eukaryote

[EU- + Greek karuōtos, having nuts (from karuon, nut).]

All living organisms are composed of cells. A eukaryotic cell is a cell with a nucleus, which contains the cell's chromosomes. Plants, animals, protists, and fungi have eukaryotic cells, unlike the Eubacteria and Archaea, whose cells do not have nuclei and are therefore termed prokaryotic. In addition to having a nucleus, eukaryotic cells differ from prokaryotic cells in being larger and much more structurally and functionally complex. Eukaryotic cells contain subcompartments called organelles, which carry out specialized reactions within their boundaries. A eukaryotic cell may be an individual organism, such as the amoeba, or a highly specialized part of a multicellular organism, such as a neuron.

Physical Characteristics

A typical eukaryotic cell is about 25 micrometers in diameter, but this average hides a large range of sizes. The smallest cell is a type of green algae, Ostreococcus tauri, with a diameter of only 0.8 micrometers, about the size of a typical bacterium. The human sperm is about 4 micrometers wide, but 40 micrometers long, while the egg is about 100 micrometers in diameter. Single neurons can be a meter or more in length. While schematic diagrams often picture cells as simple cubes or spheres, most cells have highly individual shapes. Human red blood cells are flattened disks indented on either side; muscle cells are highly elongated; neurons are long and thin with many branches on each end; and white blood cells constantly change their shapes as they crawl through the body.

Cells are also often depicted as a bag of fluid with a smattering of structures within, but this is far from the truth. Instead, the interior of the cell is a dense network of structural proteins, collectively termed the cytoskeleton, within which is embedded a large collection of organelles. The material within the cell except for the nucleus is called the cytoplasm. The nonorganelle portion of the cytoplasm is called the cytosol. The consistency of the cytoplasm is much like egg white, and not at all like freely flowing water.

Membranes

Eukaryotic cells include large amounts of membrane, which enclose the cell itself and surround each of the organelles. The membrane surrounding the cell is termed the plasma membrane. Membranes are bilayered structures, made of two layers of phospholipid molecules, built from phosphoric acids and fatty acids. One end of the phospholipid molecules (the exterior head) is hydrophilic, and it is oriented to the outer side of the membrane; the other end (the interior tails) are hydrophobic. Despite this, water molecules can pass freely through the bilayer, as can oxygen and carbon dioxide. Ions such as sodium or chloride cannot pass through, however, and neither can larger molecules such as sugars or amino acids. Instead, these materials must pass through the membrane via specialized proteins. This selective permeability allows the membrane to control the flow of materials in and out of the cell and its organelles.

Proteins and Membrane Transport

Proteins are long chains of amino acids. They have unique shapes and chemical properties that dictate their diverse functions. Proteins govern the range of materials that enter and leave the cell, relay signals from the environment to the interior, and participate in many metabolic reactions, harvesting or harnessing energy to transform raw materials into the molecules needed by the cell for growth, repair, or other functions. Cytoskeleton proteins give the cell its structure. Approximately half the weight of a membrane is due to the proteins embedded in it. Proteins give each organelle, and the cell as a whole, its unique character.

As noted, ions cannot pass freely through the cell's phospholipid membrane. Instead, most ions flow through special channels built from multiple protein subunits that together form a pore from one side of the membrane to the other. Some channels are gated, fitted with proteins that act as hinged doors, blocking the opening until stimulated to swing out of the way. Neurons, for instance, have gated sodium channels that open to allow an electrical impulse to pass and then close to recharge the cell for another firing. Molecules can also cross the membrane attached to protein pumps that are powered by ATP. Transport of scarce molecules such as sugars can also be powered indirectly, by coupling their movement to the flow of another substance. In addition to traversing the membrane directly, water passes through special channels formed by a protein called aquaporin.

Signal Transduction

Proteins, including membrane proteins, also play critical roles in signal transduction, or relay. Signals can include hormones, ions, environmental changes such as odors or light, or mechanical disturbances such as stretching. A hormone is a small molecule released by one cell in the body to influence the behavior of another. A hormone exerts its influence by binding to a protein receptor in the target cell either on the membrane or within the cytoplasm. Cells that do not make receptors for a particular hormone are not susceptible to its effects. Adrenaline and testosterone are examples of hormones that illustrate two major modes of hormone action.

