5.2.2—Cell structures in mitosis

Mitosis coordinates three structural changes: the nuclear envelope opens access to condensed chromosomes, the spindle positions and moves the chromosome copies, and the cell boundary later partitions the cytoplasm around the two new nuclei.

• Nuclear envelope: It breaks down during the early part of mitosis so spindle fibres can interact with the condensed chromosomes, then reforms around each chromosome group at the poles as new nuclei form.
• Spindle: Spindle fibres extend across the cell and attach to chromosome regions at the centromeres. Opposite-pole connections align the copies; shortening of the attached fibres contributes to poleward movement after separation.
• Chromosome link: These structural changes make accurate chromosome distribution possible: access, attachment and movement are coordinated rather than independent events.
• Cell-surface membrane: The membrane remains the cell boundary during nuclear division, then changes shape during cytokinesis. Animal cells constrict to form a cleavage furrow; plant cells build a new partition that separates the daughter cells.
• Result: Nuclear-envelope reformation creates two nuclei, and boundary/cytoplasmic partition completes the transition from one divided nucleus to two separate daughter cells.

The nuclear envelope must no longer block spindle–chromosome interaction, the spindle must provide directional forces, and the cell boundary must wait until the nuclei are organised before partitioning the cytoplasm. Together these behaviours connect chromosome movement to physical separation without treating cytokinesis as another nuclear stage.

The nuclear envelope is not simply absent for the whole cell cycle, the spindle is not a passive scaffold, and the cell-surface membrane does not disappear during mitosis. Card 4566 owns the detailed sister-chromatid movement chain; card 4568 owns the named PMAT stage labels.