= Cytokinesis
{wiki}
<Cytokinesis> partitions the <cytoplasm>. Without <cytokinesis>, two <cell nuclei> can share one <biological cell>. <Biology 2e by Mary Ann Clark and others/The Cell Cycle>[Biology 2e, §10.2, The Mitotic Phase, Figure 10.6 and Cytokinesis]{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.<Biology 2e by Mary Ann Clark and others/The Cell Cycle>[Biology 2e by Mary Ann Clark and others; §10.2, The Mitotic Phase and Cytokinesis: nuclear division versus physical daughter-cell separation]{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>.
|| Counting boundary
|| Before <cytokinesis>
|| After <cytokinesis>
| <Biological cells>
| 1
| 2
| <Cell nuclei> in each <biological cell>
| 2
| 1
| Each <cell nucleus>
| $2n=10$ <chromosomes>, $2C$
| $2n=10$ <chromosomes>, $2C$
| Nuclear <DNA> per <biological cell>
| $4C$
| $2C$
| Nuclear <DNA> across all descendants
| $4C$
| $4C$
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.
\Image[/cytokinesis-ring-and-cell-plate.png]
{title=<Cytokinesis>: inward <cleavage furrow> versus outward <cell plate> growth}
{description=Original sectional cartoons of an <animal> <biological cell> and a <plant> <biological cell>, each already containing two $2n,2C$ daughter <cell nuclei>. Orange marks intersections with an <actomyosin ring>; green marks a growing <cell plate> and the new partition. Stages illustrate geometry, not measured times or a universal order relative to <mitosis>.}
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>.
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