The Info List - Eukaryote

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Eukaryotic organisms that cannot be classified under the kingdoms Plantae, Animalia or Fungi are sometimes grouped in the kingdom PROTISTA .

A EUKARYOTE (/juːˈkæri.oʊt/ or /juːˈkæriət/ ) is any organism whose cells have a cell nucleus and other organelles enclosed within membranes . Eukaryotes belong to the taxon EUKARYA or EUKARYOTA. The defining feature that sets eukaryotic cells apart from prokaryotic cells ( Bacteria and Archaea ) is that they have membrane-bound organelles, especially the nucleus, which contains the genetic material and is enclosed by the nuclear envelope . The presence of a nucleus gives eukaryotes their name, which comes from the Greek εὖ (_eu_, "well" or "true") and κάρυον (_karyon_, "nut" or "kernel"). Eukaryotic cells also contain other membrane-bound organelles such as mitochondria and the Golgi apparatus . In addition, plants and algae contain chloroplasts . Eukaryotic organisms may be unicellular or multicellular . Only eukaryotes form multicellular organisms consisting of many kinds of tissue made up of different cell types .

Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells (haploid cells ) each with half the number of chromosomes as the original parent cell. These act as sex cells (gametes – each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes ) resulting from genetic recombination during meiosis.



* 1 History of the eukaryote concept

* 2 Cell features

* 2.1 Internal membrane * 2.2 Mitochondria and plastids * 2.3 Cytoskeletal structures * 2.4 Cell wall

* 3 Differences among eukaryotic cells

* 3.1 Animal cell * 3.2 Plant cell * 3.3 Fungal cell * 3.4 Other eukaryotic cells

* 4 Reproduction

* 5 Classification

* 5.1 Phylogeny

* 5.1.1 Five supergroups * 5.1.2 Cavalier-Smith\'s tree

* 6 Origin of eukaryotes

* 6.1 Fossils * 6.2 Relationship to Archaea * 6.3 Endomembrane system and mitochondria

* 6.4 Hypotheses for the origin of eukaryotes

* 6.4.1 Autogenous models * 6.4.2 Chimeric models

* 7 See also * 8 References * 9 External links


Konstantin Mereschkowski proposed a symbiotic origin for cells with nuclei.

In 1905 and 1910, the Russian biologist Konstantin Mereschkowsky (1855–1921) argued three things about the origin of nucleated cells. Firstly, plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic (heterotrophic ) host. Secondly, the host had earlier in evolution formed by symbiosis between an amoeba-like host and a bacteria-like ("micrococcal") cell that formed the nucleus. Thirdly, plants inherited photosynthesis from cyanobacteria.


In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in eukaryotic cells in her paper, _On the origin of mitosing cells._ In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA . This helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles , mitochondria and chloroplasts . In 1977, Woese and George Fox introduced a "third form of life", which they called the Archaebacteria; in 1990, Woese, Otto Kandler and Mark L. Wheeler renamed this the Archaea.



Life timeline view • discuss • edit -4500 — – -4000 — – -3500 — – -3000 — – -2500 — – -2000 — – -1500 — – -1000 — – -500 — – 0 — _WATER _ Single-celled life _PHOTOSYNTHESIS _ EUKARYOTES Multicellular life LAND LIFE DINOSAURS MAMMALS FLOWERS ← Earliest Earth (−4540 ) ← Earliest water ← Earliest life ← LHB meteorites ← Earliest oxygen ← Atmospheric oxygen ← Oxygen crisis ← Earliest sexual reproduction ← Ediacara biota ← Cambrian explosion ← Earliest humans P h a n e r o z o i c

P r o t e r o z o i c

A r c h e a n H a d e a n Pongola Huronian Cryogenian Andean Karoo Quaternary Axis scale : millions of years . Orange labels: known _ICE AGES_. Also see: _ Human timeline _ and _Nature timeline _



Detail of the endomembrane system and its components





Simplified structure of a mitochondrion

Mitochondria are organelles found in nearly all eukaryotes that provide energy to the cell by converting ingested sugars into ATP . They are surrounded by two membranes (each a phospholipid bi-layer ), the inner of which is folded into invaginations called cristae , where aerobic respiration takes place. Mitochondria contain their own DNA. They are now generally held to have developed from endosymbiotic prokaryotes, probably proteobacteria . Protozoa and microbes that lack mitochondria, such as the amoebozoan Pelomyxa and metamonads such as _ Giardia _ and _ Trichomonas _, have usually been found to contain mitochondrion-derived organelles, such as hydrogenosomes and mitosomes , and thus probably lost the mitochondria secondarily.

