 Cytokinesis

Cytokinesis partitions the cytoplasm. Without cytokinesis, two cell nuclei can share one biological cell. ^[ref]

Cytokinesis makes physically separate daughter biological cells by partitioning their cytoplasm and completing their cell membrane boundaries. Mitosis divides nuclear material. These processes can overlap during M phase, but completion of one does not imply completion of the other.^[ref]

For an original compartment-counting example, consider a hypothetical diploid biological cell after two daughter cell nuclei have formed. Let $n=5$ be the number of chromosomes in one haploid set, and let $C$ be that set's unreplicated nuclear DNA amount, as in cell-cycle DNA content. Assume faithful mitosis, no nuclear DNA replication during the partition, and no loss of nuclear DNA; exclude mitochondrion DNA.

The check is to follow the two existing cell nuclei: initially one boundary encloses both; finally each daughter boundary encloses one. The total remains two cell nuclei, twenty chromosomes and $2C+2C=4C$ of nuclear DNA. Thus the amount per biological cell halves because the counting boundary changes, without any new DNA replication or change of nuclear ploidy. Doubling the number of biological cells need not double their combined material.

Coordinating chromosome segregation with cytoplasm partition lets descendants inherit nuclear information inside separate working compartments. Equal nuclear DNA allocations do not require equal daughter sizes or equal allocations of every organelle. The geometry of cytokinesis connects an internal allocation to the creation of two independently bounded biological cells.