Adrenaline binds to a membrane-spanning receptor that projects both to the outside and the inside of the cell. The binding of adrenaline to the exterior portion changes the shape of the receptor, which in turn sets in motion other changes within the cell. The result is the production of a molecule called cyclic AMP (adenosine monophosphate), another form of the adenosine nucleotide. This "second messenger" binds to a variety of enzymes within the cell, activating them and leading to production of a variety of products. The exact set of enzymes turned on by cyclic AMP and the exact set of consequences depend on the particular cell. Kidney cells, for instance, increase their permeability to water, while liver cells release sugar into the bloodstream. The unique set of proteins within each cell is determined by the genes it has expressed, which in turn is determined by its own history and the hormones and other influences to which it has been exposed.

Testosterone's effects come on more slowly than adrenaline's, but last much longer. Testosterone passes through the plasma membrane and binds to a receptor in the cytosol. Once this occurs, the receptor-hormone complex is transported to the nucleus. Here, it binds to DNA, altering the rate of gene expression for a wide variety of genes. Thus, testosterone acts as a transcription factor. The prolonged action of testosterone is in part because it stimulates the production of new, long-lasting proteins that alter the cell's function for much longer than the very rapid and short-lived effects of adrenaline.

Cells continually respond to signals, and they influence other cells through the signals they release. Signaling pathways within the cell control the rate of cell division, the development and differentiation of the cell, the secretion of proteins and other molecules, and the response to injury, among many other reactions.

Metabolism

Metabolism refers to the entire set of reactions within the cell. Most reactions can be classified as either anabolic or catabolic. Anabolic reactions use stored energy to build more complex molecules from simpler ones. Protein synthesis is an example. Catabolic reactions break down complex molecules to simpler ones, releasing energy in the process that may be harvested and stored by the cell. Glucose breakdown is an example.

The energy transfer in each type of reaction almost always involves the interconversion of ATP and ADP (adenosine diphosphate). Energy is released when ATP loses a phosphate to become ADP, while energy is required to make ATP from ADP and phosphate. ATP can also be converted to AMP by the loss of two phosphates. This reaction, which releases even more energy, is used in replication of DNA and synthesis of RNA (transcription).

Mitochondrion

Glucose breakdown begins in the cytosol, but the majority of the process occurs in the mitochondrion, the energy-harvesting organelle of the cell. In addition to participating in the breakdown of glucose (and making ATP in the process), the mitochondrion is also involved in breaking down fats and amino acids. All these fuels are processed in two major steps, termed the Krebs cycle and the electron transport chain. In the Krebs cycle, the carbon skeletons are broken apart to make CO₂, while the hydrogen atoms are removed on special nucleotide carriers. In the electron transport chain, the hydrogens are stripped of their energy in a series of steps to make ATP, and in the end are reacted with oxygen to form water. The mitochondrion consumes virtually all the oxygen used by the cell. The mitochondrion also participates in many anabolic reactions, using the intermediates of the Krebs cycle as a source of carbon skeletons for creating and modifying nucleotides, amino acids, and other building blocks of the cell.

The mitochondrion is the descendant of a once free-living bacterium that took up residence inside an ancient cell, probably to take advantage of high-energy molecules the host could not metabolize. Mitochondria retain their own DNA on their own bacteria-like chromosome, although over time most of the original mitochondrion's genes were transferred to the host and now reside in the nucleus.

Chloroplast

The cells of plants and some protists possess chloroplasts, whose green chlorophyll gives plant leaves their characteristic color. Embedded in an internal membrane, chlorophyll absorbs sunlight and funnels it to a complex set of proteins nearby. Light energy is used to split water into oxygen (released as a waste product) and hydrogen, which is attached to nucleotide carriers. The hydrogen is then reacted with CO₂ from the air to form sugars, the essential high-energy product that powers all of life. Like the mitochondrion, the chloroplast is a relic of a former free-living bacterium, and has its own DNA on its own chromosome.

Nucleus

The nucleus contains the chromosomes. Chromosomes contain the genes, which are DNA sequences used to create RNA. The nucleus is bounded by a double membrane, called the nuclear envelope. Numerous large pores provide channels through which materials enter and exit. One of the chief exports of the nucleus is messenger RNA, which is used in the cytoplasm for protein construction.

