Onion Cell, S = spindle Credit: The Entangled Bank

Spindle fibers are aggregates of microtubules that move chromosomes during cell division.

Spindle fibers form at opposite poles of a cell during prophase.

During metaphase, spindle fibers extend from the cell poles toward the midpoint of the cell known as the metaphase plate.

Chromosomes are held at the metaphase plate by the equal forces of the spindle fibers pushing on the centromeres of the chromosomes.

In anaphase, spindle fibers pull the chromatids toward the spindle poles. Spindle fibers not connected to chromatids lengthen and elongate the cell.

Additional Reading:

• Stages of Mitosis
  
• Mitosis Quiz