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Cytokinesis Created 2026-10-14 Updated 2026-10-18

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.

Counting boundary	Before cytokinesis	After cytokinesis
Biological cells	1	2
Cell nuclei in each biological cell	2	1
Each cell nucleus	$2 n = 10$ chromosomes, $2 C$	$2 n = 10$ chromosomes, $2 C$
Nuclear DNA per biological cell	$4 C$	$2 C$
Nuclear DNA across all descendants	$4 C$	$4 C$

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

2 C + 2 C = 4 C

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.

Figure 1.
Cytokinesis: inward cleavage furrow versus outward cell plate growth
. Original sectional cartoons of an animal biological cell and a plant biological cell, each already containing two $2 n, 2 C$ 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. **Details**.

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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Mitotic daughter chromosome accounting 2026-10-14

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Count the same hypothetical diploid animal biological cell in three ways: chromosomes, double-stranded DNA molecules, and nuclear DNA amount. Assume one continuous double-stranded DNA molecule per unreplicated chromosome or replicated chromatid, complete S phase, and no losses or mutations. Exclude mitochondrion DNA. Here

n = 2

is the size of one haploid set, and

C

is its unreplicated nuclear DNA amount, as defined in cell-cycle DNA content. A double-stranded DNA molecule counts as one, despite containing two strands.

Stage and compartment	Chromosomes	Double-stranded DNA molecules	Nuclear DNA amount
One G1 phase parent	4 unreplicated	4	$2 C$
One prophase or metaphase biological cell	4 replicated	8 in 4 pairs of sister chromatids	$4 C$
Whole undivided anaphase biological cell	8 separated daughter chromosomes	8	$4 C$ total
One anaphase pole's allocation	4 daughter chromosomes	4	$2 C$ allocated
Each daughter after mitosis and cytokinesis	4 unreplicated	4	$2 C$ each

Figure 1.
Four replicated chromosomes, eight DNA molecules, then four of each per daughter
. Original accounting diagram for $2 n = 4$ , counting nuclear DNA only. Each colored stroke represents one double-stranded DNA molecule; the small bridge in a replicated pair indicates association, not a new DNA molecule. The eight separate strokes in the whole anaphase biological cell are already partitioning into two groups. The final row states counts per daughter, not their sum. **Details**.

The allocation can be checked copy by copy. DNA replication turns

A

into

A_{1}, A_{2}

a

into

a_{1}, a_{2}

, and similarly for

B, b

. One daughter receives

A_{1}, a_{1}, B_{1}, b_{1}

and the other

A_{2}, a_{2}, B_{2}, b_{2}

. Each therefore inherits the same four chromosome identities and the same gene and allele content as the unreplicated parent, under the stated assumptions. Across both daughters there are

4 + 4 = 8

DNA molecules and

2 C + 2 C = 4 C

: partition conserves the material present after S phase.

The transient whole-cell count of eight does not mean that each daughter's ploidy has doubled. Each completed daughter is diploid, containing both homologues of each homologous chromosome pair. This differs from separating homologous chromosomes during meiosis I. Faithful mitosis preserves a nuclear genome through growth and renewal; equal inherited genes need not imply equal gene expression or equal cell fate. Mutations or chromosome segregation errors can break the ideal inheritance pattern.

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