Translation occurs in the cytoplasm at ribosomes, large complexes made of protein and RNA. Ribosomes are assembled in the nucleus, in the region called the nucleolus. RNA is synthesized by the enzyme RNA polymerase, which unwinds DNA and transcribes short portions, known as genes. These RNA molecules are processed further before being exported as messenger RNA. Other RNAs made in the nucleus include the RNA used in ribosomes (ribosomal RNA), RNAs that carry amino acids to the ribosome (transfer RNA), and a host of small RNAs that mostly function in the nucleus to modify other RNAs.

Protein Synthesis, Modification, and Export

Messenger RNA exported from the nucleus binds to a ribosome in the cytosol, which then proceeds to translate the genetic message into a protein. Some proteins, with their ribosomes, remain free in the cytosol throughout translation, but others do not. Those that do not remain free carry a special sequence of amino acids at their leading end, called a signal peptide. This sequence directs the growing protein with its ribosome to the surface of the endoplasmic reticulum (ER), the most extensive organelle in the cell. Here, the ribosome attaches and extrudes the growing protein into the interior, or lumen, of the ER. Attachment of numerous ribosomes gives portions of the ER a rough appearance under the electron microscope. The ER also synthesizes most of the lipids used in the cell's many membranes. Lipid-synthesizing ER does not have ribosomes attached, and so appears smooth.

Many of the proteins entering the ER lumen are destined for other compartments in the cell, and contain organelle-specific targeting sequences that direct them to their final destination. Most of these proteins are first modified by the addition of branched sugar groups to make "glycoproteins." Most proteins in the plasma membrane, for instance, are glycoproteins. The full range of functions of these sugar groups is unknown, but they may help the protein to fold correctly after synthesis, act in cell-cell recognition and adhesion, and promote appropriate interactions with other proteins.

Proteins are further modified and sorted in the Golgi apparatus, a set of flattened membrane disks that is continuous with the ER. Here proteins and lipids are packaged in vesicles that bud off and travel along the cytoskeleton to their final destination. Fusion of the vesicle membrane with the target membrane delivers the contents to the target organelle. Proteins and other materials that the cell exports travel to the plasma membrane via vesicles. Fusion of the vesicle with the plasma membrane delivers the contents to the exterior of the cell.

Cell Cycle

Cells must reproduce in order for the organism to grow or repair damage. For single-celled organisms, cellular reproduction creates a new organism. Each new cell must get a complete set of chromosomes, which therefore must be duplicated and evenly divided between the two daughter cells.

The orderly series of events involving cell growth and division is termed the cell cycle. Immediately following a division, the cell grows by taking up and metabolizing nutrients, and by synthesizing the many proteins, lipids, nucleic acids, sugars, and other molecules it needs. DNA replication occurs next, making duplicate chromosomes, followed by a short period in which the cell synthesizes the numerous proteins specific for cell division itself.

Cell division includes two linked processes: mitosis, or chromosome division, and cytokinesis, or cytoplasm division. Triggered by specific protein changes, the chromosomes begin to coil up tightly and become visible under the microscope. Cytoskeleton fibers attach to them, and position the chromosomes in pairs along the cell's imaginary equator. At the same time, the nuclear envelope breaks down into numerous small vesicles. The cytoskeleton fibers (termed the spindle) pull the chromosome duplicates apart, segregating one member of each pair to opposite sides of the cell. Other cytoskeleton proteins pinch the membrane along the equator (in animal cells) or build a wall across it (in plant cells) to separate the two cell halves, ultimately forming two daughter cells. Finally, the nuclear envelope re-forms and the chromosomes uncoil, starting a new round of the cell cycle.

Bibliography

Alberts, Bruce, et al. Molecular Biology of the Cell, 3rd ed. New York: Garland Publishing, 1994.

—Richard Robinson

For more information on eukaryote, visit Britannica.com.

An organism whose cells contain a nucleus. All multicelled organisms are eukaryotes, as is one superkingdom of single-celled organisms. Eukaryotes also have organelles enclosed by membranes. (Compare prokaryote.)

An organism of the Eucaryotae, whose cells have a true nucleus bounded by a nuclear membrane and containing the chromosomes and which divide by mitosis. Eukaryotic cells also contain membrane-bound organelles, such as mitochondria, chloroplasts, lysosomes and the Golgi apparatus. Plants and animals, protozoa, fungi and algae (except blue-green algae) are eukaryotes. Other organisms (the bacteria) are prokaryotes.

