Events of Prophase:

Chromatin: Normally, the genetic material in the nucleus is in a loose thread-like form called chromatin.

Chromosomes:

At the one prophase, chromatin condenses into highly ordered structures called chromosomes. Since the genetic material has already been duplicated earlier in the S phase, each chromosome is made of two sister chromatids, bound together at the centromere. Each chromosome has a kinetochore at the centromere. A kinetochore is a complex protein structure that is the point where spindle fibers attach.

Centrosome:

Close to the nucleus are two centrioles collectively called a centrosome. Each centriole replicates and thus two daughter centrosomes are formed. Each daughter centrosome acts as a coordinating center for the cell's microtubules.

Mitotic Spindle:

The two centrosomes give rise to microtubules by polymerizing (joining monomers to form polymers) the tubulin proteins present in the cytoplasm. The microtubules thus formed are called spindle fibers, and the complete set of the spindle fibers is known as the mitotic spindle. During the formation of the mitotic spindle, the centrosomes migrate to the opposite sides of the nucleus. The nucleolus and the nuclear envelope have degraded, and spindle fibers have invaded the central space.

Aggregation of Tubulin Proteins:

In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before prophase. The cells of plants lack centrioles. Instead, spindle fibers are formed by the aggregation of tubulin proteins on the surface of the nuclear envelope during prophase.