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An actomyosin ring is a contractile assembly containing actin filaments and myosin II. In typical animal cytokinesis, it forms in the cell cortex beneath the cell membrane. Its contraction draws the boundary inward to produce a cleavage furrow; an inward groove is not yet complete separation of the daughters.^[ref]

A kinase involved in mitotic chromosome-attachment error correction and later coordination of cytokinesis, including responses to chromatin bridges.

Partition of one cell into descendant cells. Copying and allocating inherited material must be coordinated with physical separation, but nuclear division and cytokinesis are distinct events.

The cell cortex is a peripheral layer of cytoplasm beneath the cell membrane, containing the actomyosin ring during typical animal cytokinesis.

The cell cycle is the sequence of events by which a biological cell prepares for and completes division, including DNA replication and distribution of chromosomes. In Saccharomyces cerevisiae, changing bud appearance provides an accessible readout; see the Burgess, Powers and Mell account and Hartwell, Culotti and Reid's primary study cited there.

For a typical cycling diploid eukaryotic biological cell, the order is G1 phase, S phase, G2 phase, then M phase. DNA replication doubles nuclear DNA amount during S phase; mitosis distributes the copies, and cytokinesis separates the daughters. The useful distinction is between copying a library and allocating its copies: DNA replication changes how much material exists, whereas division changes which biological cell contains it.

Gene expression continues to supply RNA and proteins as the biological cell grows; it is different from duplicating the genome. In the yeast cross-species complementation experiment, restored growth requires successful repeated cell cycles, but growth alone does not locate a defect within a phase. Bud appearance in Saccharomyces cerevisiae provides another readout, while a flow cytometric DNA-content assay asks how much nuclear DNA is present and DNA pulse labeling asks whether new DNA was synthesized during an exposure.

A cell plate is an internal partition assembled during plant cytokinesis by fusion of transport vesicles. It grows outward to join the parental cell membrane, providing daughter boundaries and material for a new plant cell wall.^[ref]

A cleavage furrow is the inward indentation of the cell membrane during cytokinesis. It is the visible deformation; the actomyosin ring is machinery beneath that boundary.^[ref]

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.

A guanine-nucleotide exchange factor involved in local activation of RhoA during animal cytokinesis.

A membrane-remodelling protein system involved in processes including the final membrane constriction and separation of animal cytokinesis.

Mitosis allocates copied chromosomes to daughter cell nuclei. The spindle apparatus aligns sister chromatids before separation in anaphase.

The copying has already occurred during S phase, before mitosis. Nuclear chromosome segregation and cytokinesis, the partition of the cytoplasm, are distinct processes: two daughter cell nuclei can form inside one undivided biological cell.^[ref]

The original cartoon follows a hypothetical diploid animal biological cell with $2n=4$. Its two homologous chromosome pairs are $A,a$ and $B,b$; upper- and lower-case letters identify homologues, not dominant and recessive alleles. In prophase and metaphase, each replicated chromosome contains two sister chromatids: four replicated chromosomes contain eight chromatids, rather than eight replicated chromosomes. The X shape depicts two associated copies, not the shape of every chromosome throughout the cell cycle.

This open mitosis cartoon shows nuclear envelope breakdown and reassembly. Closed mitosis in some yeasts retains the nuclear envelope while daughter chromosomes separate inside it; the cartoon is therefore not a universal cellular architecture.^[ref]

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.

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 $2C+2C=4C$: 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.

M phase includes mitosis and ordinarily cytokinesis, which can overlap in time.

Myosin II participates in the actomyosin ring of typical animal cytokinesis.^[ref]

ToxB greatly reduces recognizable actin cables and actin patches. Untreated biological cells contain concentrated cortical structures, especially in buds, whereas treated biological cells show only weak diffuse staining even at ten times the exposure. The disappearance concerns actin filament organization and detectable F-actin, not necessarily destruction of the actin protein itself. actin monomer sequestration could leave substantial total actin present while eliminating most visible polymer.

Loss of actin cables should impair directed secretory-vesicle delivery and polarized cell growth; loss of actin patches should impair actin-dependent endocytosis. Disruption of the actomyosin machinery can also compromise cytokinesis. The rounded, poorly polarized appearance is consistent with these effects, but the images do not independently measure secretion, division rate or viability. A vehicle control and equal-exposure images are needed for quantitative fluorescence comparison; a longer exposure is useful for finding residual structures but prevents direct comparison of the displayed brightnesses.

Disrupting the dynamic actin network compromises processes required for sustained proliferation: endocytosis, directed delivery of cell membrane and cell-wall material, maintenance of polarized cell growth and successful cytokinesis. Failure of these actin-dependent functions can cause growth arrest and, under sufficiently damaging conditions, loss of viability. The altered sensitivity of actin mutants supports an actin-related mechanism, with their different assembly properties potentially changing the reserve of functional cytoskeleton.

A clear zone itself establishes failure to form a visible lawn, not necessarily death. biological cells might remain viable but fail to bud or divide while drug is present. Recovery after drug removal and subsequent colony formation would distinguish reversible arrest from killing. The experiment also does not identify which actin-dependent process first becomes limiting; matched measurements of trafficking, budding and division would be needed for that ordering.

Efficient and timely budding requires the normal actin network, but some initial polarized cell growth remains possible when visible actin filaments are greatly depleted. Around 40% of control biological cells have new buds by two hours, rising to nearly 60% by six hours. Drug-treated biological cells show almost no budding at two hours but roughly 15–20% at four to six hours, and the phase images confirm small buds despite loss of normal bright actin structures.

The result therefore supports a large delay and reduction, rather than an absolute block to every initiation event. Polarity cues or localized membrane-fusion machinery could remain spatially organized without normal long actin cables. The images and bud counts do not establish completion of a daughter-cell cycle, normal cytokinesis or indefinite proliferation. Residual actin filaments below detection remain an alternative contributor. This distinction between initial bud emergence and completion of growth was directly examined in a quiescence-exit study.^[ref]