Eukaryotes Fossil range: Proterozoic - Recent
Ostreococcus is the smallest known free living eukaryote with an average size of 0.8 µm.
Scientific classification
Domain:	Eukaryota Whittaker & Margulis,1978
Kingdoms
Animalia - Animals Fungi Amoebozoa Plantae - Plants Chromalveolata Rhizaria Excavata
Alternative phylogeny
Unikonta Opisthokonta Metazoa Mesomycetozoa Choanozoa Eumycota Amoebozoa Bikonta Apusozoa Rhizaria Excavata Archaeplastida Chromalveolata

A eukaryote (pronounced /juːˈkæriɒt/ or /juːˈkærioʊt/) is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried.^{[1] [2] [3]} The presence of a nucleus gives eukaryotes their name, which comes from the Greek ευ (eu, "good", "noble" & "true") and κάρυον (karyon, "nut" & "kernel"). Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria, chloroplasts and the Golgi apparatus. Almost all species of large organisms are eukaryotes, including animals, plants and fungi, although most species of eukaryotic protists are microorganisms.

Cell division in eukaryotes is different from that in organisms without a nucleus (prokaryotes). It involves separating the duplicated chromosomes, through movements directed by microtubules. There are two types of division processes. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.

Eukaryotes appear to be monophyletic, and so make up one of the three domains of life. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features.

Contents 1 Cell features 1.1 Internal membrane 1.2 Mitochondria and plastids 1.3 Cytoskeletal structures 1.4 Plant cell wall 2 Differences between eukaryotic cells 2.1 Animal cell 2.2 Plant cell 2.3 Fungal cell 2.4 Other eukaryotic cells 3 Reproduction 4 Origin and evolution 5 See also 6 References 7 External links

Cell features

Eukaryotic cells are typically much larger than prokaryotes. They have a variety of internal membranes and structures, called organelles, and a cytoskeleton composed of microtubules, microfilaments, and intermediate filaments, which play an important role in defining the cell's organization and shape. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubular spindle during nuclear division.

Detail of the endomembrane system and its components

Internal membrane

Eukaryotic cells include a variety of membrane-bound structures, collectively referred to as the endomembrane system. Simple compartments, called vesicles or vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and then pinches off to form a vesicle. It is probable that most other membrane-bound organelles are ultimately derived from such vesicles.

The nucleus is surrounded by a double membrane (commonly referred to as a nuclear envelope), with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form what is called the endoplasmic reticulum or ER, which is involved in protein transport and maturation. It includes the rough ER where ribosomes are attached, and the proteins they synthesize enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth ER. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles, called Golgi bodies or dictyosomes.

Vesicles may be specialized for various purposes. For instance, lysosomes contain enzymes that break down the contents of food vacuoles, and peroxisomes are used to break down peroxide, which is toxic otherwise. Many protozoa have contractile vacuoles, which collect and expel excess water, and extrusomes, which expel material used to deflect predators or capture prey. In multicellular organisms, hormones are often produced in vesicles. In higher plants, most of a cell's volume is taken up by a central vacuole, which primarily maintains its osmotic pressure.

Mitochondria structure:
1) Inner membrane
2) Outer membrane
3) Crista
4) Matrix

Mitochondria and plastids

Mitochondria are organelles found in nearly all eukaryotes. They are surrounded by double membranes (known as the phospholipid bi-layer), the inner of which is folded into invaginations called cristae, where aerobic respiration takes place. Mitochondria contain their own DNA and ribosomes and are only formed by the fission of other mitochondria. They are now generally held to have developed from endosymbiotic prokaryotes, probably proteobacteria. The few protozoa that lack mitochondria have been found to contain mitochondrion-derived organelles, such as hydrogenosomes and mitosomes.

Plants and various groups of algae also have plastids. Again, these have their own DNA and developed from endosymbiotes, in this case cyanobacteria. They usually take the form of chloroplasts, which like cyanobacteria contain chlorophyll and produce energy through photosynthesis. Others are involved in storing food. Although plastids likely had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from others through secondary endosymbiosis or ingestion.

Endosymbiotic origins have also been proposed for the nucleus, for which see below, and for eukaryotic flagella, supposed to have developed from spirochaetes. This is not generally accepted, both from a lack of cytological evidence and difficulty in reconciling this with cellular reproduction.

