5.2 Chromosome Behaviour in Mitosis

After DNA replication, each replicated chromosome contains two matching sister chromatids joined at a centromere. Mitosis uses the spindle to align and then separate those copies so each daughter nucleus receives one corresponding set of genetic material.

1. Condense and prepare: The replicated chromosomes condense so each pair of sister chromatids can be moved as a discrete structure; the spindle forms across the cell.
2. Connect: Spindle fibres attach to the chromosome region at the centromere, with the two sister chromatids connected to opposite poles. This gives the copies opposing pulling directions.
3. Align: The replicated chromosomes arrange at the cell equator. Alignment checks that each sister chromatid is positioned to move toward a different pole.
4. Separate: The centromeres divide and the sister chromatids are pulled toward opposite poles. Once separated, each chromatid is counted as an individual chromosome.
5. Reach and enclose: The chromosome groups arrive at opposite poles, decondense and become enclosed in two new nuclei. Each nucleus therefore receives one matching copy of each chromosome.

Opposite-pole spindle attachment is the key distribution logic: alignment precedes centromere separation, and separation converts one replicated chromosome into two matching chromosome copies moving apart. This is how mitosis preserves the chromosome set in each daughter nucleus rather than sending both copies to one side.

An aligned chromosome with joined sister chromatids is before anaphase; separated groups moving to opposite poles are after centromere separation. The named PMAT stage sequence belongs to 4568, while deciding a stage from a micrograph belongs to 4569; this card explains the chromosome mechanism rather than image-specific clues.

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.

Mitosis is the ordered division of one nucleus into two genetically matching nuclei. The four named stages are prophase, metaphase, anaphase and telophase (PMAT); interphase prepares the cell but is not one of these four stages.

1. Prophase — condense and prepare: Chromosomes condense and become distinct; each still has two sister chromatids joined at a centromere. The nuclear envelope breaks down and the spindle forms. The nucleus is being prepared for chromosome movement; the cytoplasm has not yet divided.
2. Metaphase — align and connect: Replicated chromosomes arrange at the cell equator. Spindle fibres attach at the centromere regions and connect the sister chromatids to opposite poles. This alignment provides the starting point for equal separation.
3. Anaphase — separate and move: Centromeres divide, so sister chromatids become individual chromosomes. Spindle fibres shorten and pull the separated chromosomes to opposite poles, forming two genetic-material groups.
4. Telophase — re-form nuclei: Chromosomes reach opposite poles and begin to decondense. A nuclear envelope reforms around each chromosome group and the spindle is dismantled. Cytokinesis then divides the cytoplasm, completing separation into two cells.

The PMAT sequence links chromosome state to movement: condensation makes chromosomes movable, spindle attachment and equatorial alignment establish opposite-pole routes, centromere division permits sister chromatids to separate, and arrival at the poles allows two nuclei to reform. Cytokinesis follows nuclear division to partition the cytoplasm, rather than being an additional PMAT stage.

Do not include interphase in PMAT: DNA replication occurs before mitosis. Card 4566 explains the chromosome-separation logic in more detail, and card 4567 covers coordinated nuclear-envelope, spindle and cell-boundary behaviour; this card is the ordered stage map. A micrograph or slide is interpreted separately in card 4569.

Stage identification in a mitosis micrograph or root-tip section is an evidence-matching task: inspect chromosome position and state first, then use nuclear-envelope and cell-separation clues when they are visible, and assign the PMAT stage only when the combined evidence fits.

1. Check the specimen and evidence: Confirm that the field is a suitable stained root-tip or other mitotic preparation. Note the cell orientation, image quality and whether the nucleus, chromosome group or cell boundary is actually visible; do not infer a hidden feature from an unclear image.
2. Inspect chromosome condensation: Look for distinct condensed chromosomes, chromosomes arranged in a line at the equator, sister chromatids separated into two groups, or chromosomes gathered at poles and beginning to decondense.
3. Inspect nuclear and spindle clues: If resolvable, check whether the nuclear envelope is breaking down or absent, whether spindle fibres are guiding an equatorial arrangement or poleward movement, and whether nuclear envelopes are reforming. These clues support the chromosome evidence rather than replacing it.
4. Match the stage: Prophase is condensation and preparation for movement; metaphase is equatorial alignment; anaphase is separated chromatids moving towards opposite poles; telophase is pole arrival with decondensation and reforming nuclei. Cytokinesis or a new cell partition is supporting evidence for completion of division, not a separate PMAT stage.
5. Cross-check before deciding: Use at least two compatible observations where possible—for example, equatorial alignment plus spindle attachment for metaphase, or two separated chromosome groups plus poleward movement for anaphase. Compare neighbouring cells only as context: a root-tip field contains cells at different stages, not one shared stage.

The decisive evidence changes through PMAT: chromosomes first become compact and movable, then align, then separate, and finally reach opposite poles while nuclei reform. This sequence explains why a cell with joined chromatids at the equator is not yet in anaphase, whereas two chromosome groups moving apart are evidence for anaphase.

Do not identify a stage from one vague shape, cell size or stain intensity alone. Section angle, overlap, image resolution and an obscured nuclear envelope can hide useful clues, so report only what the image supports and use the strongest visible combination. The ordered PMAT map belongs to card 4568; this card teaches how to apply it to an image or slide.