The cell cycles through real mitotic phases with relative timing modeled on light-microscopy observations: interphase (chromatin decondensed, nucleus intact) → prophase (chromosomes condense into visible sister-chromatid pairs joined at the centromere, centrosomes nucleate the spindle and migrate to opposite poles, the nuclear envelope breaks down) → metaphase (kinetochore microtubules from each pole capture a chromatid; chromosomes congress to the equatorial plate) → anaphase (kinetochore microtubules depolymerize, reeling sister chromatids apart toward opposite poles, while polar microtubules push the poles further apart) → telophase (chromosomes decondense at each pole, nuclear envelopes reform) → cytokinesis (an actomyosin contractile ring pinches a cleavage furrow at the equator, splitting one cell into two).
The spindle-assembly checkpoint (SAC) normally halts the metaphase→anaphase transition until every kinetochore reports proper bi-orientation (one sister chromatid's kinetochore attached to each pole). Lowering the checkpoint-stringency slider models a weakened SAC: an unattached or mono-oriented chromosome is more likely to be waved through uncorrected. When that happens both sister chromatids get reeled to the same pole instead of splitting — nondisjunction — and the two daughter cells end up aneuploid (one with an extra chromosome, one missing one), which the Daughter A / Daughter B readout shows directly after cytokinesis.
- Spindle fibers — lines from each centrosome (pole) to the kinetochore of every chromatid it has captured.
- Checkpoint delay — a high stringency value adds a metaphase pause (visible as the phase bar stalling) whenever a misattachment is detected, giving the cell more time to fix it before proceeding.
- N (chromosome number) — each chromosome shown is already replicated (a pair of sister chromatids), matching a real cell in G2/M, not two homologous chromosomes.