Cytoskeletal structures

Many eukaryotes have long slender motile cytoplasmic projections, called flagella, or similar, but shorter structures called cilia. Flagella and cilia are sometimes referred to as undulipodia, and are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin. These are entirely distinct from prokaryotic flagella. They are supported by a bundle of microtubules arising from a basal body, also called a kinetosome or centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella also may have hairs, or mastigonemes, and scales connecting membranes and internal rods. Their interior is continuous with the cell's cytoplasm.

Microfilamental structures composed by actin and actin binding proteins, e.g., α-actinin, fimbrin, filamin are present in submembraneous cortical layers and bundles, as well. Motor proteins of microtubules, e.g., dynein or kinesin and actin, e.g., myosins provide dynamic character of the network.

Centrioles are often present even in cells and groups that do not have flagella. They generally occur in groups of one or two, called kinetids, that give rise to various microtubular roots. These form a primary component of the cytoskeletal structure, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles may also be associated in the formation of a spindle during nuclear division.

Significance of cytoskeletal structures is underlined in determination of shape of the cells, as well as their being essential components of migratory responses like chemotaxis and chemokinesis. Some protists have various other microtubule-supported organelles. These include the radiolaria and heliozoa, which produce axopodia used in flotation or to capture prey, and the haptophytes, which have a peculiar flagellum-like organelle called the haptonema.

Plant cell wall

Further information: Cell wall

Plant cells have a cell wall, a fairly rigid layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. The major carbohydrates making up the primary cell wall of land plants are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.

Differences between eukaryotic cells

There are many different types of eukaryotic cells, though animals and plants are the most familiar eukaryotes, and thus provide an excellent starting point for understanding eukaryotic structure. Fungi and many protists have some substantial differences, however.

Animal cell

Structure of a typical animal cell.

Structure of a typical plant cell.

An animal cell is a form of eukaryotic cell that makes up many tissues in animals. The animal cell is distinct from other eukaryotes, most notably plant cells, as they lack cell walls and chloroplasts, and they have smaller vacuoles. Due to the lack of a rigid cell wall, animal cells can adopt a variety of shapes, and a phagocytic cell can even engulf other structures.

There are many different cell types. For instance, there are approximately 210 distinct cell types in the adult human body.

Plant cell

Further information: Plant cell

Plant cells are quite different from the cells of the other eukaryotic organisms. Their distinctive features are:

• A large central vacuole (enclosed by a membrane, the tonoplast), which maintains the cell's turgor and controls movement of molecules between the cytosol and sap
• A primary cell wall containing cellulose, hemicellulose and pectin, deposited by the protoplast on the outside of the cell membrane; this contrasts with the cell walls of fungi, which contain chitin, and the cell envelopes of prokaryotes, in which peptidoglycans are the main structural molecules
• The plasmodesmata, linking pores in the cell wall that allow each plant cell to communicate with other adjacent cells; this is different from the functionally analogous system of gap junctions between animal cells.
• Plastids, especially chloroplasts that contain chlorophyll, the pigment that gives plants their green color and allows them to perform photosynthesis
• Higher plants, including conifers and flowering plants (Angiospermae) lack the flagellae and centrioles that are present in animal cells.

Fungal cell

Fungal cells are most similar to animal cells, with the following exceptions:

• A cell wall containing chitin
• Less definition between cells; the hyphae of higher fungi have porous partitions called septa, which allow the passage of cytoplasm, organelles, and, sometimes, nuclei. Primitive fungi have few or no septa, so each organism is essentially a giant multinucleate supercell; these fungi are described as coenocytic.
• Only the most primitive fungi, chytrids, have flagella.

Other eukaryotic cells

Eukaryotes are a very diverse group, and their cell structures are equally diverse. Many have cell walls; many do not. Many have chloroplasts, derived from primary, secondary, or even tertiary endosymbiosis; and many do not. Some groups have unique structures, such as the cyanelles of the glaucophytes, the haptonema of the haptophytes, or the ejectisomes of the cryptomonads. Other structures, such as pseudopods, are found in various eukaryote groups in different forms, such as the lobose amoebozoans or the reticulose foraminiferans.

