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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.

DNA repair addresses damage to DNA. Incorporation of a labeled precursor during DNA repair need not indicate duplication of the whole genome in S phase.

A eukaryote is an organism belonging to the lineage characterized by a membrane-bounded cell nucleus and extensive cellular compartmentalization. A nuclear envelope surrounds the nuclear genome; nuclear chromosomes are typically linear. Ribosomes perform biological translation outside the cell nucleus. See OpenStax Microbiology, 3.4: nuclear organization and protein synthesis. Specialized biological cells can lose their cell nucleus, as mature mammalian red blood cells do, while remaining parts of eukaryotic organisms; see OpenStax Biology 2e, 40.2: mature red blood cells.

Yeasts, plants and animals are all eukaryotes. Typical cells in these groups have mitochondria, but chloroplasts belong to photosynthetic plant and algae lineages, not to every eukaryote. A cell wall occurs in typical plant and fungus cells but is absent from animal cells. These are comparisons of representative cells, not a checklist every specialized cell must satisfy. See OpenStax Biology 2e, 4.3: nucleus, organelles and representative cells.

The interesting feature is controlled separation: cell membranes and organelle membranes let different cellular processes occupy distinct compartments while membrane transport connects them. The evolution of eukaryotes also involved partnership: mitochondria descend from a bacterial endosymbiosis. Modern phylogenetics supports an archaeal ancestry for the nuclear lineage, alongside this bacterial contribution; see two-domain system. Thus the history of a whole eukaryote cannot be read as the history of just one of its genes.

A gene deletion removes a chosen gene sequence from a genome. An experimental gene deletion tests the consequences of that removal in a specified genetic background and environment; lack of an observed phenotype does not prove that the gene has no function under other conditions.

A genome is the complete set of genetic information under discussion. For a cellular organism it is encoded in DNA, including genes and other sequences. Specify whether a comparison concerns a nuclear genome, a mitochondrion's genome or all cellular genetic material: the cell nucleus does not contain every DNA sequence in a typical eukaryote. See NHGRI: genome.

A mitochondrion is an organelle whose inner membrane houses machinery for much of aerobic cellular respiration and ATP production in typical eukaryotic cells. Mitochondria descend from a bacterial endosymbiosis and retain a genome and ribosomes, although many required proteins are encoded by nuclear genes. This combination connects compartmentalized function to evolution. See OpenStax Biology 2e, 23.1: mitochondrial ancestry.

A multicellular organism has a body of multiple biological cells whose growth and activities are integrated into the life of the whole organism. Extensive cell differentiation is common in complex examples, but specialized biological cells are not a prerequisite for every simple form. Ask how the body develops, how its parts cooperate and how it produces descendants, rather than counting attached biological cells alone. The boundary with a biological colony depends on the degree of integration and the criteria being investigated; there is no universal cell-count threshold. See the Queller and Strassmann account cited under unicellular organism.

Specialization makes the whole capable of combinations that one general-purpose biological cell cannot achieve in the same way. In an animal, a neuron transmits signals while a red blood cell carries oxygen; each still requires a working cell membrane and metabolism. A mature mammalian red blood cell lacks a cell nucleus and mitochondria, illustrating how a useful part need not retain the equipment of a self-reproducing organism. See Clark, Douglas and Choi, Biology 2e, 40.2: red blood cell specialization. Cell differentiation generally involves different gene expression, rather than a different inherited genome for each function. Biological development must coordinate growth, position, interactions and specialization: increasing the number of biological cells is only one component. See Alberts et al., Molecular Biology of the Cell, chapter 21 (2002): construction of multicellular bodies.

