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.