Reproduction

Nuclear division is often coordinated with cell division. This generally takes place by mitosis, a process that allows each daughter nucleus to receive one copy of each chromosome. In most eukaryotes, there is also a process of sexual reproduction, typically involving an alternation between haploid generations, wherein only one copy of each chromosome is present, and diploid generations, wherein two are present, occurring through nuclear fusion (syngamy) and meiosis. There is considerable variation in this pattern, however.

Eukaryotes have a smaller surface area to volume ratio than prokaryotes, and thus have lower metabolic rates and longer generation times. In some multicellular organisms, cells specialized for metabolism will have enlarged surface areas, such as intestinal vili.

Origin and evolution

Phylogenetic tree showing the relationship between the eukaryotes and other forms of life.^[4] Eukaryotes are colored red, archaea green and bacteria blue.

One hypothesis of eukaryotic relationships

The origin of the eukaryotic cell was a milestone in the evolution of life, since they include all complex cells and almost all multi-cellular organisms. The timing of this series of events is hard to determine; Knoll (2006) suggests they developed approximately 1.6 - 2.1 billion years ago. Some acritarchs are known from at least 1650 million years ago, and the possible alga Grypania has been found as far back as 2100 million years ago. ^[5] Fossils that are clearly related to modern groups start appearing around 1.2 billion years ago, in the form of a red alga, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back to 1.6 to 1.7 billion years ago.^[6]

Biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of steranes in Australian shales indicates that eukaryotes were present 2.7 billion years ago.^[7][8]

rRNA trees constructed during the 1980s and 1990s left most eukaryotes in an unresolved "crown" group (not technically a true crown), which was usually divided by the form of the mitochondrial cristae; see crown eukaryotes. The few groups that lack mitochondria branched separately, and so the absence was believed to be primitive; but this is now considered an artifact of long-branch attraction, and they are known to have lost them secondarily.^[9][10]

More recent work has painted a different picture. Most eukaryotes are now included in one of the following supergroups, although the relationship between these groups, and the monophyly of each group, is not yet clear:^[11][12]. Some groups position in the tree is still unclear, as in Kamera lens.

Opisthokonts	Animals, fungi, choanoflagellates, etc.
Amoebozoa	Most lobose amoeboids and slime moulds
Rhizaria	Foraminifera, Radiolaria, and various other amoeboid protozoa
Excavates	Various flagellate protozoa
Archaeplastida (or Primoplantae)	Land plants, green algae, red algae, and glaucophytes
Chromalveolates	Heterokonts, Haptophytes, Cryptomonads, and Alveolates.

A further proposal is that eukaryotes can be classified into two larger clades, the unikonts and the bikonts,^[13] that derive from an ancestral uniflagellar organism and a biflagellate, respectively. In this system, the opisthokonts and amoebozoans are considered unikonts, and the rest bikonts. The correct relationships of the organisms in the Archaeplastida and Chromalveolates is far from clear, and it seems likely that neither group, as defined above, is monophyletic.^[14] Some small protist groups have not been related to any of these supergroups, in particular the characteristics of the anaerobic protist Breviata anathema seems to contradict the unikont/bikont division.^[15]

Eukaryotes are more closely related to Archaea than Bacteria, at least in terms of nuclear DNA and genetic machinery, and one controversial idea is to place them with Archaea in the clade Neomura. However, in other respects, such as membrane composition, eukaryotes are similar to Bacteria. Three main explanations for this have been proposed:

• Eukaryotes resulted from the complete fusion of two or more cells, wherein the cytoplasm formed from a eubacterium, and the nucleus from an archaeon,^[16] from a virus,^[17][18] or from a pre-cell. ^[19][20]
• Eukaryotes developed from Archaea, and acquired their eubacterial characteristics from the proto-mitochondrion.
• Eukaryotes and Archaea developed separately from a modified eubacterium.

