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A model organism is an organism used to investigate biological processes through accessible, repeatable experiments. Its value combines experimental tractability with a carefully delimited relationship to other organisms. For example, a gene deletion in Saccharomyces cerevisiae can test whether a gene is required for growth under specified conditions; adding another gene can test functional complementation. Such interventions support causal inference more directly than merely noticing that a gene and a phenotype occur together. ^[ref]

The striking opportunity is to ask two experimentally separate questions: what does this component do here, and which part of that function survives a change of species or conditions? The yeast cross-species complementation experiment makes both questions concrete. Model organism evidence transfer requires specifying the function being compared; a useful model organism need not reproduce every feature of another organism.

Evidence transfers at a stated level. Sequence alignment can suggest a candidate relationship between genes; functional complementation tests whether a candidate can supply a particular function in a chosen recipient. Neither alone establishes identical native gene expression, identical signal transduction, or identical consequences in every species. The yeast cross-species complementation experiment is especially useful because it tests a conserved function by intervention rather than assuming it from resemblance. ^[ref]

Even successful functional complementation occurs within a specific genetic background, nutrient supply and imposed gene expression. The enzyme may work with the recipient’s available substrates and partner proteins at an experimentally imposed abundance. Establishing the same role in the donor species calls for experiments there; establishing behavior under another condition calls for experiments under that condition. A conserved component can therefore coexist with context-dependent behavior of an entire pathway. This separation makes model organisms powerful: an accessible experiment supplies a precise mechanistic foothold and a better question for the next system.

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.

A Northern blot separates RNA by size and detects selected sequences through hybridization with a probe. Detection of a messenger RNA supports transcription of a gene, but does not alone establish a functional protein or its destination.

Organelle protein import brings externally synthesized proteins into an organelle. Many plastid proteins made from nuclear genes carry targeting information recognized by machinery that transports them across the envelope membranes. This delivery enables nuclear genes to supply functions inside a plastid, but the presence of targeting information is not itself evidence that a gene was acquired through endosymbiotic gene transfer. ^[ref]

A plasmid is a DNA element capable of replication separately from the main chromosomes of its host. Engineered plasmids can carry genes for genetic transformation and functional complementation.

The polymerase chain reaction amplifies a selected DNA region using repeated copying steps. Amplified material can be examined by DNA sequencing; appropriate template controls help distinguish a new rearrangement from material already present in the starting preparation.

The essential trick is to choose two PCR primers on opposite DNA strands with their extendible $3^{\prime }$ ends facing into the same interval. A DNA polymerase extends each PCR primer $5^{\prime }\to 3^{\prime }$; the resulting DNA can itself become a template. The amplicon is therefore selected by two opposed PCR primers, rather than by the DNA polymerase recognizing a named gene. This makes PCR amplification useful for inspecting one interval within a much larger genome. Here PCR means polymerase chain reaction; the statistical abbreviation PCR means principal component regression.^[ref]

For a deliberately tiny nucleotide sequence example, display the upper strand as $5^{\prime }\text{-}\mathrm{ACGT}\text{-}\mathrm{GGAATT}\text{-}\mathrm{CAGC}\text{-}{3}^{\prime }$. The left PCR primer is $5^{\prime }\text{-}\mathrm{ACGT}\text{-}{3}^{\prime }$ and binds the lower strand. The right PCR primer, written in its own $5^{\prime }$-to-$3^{\prime }$ direction, is $5^{\prime }\text{-}\mathrm{GCTG}\text{-}{3}^{\prime }$: the reverse complement of CAGC. It binds the upper strand and extends leftward in the drawing. With flanks outside this interval, the bounded amplicon contains $4+6+4=14$ base pairs, including both PCR primer sequences. These four-nucleotide PCR primers illustrate direction and counting only; they are not a proposed experimental design.

Ribonucleic acid, or RNA, is a nucleotide polymer whose nucleotide sequence can both carry information and organize a working molecular shape. Its backbone contains alternating ribose sugars and phosphate groups connected through covalent bonds. Its usual nucleobases are adenine A, uracil U, guanine G, and cytosine C. Thus RNA uses U where DNA uses T; its ribose also differs chemically from DNA's deoxyribose. See NHGRI: RNA.

