During which phase does the nuclear membrane break down?

The nuclear membrane breaks down during prophase, which is the first stage of mitosis. In this phase, chromatin condenses into visible chromosomes, and the nuclear envelope disintegrates, allowing the spindle fibers to access the chromosomes. This breakdown is a crucial step because it facilitates the separation of the duplicated chromosomes, which is essential for successful cell division. Prophase sets the stage for the following phases of mitosis, specifically metaphase and anaphase, where the chromosomes are aligned and then separated respectively. This transformation marks the transition from the interphase, where the cell prepares for division, into the actual process of mitotic division.