A motivating experiment: select for a property of groups. Ratcliff, Denison, Borrello and Travisano's 2012 study began with Saccharomyces cerevisiae growing as unicellular organisms. Daily transfer favored rapidly settling material. Their initial protocol allowed a $10\mathrm{mL}$ culture to settle for $45\min$, discarded the upper $9.9\mathrm{mL}$, and transferred the bottom $100\mu \mathrm{L}$ into fresh medium. After seven days, they instead sampled $1.5\mathrm{mL}$ and centrifuged it at $100$ times gravitational acceleration for $10\mathrm{s}$, again transferring the bottom $100\mu \mathrm{L}$. Faster settling made it more likely that a lineage would seed tomorrow's culture; the procedure selected a physical outcome without specifying how it must arise. These protocol details are explicitly recounted in Gulli, Herron and Ratcliff, Evolution of altruistic cooperation among nascent multicellular organisms (2019): original settling protocol.

The primary 2012 paper reports snowflake yeast dominating all ten initial replicate populations by 60 transfers. To distinguish the joining of previously separate biological cells from clonal development, the researchers enzymatically separated clusters and followed individual biological cells for $16\mathrm{h}$: attached descendants rebuilt clusters, with no such joining observed. Staining localized attachments at bud scars. Time-lapse observations also showed clusters releasing smaller multicellular offspring, called propagules; small clusters grew before they reproduced. See Ratcliff et al., Experimental evolution of multicellularity (2012), figures 1–3: emergence, development and reproduction.

The experimental comparisons ask different questions. Competition with the unicellular organism ancestor showed a reported $34\%$ advantage under settling selection. Without settling selection, the estimated $10\%$ disadvantage had a one-sided $P=0.06$, so it was suggestive rather than decisive evidence of a cost. Separately, three populations derived from one evolved isolate retained the cluster form through 35 transfers without settling selection. That persistence tests stability, not whether clusters always outperform single biological cells. See Ratcliff et al. (2012), figure 2 and the cluster-formation results cited above.

A 2015 follow-up added a causal test: deleting the gene ACE2 in the unicellular organism ancestor produced snowflake yeast, while supplying functional ACE2 to one evolved isolate restored separation into single biological cells. Sequence changes altering the encoded protein were found in ACE2 in five of ten examined lineages; the study did not identify that same sequence-change mechanism in every lineage. The deletion and restoration comparisons support a mechanism beyond the observation that clusters settle quickly. See Ratcliff, Fankhauser, Rogers, Greig and Travisano, Origins of multicellular evolvability in snowflake yeast (2015), figure 1.

What follows, and what does not. The evidence supports inherited cluster formation, clonal development and a reproducing collective whose properties can respond to natural selection. Calling these simple multicellular organisms recognizes those demonstrated properties; it does not turn every clump into an animal or establish animal-like cell differentiation. This laboratory route also does not reconstruct the actual ancestry of animals or show that complex developmental integration must follow. The interesting achievement is narrower and testable: a change in how descendants remain connected creates a new scale at which growth and reproductive success can be measured.

For the toy figure, let $p$ be the fraction of tracked units of type C before transfer and let $s_C,s_U$ be the respective probabilities of transfer for C and U. Assume equally weighted units and equal multiplication between transfers. When overall transfer probability is nonzero, conditioning on transfer givesWith $p=1/5$, $s_C=4/5$ and $s_U=1/10$, the numerator is $4/25$ and the denominator $6/25$, giving $p_{\mathrm{after}}=2/3$. If both types have the same nonzero transfer probability, the fraction stays $1/5$. This is conditional probability, not a model of the full experimental life cycle: real cluster sizes, growth and fragmentation affect which quantity a frequency measures. In particular, a fraction of clusters is not automatically a fraction of biological cells.

Reclinomonas americana is a microbial eukaryote whose mitochondrial genome retains an unusually rich set of bacterial features. The 1997 sequence report provides a concrete observation under evidence for bacterial ancestry of organelles.

A species is a named unit of biological classification. The useful criteria vary among groups: reproductive compatibility can inform classification in many animals, while microbial comparisons also use genomes and other characters. Escherichia coli is a species; K-12 MG1655 identifies a particular strain within it. See OpenStax Biology 2e, 18.2: species concepts.