Concept: During mitosis, the nuclear architecture must be dismantled to allow spindle fibers to access the chromosomes.

Solution: As the cell enters prophase of cell division, the nuclear envelope disintegrates and the nucleolus completely disappears. This halts rRNA synthesis so the cell can focus all resources on division.

Distractor Autopsy: The Endoplasmic Reticulum also fragments, but in classical cytological context (and specific exam criteria), the "disappearance" of the nucleolus is the most prominent visual hallmark of prophase.