In transcription, an enzyme uses one DNA strand as a template for RNA. For example, the DNA template $3^{\prime }\text{-}\mathrm{TACGG}\text{-}{5}^{\prime }$ specifies $5^{\prime }\text{-}\mathrm{AUGCC}\text{-}{3}^{\prime }$ in RNA. The base pairing rules now include A–U rather than A–T, while G–C remains. This RNA matches the other DNA strand's $5^{\prime }$-to-$3^{\prime }$ lettering after T is replaced by U. Identifying the template strand matters: replacing T by U in an arbitrarily chosen DNA strand is not a general method for deriving the RNA product. See Alberts et al., Molecular Biology of the Cell: from DNA to RNA.

RNA is often a single strand, but single-stranded does not mean straight or entirely unpaired. Complementary regions within one RNA molecule can undergo base pairing, bringing distant positions together. Consider the illustrative twelve-nucleotide nucleotide sequence $5^{\prime }\text{-}\mathrm{GCAUUUUUAUGC}\text{-}{3}^{\prime }$. Pair positions 1 with 12, 2 with 11, 3 with 10, and 4 with 9. The resulting stem-loop has four base pairs: G–C, C–G, A–U, U–A. Positions 5–8 remain unpaired in the loop. Both arms belong to the same continuous backbone, and the paired arms have the directions of antiparallel nucleic acid strands.

The diagram proposes a possible pairing pattern, not a prediction that this short RNA must adopt that shape under every condition. Actual RNA folding depends on additional interactions, temperature, ions, and associated proteins; the full three-dimensional arrangement contains more information than the drawn base pairs. The example's purpose is to show how a linear nucleotide sequence can specify contacts within itself.

Different RNA roles make this flexibility useful. Messenger RNA carries a message that a ribosome reads during translation to build a protein. Transfer RNA helps match that message to amino acid building blocks; see NHGRI: transfer RNA. Ribosomal RNA forms structural and catalytic parts of the ribosome. Other RNA participates in gene regulatory networks, changing how strongly genes are expressed. Many RNA molecules therefore function without being translated into proteins; for some genes, functional RNA is the product itself.

Most strikingly, some folded RNA acts as an enzyme: these catalytic RNA molecules are ribozymes. The ribosome's bond-forming active site is built from RNA, although the complete ribosome also contains proteins. Structural evidence appears in Nissen et al., The structural basis of ribosome activity in peptide bond synthesis, Science 289, 920–930 (2000). RNA therefore connects a readable record with physical recognition, regulation, and catalysis: the same kind of polymer can be a message, an adaptor, or part of molecular machinery.

A ribosome is the molecular machinery for biological translation: it reads messenger RNA and joins amino acids into a polypeptide that can become a protein. Both prokaryotes and eukaryotes have ribosomes. A ribosome consists of ribosomal RNA and proteins organized into small and large subunits, with no enclosing membrane. Ribosomal RNA contributes to both structure and the active site that forms peptide bonds; a ribosome is therefore not merely a collection of proteins. See Clark, Douglas and Choi, Biology 2e, 4.3: ribosome composition and Nissen et al., The structural basis of ribosome activity in peptide bond synthesis, Science 289, 920–930 (2000).

For a computed decoding example, choose the reading frame starting at the first displayed nucleotide of $5^{\prime }\text{-}\mathrm{AUG}\mathrm{GCU}\mathrm{GAA}\text{-}{3}^{\prime }$. Under the standard genetic code, AUG specifies methionine (Met), GCU specifies alanine (Ala), and GAA specifies a glutamic acid residue (Glu) in the growing polypeptide. Nine nucleotides divide into three codons, giving the ordered three-amino acid residue segment Met–Ala–Glu. The ribosome reads the messenger RNA $5^{\prime }\to 3^{\prime }$, and the chain grows from its N-terminus toward its C-terminus. If synthesis begins at this AUG in an appropriate eukaryotic initiation context, these are the first three amino acid residues before any subsequent processing. See Clark, Douglas and Choi, Biology 2e, 15.1: standard codon assignments.

For this three-amino acid residue segment, joining the three delivered amino acids requires two peptide bonds, one between Met and Ala and another between Ala and Glu. There is no stop codon in the displayed nucleotide sequence, so this is a growing segment rather than a complete released protein. An isolated nine-nucleotide RNA is not claimed to supply all signals needed for initiation. This small calculation separates three jobs: the messenger RNA specifies order, transfer RNA delivers the amino acids, and the ribosome catalyzes their joining. Each new amino acid residue is added at the C-terminus. See Clark, Douglas and Choi, Biology 2e, 15.5: initiation and chain elongation.

