure — a remarkable, temporary microtubule machine — rather than retelling the full choreography of the cell cycle. The point worth keeping is that the spindle is built from the same tubulin bricks and dynamic instability that run microtubules everywhere else; the cell simply reorganizes its everyday scaffold into a precise chromosome-sorting machine, then takes it apart again once division is done. Because this machine is essential to dividing cells, several chemotherapy drugs deliberately sabotage spindle microtubules to stop cancer cells from multiplying.
The cancer drug taxol works by freezing spindle microtubules so they can no longer shrink; chromosomes cannot be pulled apart, division stalls, and the cell is triggered to die.
The spindle must keep its microtubules dynamic; freezing them is just as crippling as dissolving them.
Here the spindle is treated as a microtubule structure, not the full cell cycle; for the timing and checkpoints of division see the cell-division entries it cross-references.