Eukarya Bikonta Apusozoa Archaeplastida Chromalveolata Rhizaria Excavata Unikonta Amoebozoa Opisthokonta Metazoa Choanozoa Eumycota One recent cladogram of Eukarya

The origins of the endomembrane system and mitochondria are also unclear.^[21] The phagotrophic hypothesis proposes that eukaryotic-type membranes lacking a cell wall originated first, with the development of endocytosis, whereas mitochondria were acquired by ingestion as endosymbionts.^[22] The syntrophic hypothesis proposes that the proto-eukaryote relied on the proto-mitochondrion for food, and so ultimately grew to surround it. Here the membranes originated after the engulfment of the mitochondrion, in part thanks to mitochondrial genes (the hydrogen hypothesis is one particular version).^[23]

In a study using genomes to construct supertrees, Pisani et al. (2007) suggest that, along with evidence that there was never a mitochondrion-less eukaryote, eukaryotes evolved from a syntrophy between an archaea closely related to Thermoplasmatales and an α-proteobacterium, likely a symbiosis driven by sulfur or hydrogen. The mitochondrion and its genome is a remnant of the α-proteobacterial endosymbiont.^[24]

See also

Wikipedia:Books has a book on: Eukaryote

• List of sequenced eukaryotic genomes
• Prokaryote

References

1. ^ Youngson, Robert M. (2006). Collins Dictionary of Human Biology. Glasgow: HarperCollins. ISBN 0-00-722134-7. 
2. ^ Nelson, David L. & Michael M. Cox (2005), Lehninger Principles of Biochemistry (4th ed.), W.H. Freeman, ISBN 0716743396
3. ^ Martin, E.A., ed (1983). Macmillan Dictionary of Life Sciences (2nd ed.). London: Macmillan Press. ISBN 0-333-34867-2. 
4. ^ Ciccarelli FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P (2006). "Toward automatic reconstruction of a highly resolved tree of life". Science 311 (5765): 1283–7. doi:10.1126/science.1123061. PMID 16513982. 
5. ^ Knoll, Andrew H.; Javaux, E.J, Hewitt, D. and Cohen, P. (2006). "Eukaryotic organisms in Proterozoic oceans". Philosophical Transactions of the Royal Society of London, Part B 361 (1470): 1023–38. doi:10.1098/rstb.2006.1843. PMID 16754612. 
6. ^ Bengtson S, Belivanova V, Rasmussen B, Whitehouse M. (2009). The controversial "Cambrian" fossils of the Vindhyan are real but more than a billion years older. Proc Natl Acad Sci U S A. 106: 7729–7734 PubMed
7. ^ Brocks JJ, Logan GA, Buick R, Summons RE (August 1999). "Archean molecular fossils and the early rise of eukaryotes". Science 285 (5430): 1033–6. doi:10.1126/science.285.5430.1033. PMID 10446042. http://www.sciencemag.org/cgi/content/full/285/5430/1033. 
8. ^ Ward P (9 Feb 2008). "Mass extinctions: the microbes strike back". New Scientist: 40–3. http://www.newscientist.com/channel/life/mg19726421.900-mass-extinctions-the-microbes-strike-back.html. 
9. ^ Tovar J, Fischer A, Clark CG (1999). "The mitosome, a novel organelle related to mitochondria in the amitochondrial parasite Entamoeba histolytica". Mol. Microbiol. 32 (5): 1013–21. doi:10.1046/j.1365-2958.1999.01414.x. PMID 10361303. 
10. ^ Boxma B, de Graaf RM, van der Staay GW, et al. (2005). "An anaerobic mitochondrion that produces hydrogen". Nature 434 (7029): 74–9. doi:10.1038/nature03343. PMID 15744302. 
11. ^ Burki F, Shalchian-Tabrizi K, Minge M, et al. (2007). "Phylogenomics reshuffles the eukaryotic supergroups". PLoS ONE 2 (8): e790. doi:10.1371/journal.pone.0000790. PMID 17726520. 
12. ^ Laura Wegener Parfrey, Erika Barbero, Elyse Lasser, Micah Dunthorn, Debashish Bhattacharya, David J Patterson, and Laura A Katz (2006 December). "Evaluating Support for the Current Classification of Eukaryotic Diversity". PLoS Genet. 2 (12): e220. doi:10.1371/journal.pgen.0020220. PMID 17194223. 
13. ^ Burki F, Pawlowski J (October 2006). "Monophyly of Rhizaria and multigene phylogeny of unicellular bikonts". Mol. Biol. Evol. 23 (10): 1922–30. doi:10.1093/molbev/msl055. PMID 16829542. http://mbe.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=16829542. 
14. ^ Kim E, Graham LE (2008). "EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata". PLoS ONE 3 (7): e2621. doi:10.1371/journal.pone.0002621. PMID 18612431. 
15. ^ Roger AJ, Simpson AGB. (2009). "Evolution: Revisiting the Root of the Eukaryote Tree". Current Biology 19 (4): R165–7. doi:10.1016/j.cub.2008.12.032. PMID 19243692. 
16. ^ Martin W (December 2005). "Archaebacteria (Archaea) and the origin of the eukaryotic nucleus". Curr. Opin. Microbiol. 8 (6): 630–7. doi:10.1016/j.mib.2005.10.004. PMID 16242992. 
17. ^ Takemura M (May 2001). "Poxviruses and the origin of the eukaryotic nucleus". J. Mol. Evol. 52 (5): 419–25. doi:10.1007/s002390010171. PMID 11443345. 
18. ^ Bell PJ (September 2001). "Viral eukaryogenesis: was the ancestor of the nucleus a complex DNA virus?". J. Mol. Evol. 53 (3): 251–6. doi:10.1007/s002390010215. PMID 11523012. 
19. ^ Wächtershäuser G (January 2003). "From pre-cells to Eukarya--a tale of two lipids". Mol. Microbiol. 47 (1): 13–22. doi:10.1046/j.1365-2958.2003.03267.x. PMID 12492850. 
20. ^ Wächtershäuser G (October 2006). "From volcanic origins of chemoautotrophic life to Bacteria, Archaea and Eukarya". Philos. Trans. R. Soc. Lond. B Biol. Sci. 361 (1474): 1787–1808. doi:10.1098/rstb.2006.1904. PMID 17008219. 
21. ^ Jékely G (2007). "Origin of eukaryotic endomembranes: a critical evaluation of different model scenarios". Adv. Exp. Med. Biol. 607: 38–51. doi:10.1007/978-0-387-74021-8_3. PMID 17977457. 
22. ^ Cavalier-Smith T (01 March 2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". Int. J. Syst. Evol. Microbiol. 52 (Pt 2): 297–354. PMID 11931142. http://ijs.sgmjournals.org/cgi/pmidlookup?view=long&pmid=11931142. 
23. ^ Martin W, Müller M (March 1998). "The hydrogen hypothesis for the first eukaryote". Nature 392 (6671): 37–41. doi:10.1038/32096. PMID 9510246. 
24. ^ Pisani D, Cotton JA, McInerney JO (2007). "Supertrees disentangle the chimerical origin of eukaryotic genomes". Mol Biol Evol. 24 (8): 1752–60. doi:10.1093/molbev/msm095. PMID 17504772. 