The beauty of a ribosome is its reusable chemistry: the same machinery can assemble many different proteins by following different messenger RNA templates. The genetic code connects an alphabet of nucleotides to an alphabet of amino acids, while transfer RNA links recognition of the message to delivery of building material. Changing the message can therefore change the product without building a different ribosome for every protein.

In 1955 George E. Palade described small particles in the cytoplasm, both free and associated with membranes, in A small particulate component of the cytoplasm, Journal of Biophysical and Biochemical Cytology 1, 59–68. The paper documents the structural observations, rather than the full modern decoding mechanism. Complementary biochemical work by Hoagland, Stephenson, Scott, Hecht and Zamecnik described a soluble RNA intermediate in protein synthesis in 1958, helping establish the role now assigned to transfer RNA; see A soluble ribonucleic acid intermediate in protein synthesis, Journal of Biological Chemistry 231, 241–257. Together, structural observations and biochemical experiments connected visible cellular machinery with its working parts.

Saccharomyces cerevisiae is a particular species of yeast, used in baking, brewing and laboratory research. During ordinary vegetative growth it reproduces by budding: a smaller daughter grows from the mother before separation. Laboratory strains can be maintained as either haploid or diploid biological cells. This makes it possible to investigate genes in a single eukaryote, then use controlled crosses and meiosis to connect a genotype to a phenotype. See Burgess, Powers and Mell, Budding Yeast Saccharomyces cerevisiae as a Model Genetic Organism (2017), pp. 1–2.

The practical value of Saccharomyces cerevisiae as a model organism is the combination of easy laboratory cultivation, accessible genetics, and cellular functions shared with other eukaryotes. A haploid strain has one set of chromosomes, so a changed allele is not accompanied by a second copy at that genetic locus. But a mutation need not produce an observable phenotype under every condition. The yeast is also a whole organism, not a miniature human: transferring a result requires evidence that the relevant function is shared in the target system. See Duina, Miller and Keeney, Budding Yeast for Budding Geneticists (2014): cultivation, genetic tools and conserved functions.

For a concrete example, Hartwell, Culotti and Reid's 1970 work followed temperature-sensitive Saccharomyces cerevisiae strains with successive microscope photographs. Failure to complete a particular part of the cell cycle helped identify required gene functions. Their distinction between when a function is required and where a cell finally stops is especially instructive: the visible endpoint need not be the time of the underlying defect. See Hartwell, Culotti and Reid, Genetic control of the cell-division cycle in yeast. I. Detection of mutants (1970): primary abstract. This is a useful way to turn an observable change in a biological cell into a question about genes and causal order.

In alcoholic fermentation, Saccharomyces cerevisiae converts sugars into ethanol and carbon dioxide. The carbon dioxide helps dough rise; the ethanol is useful in alcoholic drinks. The metabolism also supports production of ATP, a usable cellular energy carrier. This is one metabolic ability of one species, not the definition of every yeast. See Clark, Douglas and Choi, Biology 2e, 7.5: alcohol fermentation and the Burgess, Powers and Mell account above.

For glucose, the ideal net stoichiometry isOne molecule of glucose supplies six carbon chemical atoms: four end up in the two ethanol molecules and two in the two carbon dioxide molecules. The twelve hydrogen and six oxygen chemical atoms also balance. This is a net material balance across several enzyme-catalyzed steps, not a single chemical reaction step or the complete bookkeeping for cellular energy carriers. The standard pathway accounting yields a net two ATP per glucose: the initial breakdown produces the ATP, while the later conversion to ethanol regenerates a coenzyme needed to keep the breakdown going. See Clark, Douglas and Choi, Biology 2e, 7.2: net ATP accounting and the section 7.5 cited above.

At complete conversion, this toy gives $20\mathrm{mmol}$ of ethanol and $20\mathrm{mmol}$ of carbon dioxide. The carbon balance is $6\times 10=2\times 20+1\times 20=60\mathrm{mmol}$ of carbon chemical atoms. A real culture also builds cellular material, can use other metabolic routes, and may retain dissolved carbon dioxide; the toy is a reference balance rather than a prediction of collected gas or beverage yield.

