82-547: The cryptomonads-haptophytes assemblage is a proposed but disputed monophyletic grouping of unicellular eukaryotes that are not included in the SAR supergroup . Several alternative names have been used for the group, including Hacrobia (derived from "ha-" referring to Haptophyta, "-cr-" referring to cryptomonads, and "-bia" as a general suffix referring to life ); CCTH (standing for Cryptophyta, Centrohelida, Telonemia and Haptophyta); and "Eukaryomonadae". As of February 2012, it
164-451: A cytosolic tRNA-Glu as trnE is highly conserved; single base changes in trnE have resulted in the loss of haem synthesis. The gene for tRNA- formylmethionine (tRNA-fmet) is also encoded in the plastid genome and is required for translation initiation in both plastids and mitochondria. A plastid is required to continue expressing the gene for tRNA-fmet so long as the mitochondrion is translating proteins. The limited window hypothesis offers
246-405: A microtubular spindle during nuclear division, in the distinctively eukaryotic process of mitosis . Eukaryotes differ from prokaryotes in multiple ways, with unique biochemical pathways such as sterane synthesis. The eukaryotic signature proteins have no homology to proteins in other domains of life, but appear to be universal among eukaryotes. They include the proteins of the cytoskeleton,
328-513: A "symbiosis-based phylogeny", giving the description "Eukarya (symbiosis-derived nucleated organisms)". By 2014, a rough consensus started to emerge from the phylogenomic studies of the previous two decades. The majority of eukaryotes can be placed in one of two large clades dubbed Amorphea (similar in composition to the unikont hypothesis) and the Diphoda (formerly bikonts), which includes plants and most algal lineages. A third major grouping,
410-430: A body, with its cells dividing by mitosis , and at some stage produce haploid gametes through meiosis , a division that reduces the number of chromosomes and creates genetic variability . There is considerable variation in this pattern. Plants have both haploid and diploid multicellular phases . Eukaryotes have lower metabolic rates and longer generation times than prokaryotes, because they are larger and therefore have
492-479: A bundle of microtubules arising from a centriole , characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs ( mastigonemes ), as in many stramenopiles . Their interior is continuous with the cell's cytoplasm . Centrioles are often present, even in cells and groups that do not have flagella, but conifers and flowering plants have neither. They generally occur in groups that give rise to various microtubular roots. These form
574-411: A cell by another free living organism. Secondary endosymbiosis occurs when the product of primary endosymbiosis is itself engulfed and retained by another free living eukaryote. Secondary endosymbiosis has occurred several times and has given rise to extremely diverse groups of algae and other eukaryotes. Some organisms can take opportunistic advantage of a similar process, where they engulf an alga and use
656-468: A common ancestor. Another theory behind nucleation is that early nuclear membrane proteins caused the cell membrane to fold and form a sphere with pores like the nuclear envelope . As a way of forming a nuclear membrane, endosymbiosis could be expected to use less energy than if the cell was to develop a metabolic process to fold the cell membrane for the purpose. Digesting engulfed cells without energy-producing mitochondria would have been challenging for
738-439: A more general explanation for the retention of genes in non-photosynthetic plastids. According to this hypothesis, genes are transferred to the nucleus following the disturbance of organelles. Disturbance was common in the early stages of endosymbiosis, however, once the host cell gained control of organelle division, eukaryotes could evolve to have only one plastid per cell. Having only one plastid severely limits gene transfer as
820-406: A primary component of the cytoskeleton, 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 produce the spindle during nuclear division. The cells of plants, algae, fungi and most chromalveolates , but not animals, are surrounded by a cell wall. This is a layer outside the cell membrane , providing
902-502: A processed mRNA intermediate. The cDNA hypothesis has since been revised as edited mitochondrial cDNAs are unlikely to recombine with the nuclear genome and are more likely to recombine with their native mitochondrial genome. If the edited mitochondrial sequence recombines with the mitochondrial genome, mitochondrial splice sites would no longer exist in the mitochondrial genome. Any subsequent nuclear gene transfer would therefore also lack mitochondrial splice sites. The bulk flow hypothesis
