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The nuclear envelope reforms during telophase, which is the final stage of mitosis. Throughout the previous phases of mitosis—prophase, metaphase, and anaphase—the nuclear envelope breaks down. In prophase, the chromatin condenses into visible chromosomes, and the nuclear envelope disintegrates, allowing the spindle fibers to attach to the chromosomes. Metaphase occurs when the chromosomes align at the cell's equatorial plane, and anaphase involves the separation of sister chromatids as they are pulled toward opposite poles of the cell.

As telophase begins, the separated chromatids reach the poles of the cell, and the process of reforming the nuclear envelope starts around each set of chromosomes. This leads to the re-establishment of the distinct nucleus in each daughter cell, allowing for normal cellular function to resume after mitosis is complete. This is a critical step for ensuring that each daughter cell inherits a complete set of chromosomes encased within a nucleus, which sets the stage for subsequent cell division and proper functioning of the cells.