Although alcoholic fermentation does not require molecular oxygen as a reactant, the presence of molecular oxygen does not force Saccharomyces cerevisiae to stop fermenting. De Deken's 1966 experiments reported predominantly fermentative glucose use during exponential growth in air. Consequently, bubbles do not establish either absence of molecular oxygen or an exclusively fermentative metabolism. See De Deken, The Crabtree Effect: A Regulatory System in Yeast (1966): primary abstract.

Schizosaccharomyces pombe is a yeast species used as a model organism. It supplies the candidate genes in the yeast cross-species complementation experiment; the recipient remains Saccharomyces cerevisiae.

Species delimitation infers the boundaries and number of species from evidence. No single arbitrary percentage of nucleotide sequence difference is a universal boundary. Divergence within an old, widespread species can exceed divergence between recently separated species; geography and multiple kinds of evidence matter. ^[ref]

An original counterexample to a universal percentage rule: an assay could examine a strongly conserved gene in one comparison and a rapidly changing nucleotide sequence in another. Equal percentages then answer different historical questions. Even the denominator needs specifying: all sites, aligned sites, or only a filtered region? A threshold is an operational rule for a stated assay and group of organisms, whose errors need evaluating. It is not a definition that automatically transfers to every species.

The diagram's hypothetical pair B has similar morphology but reduced mating and distinct ancestry evidence. A morphological species concept could miss that distinction. Pair C instead has different morphology but compatible mating and ancestry that fails to separate the samples. That combination calls for checking variation in phenotype within a species. Pair A has only a structural difference observed: the other criteria remain untested. These examples show why missing evidence, disagreeing evidence and agreeing evidence require different interpretations.

Terminal differentiation is advanced specialization at the end of a developmental lineage. It often includes withdrawal from the cell cycle; the term does not mean that every nuclear gene needed for another cell fate has been deleted. A mature mammalian red blood cell is an especially restrictive example because it lacks a cell nucleus; the intestinal nucleus transfer experiment used nucleated donors.

Transcription is synthesis of RNA using one DNA strand as a template by an RNA polymerase. During elongation the RNA polymerase reads that template $3^{\prime }\to 5^{\prime }$ while extending RNA $5^{\prime }\to 3^{\prime }$. Only the selected region is transcribed; other regions need not be active, and the template strand can differ between genes. See Alberts et al.: RNA polymerase and selection of transcribed DNA regions.

For the displayed example, align these three nucleotide sequences in the same left-to-right positions:The first template triplet TAC selects AUG: T pairs with A, A with U, and C with G. The next CGA selects GCU, and ATT selects UAA. The coding strand therefore matches the RNA lettering except for T versus U, whereas the template is complementary. Reading that template in its own $5^{\prime }\to 3^{\prime }$ direction gives $5^{\prime }\text{-}\mathrm{TTAAGCCAT}\text{-}{3}^{\prime }$; orientation cannot be discarded. The gene expression diagram keeps all three rows aligned. See Clark, Douglas and Choi, Biology 2e: template and coding strands in transcription.

This correspondence makes nucleotide sequences interpretable, but replacing T by U is valid only after identifying the coding strand. Moreover, RNA may be processed after transcription, so the initial product and mature messenger RNA need not have identical uninterrupted nucleotide sequences. Transcription also makes functional RNA that will not undergo biological translation. See Alberts et al.: RNA processing and RNA as the final gene product.

The two-domain system recognizes bacteria and archaea as the two primary lineages, placing the nuclear ancestry of eukaryotes within archaea. In this model, archaea as a complete clade includes that eukaryotic lineage; restricting the name to prokaryotic members describes a different grouping. Williams and colleagues' study, published online in 2019 and in a 2020 journal issue, found support for this model and a close relationship to Asgard archaea. See Williams et al., 2020: Phylogenomics provides robust support for a two-domains tree of life.

This updates the interpretation of the 1977 and 1990 classifications rather than undoing their discovery of deep differences between bacteria and archaea. Deep phylogenetic trees depend on the sampled genes, organisms and inference model; the precise closest archaeal relatives and early sequence of events require further evidence. Bacterial ancestry of mitochondria through endosymbiosis is a separate part of eukaryotic history.