Spindle fibers are known for forming a protein structure responsible for splitting genetic materials in a cell. In the nuclear division, fibers are also required in chromosome division from a parental cell into multiple daughter cells. During the mitosis division, the spindle fibers are called mitotic and meiotic in the meiosis split. At the start of the nuclear split, double wheel-shaped structures, commonly known as centrioles, tend to place themselves on the cell ends, making cell poles. After this, protein fibers extend in all directions from centrioles making what we refer to as a spindle.
Microtubules connect themselves to the chromosomes poles by linking to kinetochores which are protein complexes. Kinetochores are known as protein deposits found on every side of the chromosome at a center region of the chromosome. In the phase of cell division, the microtubules are responsible for pulling chromosomes in every direction until they get a proper alignment at the equatorial plane (Nazockdast & Redemann, 2020). The cell is taken through a significant checkpoint during this occurrence, ensuring that chromosomes are intact and held by the spindle. This is done to prepare chromosomes before the start of the necessary division. In addition, during the anaphase, chromosomes go through concurrent separation as they get pulled by the spindle at the opposite ends of the cell.
The spindle normally begins forming during the prophase when microtubules gather between two centrosomes. In this process, they make a connection that later separates duplicated chromosomes. During these several multi-steps, cell chromosomes tend to shrink while the spindle assemblies. Replicated chromosomes are then linked to the spindle, gather at the cell equator, and then separate as the microtubules of the spindle move in the opposite direction of the cell. In this process, chromosomes are engulfed by a membrane of nuclear which makes the original cell split into two daughter cells.
Reference
Nazockdast, E., & Redemann, S. (2020). Mechanics of the spindle apparatus. In Seminars in Cell & Developmental Biology. Academic Press.