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#1732869189437984-477: A result, many plastid and mitochondrial processes are driven by nuclear encoded gene products. In addition, many nuclear genes originating from endosymbionts have acquired novel functions unrelated to their organelles. The mechanisms of gene transfer are not fully known; however, multiple hypotheses exist to explain this phenomenon. The possible mechanisms include the Complementary DNA (cDNA) hypothesis and
1066-433: A result, plastids must make their own functional RNAs or import nuclear counterparts. The genes encoding tRNA-Glu and tRNA-fmet, however, appear to be indispensable. The plastid is responsible for haem biosynthesis, which requires plastid encoded tRNA-Glu (from the gene trnE) as a precursor molecule. Like other genes encoding RNAs, trnE cannot be transferred to the nucleus. In addition, it is unlikely trnE could be replaced by
1148-761: A resurrection of the idea of symbiogenesis in the 1960s. Lynn Margulis advanced and substantiated the theory with microbiological evidence in a 1967 paper, On the origin of mitosing cells. In her 1981 work Symbiosis in Cell Evolution she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic spirochaetes that developed into eukaryotic flagella and cilia . This last idea has not received much acceptance, because flagella lack DNA and do not show ultrastructural similarities to bacteria or to archaea (see also: Evolution of flagella and Prokaryotic cytoskeleton ). According to Margulis and Dorion Sagan , "Life did not take over
1230-467: A single origin of all extant plastids – although this theory is still debated. Some species including Pediculus humanus (lice) have multiple chromosomes in the mitochondrion. This and the phylogenetics of the genes encoded within the mitochondrion suggest that mitochondria have multiple ancestors, that these were acquired by endosymbiosis on several occasions rather than just once, and that there have been extensive mergers and rearrangements of genes on
1312-452: A small portion of their genome; however no one hypothesis will apply to all organisms, and the topic is still quite controversial. The hydrophobicity hypothesis states that highly hydrophobic (water hating) proteins (such as the membrane bound proteins involved in redox reactions) are not easily transported through the cytosol and therefore these proteins must be encoded in their respective organelles. The code disparity hypothesis states that
1394-510: A smaller surface area to volume ratio. The evolution of sexual reproduction may be a primordial characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks and Roger have proposed that facultative sex was present in the group's common ancestor. 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
1476-402: A sophisticated, morphology-generating cytoskeleton sustained by mitochondria. This fossil evidence indicates that endosymbiotic acquisition of alphaproteobacteria must have occurred before 1.6 Gya. Molecular clocks have also been used to estimate the last eukaryotic common ancestor, however these methods have large inherent uncertainty and give a wide range of dates. Reasonable results include
1558-649: A symbiotic relationship with other one-celled organisms dates back to the 19th century, when it was espoused by researchers such as Andreas Schimper . The endosymbiotic theory was articulated in 1905 and 1910 by the Russian botanist Konstantin Mereschkowski , and advanced and substantiated with microbiological evidence by Lynn Margulis in 1967. Among the many lines of evidence supporting symbiogenesis are that mitochondria and plastids contain their own chromosomes and reproduce by splitting in two , parallel but separate from
1640-417: A system of domains rather than kingdoms as top level rank being put forward by Carl Woese , Otto Kandler , and Mark Wheelis in 1990, uniting all the eukaryote kingdoms in the domain "Eucarya", stating, however, that " 'eukaryotes' will continue to be an acceptable common synonym". In 1996, the evolutionary biologist Lynn Margulis proposed to replace kingdoms and domains with "inclusive" names to create
1722-610: Is closer in structure to bacterial RNA than to eukaryote RNA. Some eukaryotes, such as the metamonads Giardia and Trichomonas , and the amoebozoan Pelomyxa , appear to lack mitochondria, but all contain mitochondrion-derived organelles, like hydrogenosomes or mitosomes , having lost their mitochondria secondarily. They obtain energy by enzymatic action in the cytoplasm. Plants and various groups of algae have plastids as well as mitochondria. Plastids, like mitochondria, have their own DNA and are developed from endosymbionts , in this case cyanobacteria . They usually take
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#17328691894371804-415: Is more difficult to control in the cytoplasm. The majority of the genes in the mitochondria and plastids are related to the expression (transcription, translation and replication) of genes encoding proteins involved in either photosynthesis (in plastids) or cellular respiration (in mitochondria). One might predict that the loss of photosynthesis or cellular respiration would allow for the complete loss of
