In this stage following changes occur:

• Chromatin material condenses and appears as thread like structure called chromosomes.

• Centrosomes divided into two and move to the opposite side of the nucleus and give  rise to spindle fibers with attachments of each chromatid.

• Centrosomes become shorter and thicker.
• Each chromosomes splits longitudinally into two chromatids but held together at a point called Centromere.

• Nuclear membrane and nucleolus start disappearing and disappear completely at the end of prophase.