Chromatin condensation

An important organelle in mitosis is the centrosome, the microtubule organizing center in metazoans. During prophase, the two centrosomes, which replicate independently of mitosis, have their microtubule-activity increased due to the recruitment of γ-tubulin. The centrosomes will be pushed apart to opposite ends of the cell nucleus by the action of molecular motors acting on the microtubules.

At the end of prophase, the tiny nucleolus within the nucleus dissolves. The nucleolus starts moving towards the nuclear membrane at the beginning of prophase, and its disintegration occurs near the nuclear envelope. Its contents are dispersed in masses.

Basically, the chromatin in the nucleus coils into chromosomes. The centrosomes move to opposite poles of the cell, forming a bridge of spindle fibers. At the end of prophase, the nucleolus disperses.

In prometaphase, the next step of mitosis, the nuclear membrane breaks apart and the chromosomes are captured by microtubules attached to centromeres.

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