1886-618: Is much larger than that of prokaryotes. The eukaryotes seemingly emerged within the Asgard archaea , and are closely related to the Heimdallarchaeia . This implies that there are only two domains of life , Bacteria and Archaea, with eukaryotes incorporated among the Archaea. Eukaryotes first emerged during the Paleoproterozoic , likely as flagellated cells. The leading evolutionary theory
1968-532: Is that their cells have nuclei . This gives them their name, from the Greek εὖ ( eu , "well" or "good") and κάρυον ( karyon , "nut" or "kernel", here meaning "nucleus"). Eukaryotic cells have a variety of internal membrane-bound structures, called organelles , and a cytoskeleton which defines the cell's organization and shape. The nucleus stores the cell's DNA , which is divided into linear bundles called chromosomes ; these are separated into two matching sets by
2050-463: Is the alternative to the cDNA hypothesis, stating that escaped DNA, rather than mRNA, is the mechanism of gene transfer. According to this hypothesis, disturbances to organelles, including autophagy (normal cell destruction), gametogenesis (the formation of gametes), and cell stress release DNA which is imported into the nucleus and incorporated into the nuclear DNA using non-homologous end joining (repair of double stranded breaks). For example, in
2132-719: Is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria , plastids such as chloroplasts , and possibly other organelles of eukaryotic cells are descended from formerly free-living prokaryotes (more closely related to the Bacteria than to the Archaea ) taken one inside the other in endosymbiosis . Mitochondria appear to be phylogenetically related to Rickettsiales bacteria, while chloroplasts are thought to be related to cyanobacteria . The idea that chloroplasts were originally independent organisms that merged into
2214-427: Is the sister clade to Cryptista + Archaeplastida. Based on work done by Silar 2016. Alveidea Centrohelea Rappephyceae Pavlovophyceae Prymnesiophyceae Endohelea Picomonadea Telonemea Palpitea Leucocryptea Goniomonadea Cryptophyceae Eukaryote The eukaryotes ( / j uː ˈ k ær i oʊ t s , - ə t s / yoo- KARR -ee-ohts, -əts ) constitute
2296-864: Is they were created by symbiogenesis between an anaerobic Asgard archaean and an aerobic proteobacterium , which formed the mitochondria . A second episode of symbiogenesis with a cyanobacterium created the plants, with chloroplasts . Eukaryotic cells contain membrane-bound organelles such as the nucleus , the endoplasmic reticulum , and the Golgi apparatus . Eukaryotes may be either unicellular or multicellular . In comparison, prokaryotes are typically unicellular. Unicellular eukaryotes are sometimes called protists . Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion ( fertilization ). Eukaryotes are organisms that range from microscopic single cells , such as picozoans under 3 micrometres across, to animals like
2378-443: Is thought to occur in tobacco plants, which show a high rate of gene transfer and whose cells contain multiple chloroplasts. In addition, the bulk flow hypothesis is also supported by the presence of non-random clusters of organelle genes, suggesting the simultaneous movement of multiple genes. Ford Doolittle proposed that (whatever the mechanism) gene transfer behaves like a ratchet, resulting in unidirectional transfer of genes from
2460-495: Is unclear whether this group is monophyletic or not; results of phylogenetic studies are "often dependent on the selection of taxa and gene data set". Two 2012 studies produced opposite results. In the past, heterokonts , haptophytes , and cryptomonads have sometimes been grouped together in a group known as chromists . Though the heterokonts are now split out, Cryptophyta and Haptophyta are considered in some studies to be closely related (and are sometimes simply referred to as
2542-1241: The Excavata , has been abandoned as a formal group as it is paraphyletic . The proposed phylogeny below includes only one group of excavates ( Discoba ), and incorporates the 2021 proposal that picozoans are close relatives of rhodophytes. The Provora are a group of microbial predators discovered in 2022. Ancyromonadida [REDACTED] Malawimonada [REDACTED] CRuMs [REDACTED] Amoebozoa [REDACTED] Breviatea [REDACTED] Apusomonadida [REDACTED] Holomycota (inc. fungi) [REDACTED] Holozoa (inc. animals) [REDACTED] ? Metamonada [REDACTED] Discoba [REDACTED] Cryptista [REDACTED] Rhodophyta (red algae) [REDACTED] Picozoa [REDACTED] Glaucophyta [REDACTED] Viridiplantae (plants) [REDACTED] Hemimastigophora [REDACTED] Provora [REDACTED] Haptista [REDACTED] Telonemia [REDACTED] Rhizaria [REDACTED] Alveolata [REDACTED] Stramenopiles [REDACTED] [REDACTED] Symbiogenesis Symbiogenesis ( endosymbiotic theory , or serial endosymbiotic theory )
