During which phase does the nuclear envelope break down?

The correct choice is prophase. During this phase of cell division, particularly in mitosis, the nuclear envelope begins to disintegrate, allowing the mitotic spindle to interact with the chromosomes. Prophase is characterized by the condensation of chromatin into visible chromosomes, which can be seen under a microscope, and the breakdown of the nuclear envelope facilitates the movement and alignment of these chromosomes later in the process. This breakdown is crucial for the subsequent phases of mitosis, as it ensures that the chromosomes can be properly aligned and separated into the daughter cells.