In 2016, _ Monocercomonoides _, a metamonad flagellate which resides in the intestines of the chinchilla , has been found to lack mitochondria entirely. _Monocercomonoides_ obtains its energy by enzymatic action on nutrients absorbed from the environment. It has also acquired, by lateral gene transfer , a cytosolic sulphur mobilisation system which provides the clusters of iron and sulfur required for protein synthesis. The normal mitochondrial iron-sulphur cluster pathway is considered to have been lost secondarily.




Main article: Cytoskeleton _ Longitudinal section through the flagellum of Chlamydomonas reinhardtii _






Main article: Cell wall






Structure of a typical animal cell Play media 3D Simulation of animal cell Structure of a typical plant cell


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


Main article: Plant cell


* 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 * Bryophytes and seedless vascular plants lack flagellae and centrioles except in the sperm cells. Sperm of cycads and _ Ginkgo _ are large, complex cells that swim with hundreds to thousands of flagellae. * Conifers (Pinophyta) and flowering plants (Angiospermae) lack the flagellae and centrioles that are present in animal cells .



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

* A cell wall that contains 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.




_ This diagram illustrates the twofold cost of sex_. If each individual were to contribute to the same number of offspring (two), _(a)_ the sexual population remains the same size each generation, where the _(b)_ asexual population doubles in size each generation.

Cell division generally takes place asexually 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 copies of each chromosome are present, occurring through meiosis . There is considerable variation in this pattern.


The evolution of sexual reproduction may be a primordial and fundamental characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks and Roger proposed that facultative sex was present in the common ancestor of all eukaryotes. A core set of genes that function in meiosis is present in both _ Trichomonas vaginalis _ and _ Giardia intestinalis _, two organisms previously thought to be asexual. Since these two species are descendants of lineages that diverged early from the eukaryotic evolutionary tree, it was inferred that core meiotic genes, and hence sex, were likely present in a common ancestor of all eukaryotes. Other studies on eukaryotic species once thought to be asexual have revealed evidence for a sexual cycle. For instance, parasitic protozoa of the genus Leishmania have recently been shown to have a sexual cycle. Also, evidence now indicates that amoeba, that were previously regarded as asexual, are anciently sexual and that the majority of present-day asexual groups likely arose recently and independently.


Further information: wikispecies: Eukaryota Phylogenetic and symbiogenetic tree of living organisms, showing a view of the origins of eukaryotes "> One hypothesis of eukaryotic relationships. The Opisthokonta group includes both animals (Metazoa) and fungi. Plants (Plantae) are placed in Archaeplastida . A pie chart of described eukaryote species (except for Excavata), together with a tree showing possible relationships between the groups

In antiquity , the two clades of animals and plants were recognized. They were given the taxonomic rank of Kingdom by Linnaeus . Though he included the fungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom, the composition of which was not entirely clear until the 1980s. The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1830, the German biologist Georg A. Goldfuss coined the word _protozoa _ to refer to organisms such as ciliates , and this group was expanded until it encompassed all single-celled eukaryotes, and given their own kingdom, the Protista , by Ernst Haeckel in 1866. The eukaryotes thus came to be composed of four kingdoms:

* Kingdom Protista * Kingdom Plantae * Kingdom Fungi * Kingdom Animalia

The protists were understood to be "primitive forms", and thus an evolutionary grade , united by their primitive unicellular nature. The disentanglement of the deep splits in the tree of life only really started with DNA sequencing , leading to a system of domains rather than kingdoms as top level rank being put forward by Carl Woese , uniting all the eukaryote kingdoms under the eukaryote domain. At the same time, work on the protist tree intensified, and is still actively going on today. Several alternative classifications have been forwarded, though there is no consensus in the field.

A classification produced in 2005 for the International Society of Protistologists , which reflected the consensus of the time, divided the eukaryotes into six supposedly monophyletic 'supergroups'. However, in the same year (2005), doubts were expressed as to whether some of these supergroups were monophyletic , particularly the Chromalveolata , and a review in 2006 noted the lack of evidence for several of the supposed six supergroups. A revised classification in 2012 recognizes five supergroups.



Excavata Various flagellate protozoa

Amoebozoa Most lobose amoeboids and slime molds

Opisthokonta Animals , fungi , choanoflagellates , etc.

There are also smaller groups of eukaryotes whose position is uncertain or seems to fall outside the major groups — in particular, Haptophyta , Cryptophyta , Centrohelida , Telonemia , Picozoa , Apusomonadida , Ancyromonadida , Breviatea , and the genus _ Collodictyon _. Overall, it seems that, although progress has been made, there are still very significant uncertainties in the evolutionary history and classification of eukaryotes. As Roger 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.