Hacrobia - Misplaced Pages Continue
2624-516: The SAR group , the latter cluster with the Archaeplastida (plants in the broad sense). As of February 2012, it remains unclear whether the Hacrobia forms a monophyletic group. Another study suggested the following arrangement: centrohelids are related to haptophytes and form the clade Haptista ; Haptista is the sister group to SAR; Cryptista are related to Archaeplastida; and Haptista + SAR
2706-518: The archaea —having a volume of around 10,000 times greater. Eukaryotes represent a small minority of the number of organisms , but, as many of them are much larger, their collective global biomass (468 gigatons) is far larger than that of prokaryotes (77 gigatons), with plants alone accounting for over 81% of the total biomass of Earth . The eukaryotes are a diverse lineage, consisting mainly of microscopic organisms . Multicellularity in some form has evolved independently at least 25 times within
2788-583: The blue whale , weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long, or plants like the coast redwood , up to 120 metres (390 ft) tall. Many eukaryotes are unicellular; the informal grouping called protists includes many of these, with some multicellular forms like the giant kelp up to 200 feet (61 m) long. The multicellular eukaryotes include the animals, plants, and fungi , but again, these groups too contain many unicellular species . Eukaryotic cells are typically much larger than those of prokaryotes —the bacteria and
2870-449: The domain of Eukaryota or Eukarya , organisms whose cells have a membrane-bound nucleus . All animals , plants , fungi , and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes : the Bacteria and the Archaea . Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collective global biomass
2952-673: The endoplasmic reticulum and the Golgi apparatus , both being parts of the endomembrane system. The syntrophy hypothesis, proposed by López-García and Moreira around the year 2000, suggested that eukaryotes arose by combining the metabolic capabilities of an archaean, a fermenting deltaproteobacterium, and a methanotrophic alphaproteobacterium which became the mitochondrion. In 2020, the same team updated their syntrophy proposal to cover an Asgard archaean that produced hydrogen with deltaproteobacterium that oxidised sulphur. A third organism, an alphaproteobacterium able to respire both aerobically and anaerobically, and to oxidise sulphur, developed into
3034-508: The sexual reproduction of the rest of the cell; that the chromosomes of some mitochondria and plastids are single circular DNA molecules similar to the circular chromosomes of bacteria; that the transport proteins called porins are found in the outer membranes of mitochondria and chloroplasts, and also bacterial cell membranes; and that cardiolipin is found only in the inner mitochondrial membrane and bacterial cell membranes. The Russian botanist Konstantin Mereschkowski first outlined
3116-510: The taxonomic rank of kingdom by Linnaeus in the 18th century. 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 various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologist Georg A. Goldfuss coined the word Protozoa to refer to organisms such as ciliates , and this group
3198-451: The "Cryptophyta+Haptophyta" group). A 2009 paper suggested that the Telonemia and centrohelids may form a clade with the cryptophytes and haptophytes. The picobiliphytes may belong in this group but are too poorly known to be classified with confidence. Several recent studies have concluded that Haptophyta and Cryptophyta do not form a monophyletic group. The former are a sister group to
3280-468: The French scientist Paul Jules Portier published Les Symbiotes , in which he claimed that the mitochondria originated from a symbiosis process. Ivan Wallin advocated the idea of an endosymbiotic origin of mitochondria in the 1920s. The Russian botanist Boris Kozo-Polyansky became the first to explain the theory in terms of Darwinian evolution . In his 1924 book A New Principle of Biology. Essay on
3362-473: The Great Oxidation Event might be a consequence of eukaryogenesis, and its impact on the export and burial of organic carbon. Some endosymbiont genes remain in the organelles. Plastids and mitochondria retain genes encoding rRNAs, tRNAs, proteins involved in redox reactions, and proteins required for transcription, translation, and replication. There are many hypotheses to explain why organelles retain
Hacrobia - Misplaced Pages Continue
3444-466: The Theory of Symbiogenesis , he wrote, "The theory of symbiogenesis is a theory of selection relying on the phenomenon of symbiosis." These theories did not gain traction until more detailed electron-microscopic comparisons between cyanobacteria and chloroplasts were made, such as by Hans Ris in 1961 and 1962. These, combined with the discovery that plastids and mitochondria contain their own DNA, led to
