In mitosis, the nuclear membrane disappears during the prophase stage. This is the first stage of mitosis, where the chromatin condenses into visible chromosomes. As the chromosomes condense, the nuclear membrane begins to break down and eventually disappears completely. This allows the chromosomes to move freely within the cell and align themselves properly for the subsequent stages of mitosis.
The breakdown of the nuclear membrane is a crucial step because it ensures that the chromosomes can be accurately separated into two daughter cells. Without the nuclear membrane, the spindle fibers can attach to the chromosomes and pull them apart during the metaphase and anaphase stages.
In summary, the nuclear membrane disappears during prophase to facilitate the proper alignment and separation of chromosomes, ensuring that each daughter cell receives an identical set of chromosomes.