As of 2011 , there is widespread agreement that the Rhizaria belong with the Stramenopiles and the Alveolata, in a clade dubbed the SAR supergroup , so that Rhizaria is not one of the main eukaryote groups; also that the Amoebozoa and Opisthokonta are each monophyletic and form a clade, often called the unikonts . Beyond this, there does not appear to be a consensus.

It has been estimated that there may be 75 distinct lineages of eukaryotes. Most of these lineages are protists.

The known eukaryote genome sizes vary from 8.2 megabases (Mb) in _ Babesia bovis _ to 112,000–220,050 Mb in the dinoflagellate _ Prorocentrum micans _, suggesting that the genome of the ancestral eukaryote has undergone considerable variation during its evolution. The last common ancestor of all eukaryotes is believed to have been a phagotrophic protist with a nucleus, at least one centriole and cilium , facultatively aerobic mitochondria, sex (meiosis and syngamy ), a dormant cyst with a cell wall of chitin and/or cellulose and peroxisomes . Later endosymbiosis led to the spread of plastids in some lineages.

Five Supergroups

A global tree of eukaryotes from a consensus of phylogenetic evidence (in particular, phylogenomics), rare genomic signatures, and morphological characteristics is presented in Adl _et al._ 2012 and Burki 2014.





Red algae (Rhodophyta)

Green plants (Viridiplantae)












Holomycota (inc. fungi)

Holozoa (inc. animals)


In some analyses, the Hacrobia group ( Haptophyta + Cryptophyta ) is placed next to Archaeplastida , but in other ones it is nested inside the Archaeplastida. However, several recent studies have concluded that Haptophyta and Cryptophyta do not form a monophyletic group. The former could be a sister group to the SAR group , the latter cluster with the Archaeplastida (plants in the broad sense).


Cavalier-Smith\'s Tree

Cavalier-Smith 2010, 2013, and 2014 places the eukaryotic tree's root between Excavata (with ventral feeding groove supported by a microtubular root) and the grooveless Euglenozoa :



Excavata (paraphyletic)








The three-domains tree and the Eocyte hypothesis . Phylogenetic tree showing the relationship between the eukaryotes and other forms of life. Eukaryotes are colored red, archaea green and bacteria blue.


The origin of the eukaryotic cell is considered a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular 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 1.65 billion years ago, and the possible alga _Grypania _ has been found as far back as 2.1 billion years ago. The _Geosiphon _-like fossil fungus _ Diskagma _ has been found in paleosols 2.2 billion years old





* Eukaryotes resulted from the complete fusion of two or more cells, wherein the cytoplasm formed from a eubacterium , and the nucleus from an archaeon, from a virus , or from a pre-cell . * Eukaryotes developed from Archaea, and acquired their eubacterial characteristics through the endosymbiosis of a proto-mitochondrion of eubacterial origin. * Eukaryotes and Archaea developed separately from a modified eubacterium.

An additional proposal, the chronocyte hypothesis, postulates that a primitive eukaryotic cell was formed by the endosymbiosis of both archaea and bacteria by a third type of cell, termed a chronocyte.

A possible cladogram for the positioning of eukaryotes within Archaea, based on phylogenomic analyses of the Asgard archaea:

Asgard Archaea







The origins of the endomembrane system and mitochondria are also unclear. 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. 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).

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.



Autogenous Models

An autogenous model for the origin of eukaryotes.

Autogenous models propose that a proto-eukaryotic cell containing a nucleus existed first, and later acquired mitochondria . According to this model, a large prokaryote developed invaginations in its plasma membrane in order to obtain enough surface area to service its cytoplasmic volume. As the invaginations differentiated in function, some became separate compartments—giving rise to the endomembrane system , including the endoplasmic reticulum , golgi apparatus , nuclear membrane , and single membrane structures such as lysosomes . Mitochondria are proposed to come from the endosymbiosis of an aerobic proteobacterium , and it's assumed that all the eukaryotic lineages that did not acquire mitochondria became extinct. Chloroplasts came about from another endosymbiotic event involving cyanobacteria . Since all eukaryotes have mitochondria, but not all have chloroplasts, mitochondria are thought to have come first. This is the serial endosymbiosis theory.


Chimeric Models


Based on the process of mutualistic symbiosis , the hypotheses can be categorized as – the serial endosymbiotic theory (SET), the hydrogen hypothesis (mostly a process of symbiosis where hydrogen transfer takes place among different species), and the syntrophy hypothesis.






* Biology portal

* Book: Eukaryote

* Evolution of sexual reproduction * List of sequenced eukaryotic genomes * Parikaryote * Prokaryote * Thaumarchaeota * Vault (organelle)


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