What distinguishes an atom of one element from an atom of another element?

The right answer is Spindle.

In cell division, the mitotic spindle or achromatic mitotic apparatus of a cell refers to a structure of microtubules arranged in a spindle-shaped (i.e. swollen in the middle) bipolar assembly.

The achromatic spindle, meiotic spindle or mitotic spindle, is the set of microtubules that flow from the centrioles during the processes of cell division, whether mitosis (mitotic spindle) or meiosis (achromatic spindle or meiotic spindle), and range from centromeres of chromosomes to centrioles at the poles. They come from the centrosome (in the animal cell) or from the microtubule organizing center (in the plant cell). In the metaphase, all chromosomes are arranged in the equatorial plane of the dividing cell, and during anaphase, each of the two chromatids in which a chromosome is divided is driven to one of the two poles of the cell by said microtubules. The kinetochores are proteinaceous leaves located in the centromeres of the chromosomes in which the microtubules of the mitotic or achromatic spindle are anchored.