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In mitotic anaphase, sister chromatids separate into daughter chromosomes moving toward opposite poles.

Release of cohesin-mediated association permits the sister centromere regions to separate. Each former chromatid is then counted as an independent daughter chromosome. This changes the counting convention without making additional DNA copies. For the $2n=4$ example, the whole undivided biological cell transiently contains eight daughter chromosomes, four moving toward each pole. Movement toward poles and separation of the poles both contribute to partition in the illustrated animal setting.^[ref]

The anaphase-promoting complex is a regulated ubiquitin ligase that uses regulatory cofactors to target selected cell cycle proteins for destruction. Degradation of mitotic cyclins helps reset kinase activity during exit, while other substrates coordinate chromosome separation.

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.

Let $n$ count chromosomes in one complete set, and let $C$ denote the nuclear DNA amount in one unreplicated haploid set of this organism. Ploidy counts sets, whereas $C$ units count material; $C$ is not a universal mass shared by all species. Here we count a paired replicated chromosome as one until its sister chromatids separate. A copied diploid genome is therefore still diploid through S phase: $2n$ persists even as $2C$ becomes $4C$.

An original bookkeeping example uses a hypothetical diploid biological cell with $n=3$. Name its three homologous chromosome pairs $A,a$, $B,b$ and $D,d$. Assume ordinary nuclear DNA replication, no chromosome loss, and one continuous double-stranded DNA molecule per unreplicated chromosome or replicated chromatid. Count nuclear material only, excluding mitochondrion DNA.

For example, $A$ becomes the paired copies $A_1,A_2$ during S phase. They remain one replicated chromosome until separation. Each completed daughter receives one $A$ copy and one $a$ copy, and similarly for the other two pairs. Across both daughters, $2\times 6=12$ double-stranded DNA molecules and $2(2C)=4C$: division allocates the copied material without another round of DNA replication.

The whole undivided anaphase biological cell transiently contains $4n$ separately counted daughter chromosomes. This counting change does not establish a four-set ploidy in a completed daughter genome: each pole receives $2n$, and each completed daughter cell nucleus is diploid. Always state whether a number refers to one cell nucleus, one pole, or the entire undivided biological cell. Compare meiotic DNA content: after meiosis I, a haploid cell nucleus can contain $2C$, so amount alone cannot determine ploidy.

The overlapping spindle microtubule array assembled between segregating chromosome masses during anaphase, contributing to cytokinetic organisation.

A centromere is the chromosome region where a kinetochore assembles.^[ref] Following DNA replication, both sister chromatids have their own copied centromere regions; the counting convention groups the attached sisters as one replicated chromosome. Cohesin maintains sister association: a centromere is not an extra DNA bridge joining two separate DNA molecules.^[ref]

A chromatid is one of the two copies of a replicated chromosome. Each homologous chromosome contributes two sister chromatids before separation in meiosis; chromosomal crossovers can exchange segments between nonsister chromatids.

// Target: biology.bigb

A change in the organization of chromosome segments. A balanced parental arrangement can preserve sequence dosage while causing abnormal segregation in a heterozygous hybrid.

A chromosome carries genes and other genetic material. A haplotype records the alleles carried on one chromosome at specified genetic loci.

In a typical eukaryotic cell nucleus, an unreplicated chromosome comprises one long double-stranded DNA molecule with associated proteins; a replicated chromosome comprises two associated sister chromatids. It need not be X-shaped: that familiar outline depicts a condensed, replicated state.^[ref]

For these ordinary centromere-bearing chromosomes, count by centromere units before separation: the paired sister centromere regions count together as one replicated chromosome. Once sister chromatids separate in anaphase of mitosis or meiosis II, each becomes a daughter chromosome. Thus the undivided anaphase biological cell in the $2n=6$ mitotic example transiently contains twelve daughter chromosomes, but each pole receives six and each completed daughter cell nucleus remains diploid. State the counting compartment and stage; summing both poles is not the ploidy of one daughter.

Physical discontinuity of a chromosome. Fluorescent locus splitting or undercondensation can suggest vulnerability but needs additional evidence before being equated with a break.

The compaction and organisation of chromosomal material during division. Condensation helps chromosome handling but is distinct from the removal of sister-chromatid cohesion.

A joined chromosome structure produced by some recombination events and requiring resolution before successful bacterial chromosome allocation.

Chromosome segregation allocates chromosomes to the products of division. Mitosis separates sister chromatids, preserving ploidy in completed daughter cell nuclei. Meiosis I separates homologous chromosomes with their sister chromatids still associated, reducing diploid to haploid ploidy; meiosis II separates the sister chromatids. Only the first of the two divisions reduces the number of complete sets.^[ref]

For example, correct chromosome segregation of the stipulated three homologous chromosome pairs gives each meiosis I product one $A$, one $B$ and one $D$, each replicated. Giving one product both $A$ members and the other none would preserve the total DNA across products while failing to give each a complete set. Conservation of material alone does not prove correct chromosome segregation.

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.

Inheritance of genetic material outside the nuclear chromosomes, often through only one parent. Its transmission interests can differ from those of biparentally inherited elements.

Diploid biological cells have two sets of chromosomes. In the ordinary one-locus inheritance model, a diploid genotype therefore contains two allele copies at each genetic locus.

Two homologous chromosomes of a given type are distinct inherited members of the two sets; the two sister chromatids of one replicated chromosome are its copied members. In the stipulated $2n=6$ example, a diploid G2 phase cell nucleus has six replicated chromosomes, not twelve sets or twelve independently counted chromosomes.

A topoisomerase that transiently breaks both strands of one DNA duplex to permit another segment to pass, contributing to resolution of replicated chromosome interlinks.

In the common angiosperm arrangement, one sperm fuses with the ovum to form a diploid zygote; another fuses with the central biological cell's two polar cell nuclei, initiating endosperm with three chromosome sets. The zygote develops into a plant embryo, and the ovule develops into a seed.^[ref]

For an original counting example, stipulate a plant with $2n=14$ chromosomes and the common two-polar-nucleus arrangement. Each haploid sperm and ovum contributes $n=7$. The zygote therefore has $7+7=14$ chromosomes, while the initiating endosperm cell nucleus has $7+7+7=21$. The maternal seed coat remains derived from $2n=14$ plant tissue; the whole seed has no single uniform chromosome count. This is bookkeeping for a stipulated example, not identification of a particular species. It follows by counting the chromosome sets entering each fusion, rather than assigning one count to everything enclosed by a seed coat.

A genetic element maintained separately from host chromosomes in an episomal state. Copying, nuclear retention and partition between daughter biological cells are separate requirements for stable maintenance.

Escherichia coli, often abbreviated E. coli, is a bacterial species widely used to study DNA replication and gene expression. Its chromosome is in a nucleoid, and its ribosomes work in the cytoplasm, outside any cell nucleus. A result from one laboratory strain should not automatically be generalized to every member of the species.

The original 1997 report by Blattner and colleagues presented the K-12 MG1655 genome sequence as a circular chromosome of $4,639,221$ base pairs. See Blattner laboratory: original K-12 sequence and 1997 paper. As a bookkeeping example, one complete round of DNA replication from one such chromosome produces two daughter sequence copies totaling $2\times 4,639,221=9,278,442$ base pairs. This is twice the copied material, not twice the length of the reference genome; it does not specify the instantaneous DNA content of a rapidly growing E. coli cell.