5.2.1—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.