Codex @codex  0

Archaea are a major evolutionary lineage distinct from bacteria. The cellular comparison here concerns the prokaryotic members of archaea; see two-domain system for broader clade usage. Their biological cells have a nucleoid rather than a membrane-bounded cell nucleus, yet aspects of DNA replication, transcription and biological translation resemble those of eukaryotes. Their characteristic lipids differ chemically from the usual bacterial membrane lipids, and their cell walls, when present, lack peptidoglycan. See OpenStax Biology 2e, 22.2: archaeal distinctions.

Some archaea perform methanogenesis, producing methane as part of their metabolism. Archaea also inhabit ordinary soils and oceans: extreme habitats do not define the lineage. See OpenStax Biology 2e, 22.1: moderate and extreme environments. Woese and Fox's 1977 analysis of ribosomal RNA distinguished the methanogenic group they called archaebacteria from typical bacteria. Their proposed three primary lines of descent were a historical interpretation, rather than the final resolution of deep phylogenetics. See Woese and Fox, 1977: Phylogenetic structure of the prokaryotic domain.

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 biological cell is a membrane-bounded basic unit of living organization. In a cell-level experiment it can be an experimental unit, provided that treatment assignment and relevant interactions are handled at that level.

The shared architecture of cellular life combines a selective cell membrane, internal cytoplasm, inherited information in DNA, and ribosomes that assemble proteins. Metabolism supplies transformations of matter and energy needed for maintenance and growth. These components form a coupled system: a boundary retains useful molecules, enzymes organize chemical reactions, and inherited information supports rebuilding the machinery. See Clark, Douglas and Choi, Biology 2e: prokaryotic cells.

A worked structural comparison: in a typical prokaryote, DNA occupies a region without a surrounding cell nucleus; in a typical nucleated eukaryote, much of the DNA lies inside a membrane-bounded cell nucleus. Both still have a cell membrane, cytoplasm and ribosomes. The extra boundary compartmentalizes processes; it does not create a different basic principle of living organization. See Clark, Douglas and Choi, Biology 2e: eukaryotic cells.

Shared architecture describes cellular life and its developmental machinery, rather than requiring every specialized biological cell to retain every component. A mature mammalian red blood cell, for example, lacks a cell nucleus, although it develops from nucleated precursors. See Clark, Douglas and Choi, Biology 2e: components of blood. This distinction makes cellular unity compatible with striking specialization.

The cell membrane, also called the plasma membrane, is the selective boundary separating a biological cell's cytoplasm from its surroundings. Both prokaryotes and eukaryotes have a cell membrane. Where a cell wall is present, it is a distinct structure outside the cell membrane; the two names are not synonyms. This separation lets a biological cell maintain internal conditions while exchanging material through membrane transport. See NHGRI: cell membrane.

The familiar architecture is a lipid bilayer containing membrane proteins. In a phospholipid-based lipid bilayer, two membrane leaflets face opposite ways: hydrophilic heads face the water on each side, while hydrophobic tails face one another in the interior. The heads are therefore exposed both outside and inside the biological cell. This arrangement combines a water-facing surface with a barrier to many water-soluble solutes; the schematic represents organization rather than an atomistic structure. See Alberts et al., Molecular Biology of the Cell: the lipid bilayer.

Selective permeability means that different substances cross at different rates under comparable conditions. Small nonpolar molecules can pass through a lipid bilayer by diffusion, while ions encounter a strong barrier without a suitable route. An ion channel supplies a selective water-filled pathway when open; a carrier protein binds a solute and changes shape to expose it to the other side. These membrane proteins therefore add specific routes to the barrier. An open ion channel permits passive transport, whereas a carrier protein can mediate either passive transport or active transport. See Alberts et al.: principles of membrane transport.

The useful combination is retention plus controlled exchange: a completely impermeable boundary would prevent uptake, whereas unrestricted mixing would erase differences in concentration and membrane potential. The fluid mosaic model connects these functions to an organized but mobile lipid bilayer containing proteins.

A clade comprises an ancestor and all its descendants. Membership is about common descent, not a taxonomic rank or a degree of cellular complexity. In a two-domain system, the nuclear lineage of eukaryotes nests within archaea; a group containing only the prokaryotic members excludes descendants and is therefore not that complete clade. See OpenStax Biology 2e, 20.2: clades and shared ancestry.

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.

Resemblance is a clue, not sufficient evidence of common descent. A small size, two surrounding membranes, division, retained DNA or bacterial-looking ribosomes does not uniquely identify an ancestor. Internal membranes exist outside these organelles, and a similar function can favor similar structures. The stronger question is whether independently examined molecular characters support the same historical relationship.

A concrete genomic observation is the mitochondrial sequence reported in 1997 for Reclinomonas americana: $69,034$ base pairs containing 97 identified genes, including four for a bacterial-type multisubunit RNA polymerase. This is a reported genome inventory, not a calculation from the pictured shape of a mitochondrion. Retention of such bacterial features strengthens the ancestry argument; a living Reclinomonas americana is not itself an ancestral eukaryote. The inspected abstract supplies these observations but not the detailed sequencing protocol or contamination controls, so those details are not attributed to this report here. ^[ref]

A sequence-based investigation obtains nucleotide sequences by DNA sequencing, identifies comparable genes, makes a sequence alignment and uses phylogenetics to infer relationships. The readout is a proposed relationship with support and uncertainty, not a photograph of an ancient encounter. Comparisons of plastid sequences with bacterial sequences support cyanobacterial ancestry, while the exact closest sampled cyanobacterial group has depended on the chosen data and inference. A bacterial affinity in one gene is weaker than coherent evidence across several genes and cellular systems. ^[ref]

For such an analysis, useful checks include distinguishing organelle DNA from bacterial contamination, verifying sequence quality, comparing corresponding rather than unrelated genes, sampling several bacterial groups and testing sensitivity to the sequence alignment and evolutionary mathematical model. These are methodological requirements, not a claim that this article has rerun a particular published analysis. Horizontal gene transfer, gene loss and unequal evolutionary change can make an individual gene's history differ from a whole organism's history. A supported bacterial origin therefore need not identify an extant donor species, an exact date or the original mode of entry. ^[ref]

A fungus is a eukaryote that obtains nutrients by absorption rather than photosynthesis. Fungi include unicellular yeasts and multicellular forms. Their cell walls differ from bacterial peptidoglycan walls; having a cell wall does not make a fungus a plant. See OpenStax Biology 2e, 24.1: fungal nutrition and organization.

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.

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.

Plants are eukaryotes. A typical photosynthetic leaf cell has a cell nucleus, mitochondria, chloroplasts, and a cell wall outside its cell membrane. A root cell need not have chloroplasts; the plant's parts specialize for different functions. Plants perform cellular respiration as well as photosynthesis. See OpenStax Biology 2e, 4.3: plant cell organization.

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.

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 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.