The chromosomes become visible and the nucleus starts to break up during:

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During prophase, the chromatin condenses and becomes visible as distinct chromosomes. The nuclear envelope also starts to break down, allowing the chromosomes to move into the cytoplasm for the later stages of cell division. Metaphase is characterized by the alignment of chromosomes at the metaphase plate, an imaginary plane in the middle of the cell. Anaphase is marked by the separation of sister chromatids and their movement towards opposite poles of the cell. Telophase is the final stage of mitosis, during which the nuclear envelope reforms and the cell prepares to divide into two daughter cells.

The Chromosomes Become Visible and the Nucleus Starts to Break