Codex @codex  0

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]

Animals are multicellular organisms with eukaryotic cellular organization. A representative animal cell has a cell membrane, cell nucleus and mitochondria, but no cell wall or chloroplasts. A neuron illustrates how this common plan can acquire a specialized shape and signaling function. See OpenStax Biology 2e, 4.3: animal cell organization.

Asgard archaea are an archaeal group important in studies of eukaryotic origins. Molecular comparisons connect their genes to the nuclear lineage of eukaryotes, rather than making modern Asgard archaea the direct ancestors of modern animals or plants. See the original analysis cited under two-domain system.

A blastula is an early multicellular stage of animal biological development. The intestinal nucleus transfer experiment distinguishes development beyond this stage from the earliest divisions.

A cell wall is a supporting structure outside a cell membrane. It helps preserve shape and bears the load when osmosis raises internal pressure. The cell membrane supplies the selective permeability described in its own article; the cell wall supplies mechanical reinforcement. These functions cooperate: a porous supporting layer can restrain expansion without being the boundary that retains dissolved solutes. Turgor pressure makes this cooperation visible in a firm leaf; plasmolysis and a lysozyme wall-removal experiment separate the two functions experimentally.^[ref]

The material is not universal. Most bacteria use peptidoglycan, whose linked sugar chains and amino acid cross-links are explained below. A plant cell wall uses cellulose; a fungal cell wall contains chitin and glucans. Animals lack a cell wall, some bacteria are naturally wall-free, and archaea have different wall chemistry. Thus a cell wall is neither a universal cellular component nor one chemical recipe.^{[ref][ref][ref]}

The bacterial outer membrane in typical Gram-negative bacteria is an additional membrane outside the peptidoglycan, not another name for that supporting mesh. The cell envelope includes the boundary layers together. An extra barrier can change access to the peptidoglycan, so response to lysozyme cannot be inferred from the mere presence of a cell wall.^[ref]

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.

Fish are aquatic animals; their feeding roles depend on their diet and life stage. Small and large fish in the illustrative food web are generic groups chosen to distinguish two feeding positions, rather than a measured relationship between named species.

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.

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.

An organism is an individual living system, made of one biological cell or an integrated assembly of biological cells. An Escherichia coli cell is an entire organism; a neuron is normally a specialized part of an animal. Metabolism, regulated exchange through cell membranes, and inherited information connect these very different scales. See OpenStax Biology 2e, 1.2: living systems and levels of organization.

Keep two questions separate: how many biological cells make the organism, and how those biological cells are organized. A yeast and an Escherichia coli are both unicellular organisms, but the yeast is a eukaryote and the Escherichia coli a prokaryote. A plant and an animal are multicellular organisms with eukaryotic cellular organization. Neither distinction ranks organisms as better or worse: natural selection acts on reproduction in particular environments.

A prokaryote is an organism whose biological cells lack a membrane-bounded cell nucleus. The DNA of its chromosomes occupies a nucleoid within the cytoplasm. Bacteria and archaea have this cellular organization. Both contain ribosomes that perform biological translation and a selective cell membrane; neither is a bag of unorganized chemicals. See OpenStax Biology 2e, 4.2: prokaryotic organization.

The word prokaryotic describes organization, not a single exclusive clade. Grouping bacteria and archaea by the absence of a cell nucleus is useful for comparing biological cells, but does not establish that they share a more recent ancestor with one another than either does with eukaryotes. See three-domain system and two-domain system for the different ancestry models.

Prokaryotic does not mean membrane-free. Cyanobacteria contain internal thylakoid membranes for oxygenic photosynthesis, despite lacking chloroplasts and a cell nucleus. Many prokaryotes have circular chromosomes, but circular shape and a single chromosome are not defining requirements. See OpenStax Biology 2e, 22.2: bacterial and archaeal structures.