At the beginning of the process in an animal cell, the partition of the centriole takes place, which has been duplicated during Interphase but were in the same centrosome.
Bipolarity:
Early in mitosis the two pairs of centriole separate and migrate to opposite sides of the nucleus, establishing the bipolarity of the dividing cells.
Formation of Microtubules:
Three sets of microtubules (fibers) originated from each pair of centrioles. One set the astral microtubules; radiate outward and form aster, other two sets of microtubules compose the spindle.
Types of Microtubules:
The Kinetochore Microtubules attach to the chromosomes at kinetochore and Polar Microtubules do not interact the chromosomes but instead interdigiate with polar microtubules from the opposite pole.
Composition of Microtubules:
The microtubules are composed of a protein tubulin and traces of RNA.
Mitotic Apparatus:
This specialized microtubules structure including aster and spindle is called mitotic apparatus.
Role of Mitotic Apparatus:
This is larger than the nucleus, and is designed to attach and capture chromosomes, align them and finally separating them so that equal distribution of chromosomes is ensured.