During animal cell division, the organelle responsible for making up spindle fibers is the centrosome. The centrosome is composed of two centrioles that are located at right angles to each other. These centrioles play a vital role in organizing microtubules, which are the building blocks of spindle fibers.
When a cell is preparing to divide, the centrosome duplicates, and each centrosome moves to opposite sides of the cell. The microtubules then extend from these centrosomes and form the mitotic spindle, which is crucial for separating chromosomes during mitosis.
The spindle fibers attach to the chromosomes at their kinetochores, ensuring that each daughter cell receives an equal set of chromosomes. Thus, the centrosome and the microtubules it organizes are key components in the successful division of animal cells.