3526-511: The bulk flow hypothesis. The cDNA hypothesis involves the use of messenger RNA (mRNAs) to transport genes from organelles to the nucleus where they are converted to cDNA and incorporated into the genome. The cDNA hypothesis is based on studies of the genomes of flowering plants. Protein coding RNAs in mitochondria are spliced and edited using organelle-specific splice and editing sites. Nuclear copies of some mitochondrial genes, however, do not contain organelle-specific splice sites, suggesting
3608-403: The cell can 'try again' to have successful transfer of genes to the nucleus. In this ratchet-like way, genes from an organelle would be expected to accumulate in the nuclear genome over evolutionary time. Endosymbiotic theory for the origin of mitochondria suggests that the proto-eukaryote engulfed a protomitochondrion, and this endosymbiont became an organelle, a major step in eukaryogenesis ,
3690-709: The cell to move, change shape, or transport materials. The motor structures are microfilaments of actin and actin-binding proteins , including α- actinin , fimbrin , and filamin are present in submembranous cortical layers and bundles. Motor proteins of microtubules, dynein and kinesin , and myosin of actin filaments, provide dynamic character of the network. Many eukaryotes have long slender motile cytoplasmic projections, called flagella , or multiple shorter structures called cilia . These organelles are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin , and are entirely distinct from prokaryotic flagella. They are supported by
3772-404: The cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. The major polysaccharides making up the primary cell wall of land plants are cellulose , hemicellulose , and pectin . The cellulose microfibrils are linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in
3854-784: The chromatophore (1.02 Mb) and found that only 867 proteins were encoded by these photosynthetic cells. Comparisons with their closest free living cyanobacteria of the genus Synechococcus (having a genome size 3 Mb, with 3300 genes) revealed that chromatophores had undergone a drastic genome shrinkage. Chromatophores contained genes that were accountable for photosynthesis but were deficient in genes that could carry out other biosynthetic functions; this observation suggests that these endosymbiotic cells are highly dependent on their hosts for their survival and growth mechanisms. Thus, these chromatophores were found to be non-functional for organelle-specific purposes when compared with mitochondria and plastids. This distinction could have promoted
3936-427: The complex transcription machinery, the membrane-sorting systems, the nuclear pore , and some enzymes in the biochemical pathways. Eukaryote cells include a variety of membrane-bound structures, together forming the endomembrane system. Simple compartments, called vesicles and vacuoles , can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis , where
4018-452: The creation of the eukaryotes. Mitochondria are organelles that synthesize the energy-carrying molecule ATP for the cell by metabolizing carbon-based macromolecules . The presence of DNA in mitochondria and proteins, derived from mtDNA , suggest that this organelle may have been a prokaryote prior to its integration into the proto- eukaryote . Mitochondria are regarded as organelles rather than endosymbionts because mitochondria and
4100-622: The cytoplasm. Mitochondria are organelles in eukaryotic cells. The mitochondrion is commonly called "the powerhouse of the cell", for its function providing energy by oxidising sugars or fats to produce the energy-storing molecule ATP . Mitochondria have two surrounding membranes , each a phospholipid bilayer , the inner of which is folded into invaginations called cristae where aerobic respiration takes place. Mitochondria contain their own DNA , which has close structural similarities to bacterial DNA , from which it originated, and which encodes rRNA and tRNA genes that produce RNA which
4182-413: The early evolution of photosynthetic organelles. The loss of genetic autonomy, that is, the loss of many genes from endosymbionts, occurred very early in evolutionary time. Taking into account the entire original endosymbiont genome, there are three main possible fates for genes over evolutionary time. The first is the loss of functionally redundant genes, in which genes that are already represented in
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#17328691894374264-503: The estimate of c. 1.8 Gya. A 2.3 Gya estimate also seems reasonable, and has the added attraction of coinciding with one of the most pronounced biogeochemical perturbations in Earth history, the early Palaeoproterozoic Great Oxygenation Event . The marked increase in atmospheric oxygen concentrations at that time has been suggested as a contributing cause of eukaryogenesis, inducing the evolution of oxygen-detoxifying mitochondria. Alternatively,