This article contains material from the Science Primer published by the NCBI, which, as a U.S. government publication, is in the public domain.

External links

Wikispecies has information related to: Eukaryota

• Eukaryotes (Tree of Life web site)
• Prokaryote versus eukaryote, BioMineWiki
• Eukaryote at the Encyclopedia of Life

v • d • e Eukaryota classification Domain : Archaea - Bacteria - Eukaryota Bikonta AH/SAR AH Archaeplastida, or Plantae sensu lato Viridiplantae/Plantae sensu stricto · Rhodophyta  · Glaucocystophyceae Hacrobia, or non-SAR chromalveolata Haptophyta · Cryptophyta · Centroheliozoa SAR SA Heterokont ("S") Ochrophyta · Bigyra · Pseudofungi Alveolata Ciliates · Myzozoa (Apicomplexa, Dinoflagellata) Rhizaria Cercozoa · Retaria (Foraminifera, Radiolaria) Excavata Discoba (Euglenozoa, Percolozoa) · Metamonad · Malawimonas Apusozoa Apusomonadida (Apusomonas) · Ancyromonadida (Ancyromonas) · Hemimastigida (Hemimastix, Spironema, Stereonema) Unikonta Amoebozoa Lobosea · Conosa · Phalansterium Opisthokonta Holozoa Mesomycetozoea Dermocystida · Ichthyophonida Filozoa Filasterea Capsaspora · Ministeria Choanoflagellatea Codonosigidae Metazoa or "Animalia" Eumetazoa (Bilateria, Cnidaria, Ctenophora) · Mesozoa · Parazoa (Placozoa, Porifera) Fungi Dikarya (Ascomycota, Basidiomycota) · Glomeromycota · Zygomycota · Blastocladiomycota  · Chytridiomycota/Neocallimastigomycota  · Microsporidia sister: Nucleariidae (Nuclearia, Micronuclearia)

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)