4346-406: The eukaryotes. Complex multicellular organisms, not counting the aggregation of amoebae to form slime molds , have evolved within only six eukaryotic lineages: animals , symbiomycotan fungi , brown algae , red algae , green algae , and land plants . Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics. The defining feature of eukaryotes
4428-484: The eukaryotic evolutionary tree, core meiotic genes, and hence sex, were likely present in the common ancestor of eukaryotes. Species once thought to be asexual, such as Leishmania parasites, have a sexual cycle. Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently. In antiquity , the two lineages of animals and plants were recognized by Aristotle and Theophrastus . The lineages were given
4510-432: The example of the freshwater amoeboid , however, Paulinella chromatophora , which contains chromatophores found to be evolved from cyanobacteria, Keeling and Archibald argue that this is not the only possible criterion; another is that the host cell has assumed control of the regulation of the former endosymbiont's division, thereby synchronizing it with the cell's own division . Nowack and her colleagues gene sequenced
4592-646: The form of chloroplasts which, like cyanobacteria, contain chlorophyll and produce organic compounds (such as glucose ) through photosynthesis . Others are involved in storing food. Although plastids probably 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. The capture and sequestering of photosynthetic cells and chloroplasts, kleptoplasty , occurs in many types of modern eukaryotic organisms. The cytoskeleton provides stiffening structure and points of attachment for motor structures that enable
4674-532: The free-living organisms most closely related to plastids. Both cyanobacteria and alphaproteobacteria maintain a large (>6 Mb ) genome encoding thousands of proteins. Plastids and mitochondria exhibit a dramatic reduction in genome size when compared with their bacterial relatives. Chloroplast genomes in photosynthetic organisms are normally 120–200 kb encoding 20–200 proteins and mitochondrial genomes in humans are approximately 16 kb and encode 37 genes, 13 of which are proteins. Using
4756-420: The globe by combat, but by networking" (i.e., by cooperation). Christian de Duve proposed that the peroxisomes may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular oxygen in the Earth's atmosphere. However, it now appears that peroxisomes may be formed de novo , contradicting the idea that they have a symbiotic origin. The fundamental theory of symbiogenesis as
4838-503: The heterotrophic protist Hatena . This organism behaves like a predator until it ingests a green alga , which loses its flagella and cytoskeleton but continues to live as a symbiont. Hatena meanwhile, now a host, switches to photosynthetic nutrition, gains the ability to move towards light, and loses its feeding apparatus. Despite the diversity of organisms containing plastids, the morphology, biochemistry, genomic organisation, and molecular phylogeny of plastid RNAs and proteins suggest
4920-432: The host cell. On this view, membrane-bound bubbles or vesicles leaving the protomitochondria may have formed the nuclear envelope. The process of symbiogenesis by which the early eukaryotic cell integrated the proto- mitochondrion likely included protection of the archaeal host genome from the release of reactive oxygen species . These would have been formed during oxidative phosphorylation and ATP production by
5002-446: The host cells share some parts of their genome , undergo division simultaneously, and provide each other with means to produce energy. The endomembrane system and nuclear membrane were hypothesized to have derived from the protomitochondria . The presence of a nucleus is one major difference between eukaryotes and prokaryotes . Some conserved nuclear proteins between eukaryotes and prokaryotes suggest that these two types had
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#17328691894375084-426: The incorporation of symbiotic bacteria to form successively nuclei and chloroplasts . Mereschkowski knew of the work of botanist Andreas Schimper . In 1883, Schimper had observed that the division of chloroplasts in green plants closely resembled that of free-living cyanobacteria . Schimper had tentatively proposed (in a footnote) that green plants had arisen from a symbiotic union of two organisms. In 1918
5166-400: The initial stages of endosymbiosis, due to a lack of major gene transfer, the host cell had little to no control over the endosymbiont. The endosymbiont underwent cell division independently of the host cell, resulting in many "copies" of the endosymbiont within the host cell. Some of the endosymbionts lysed (burst), and high levels of DNA were incorporated into the nucleus. A similar mechanism
5248-442: The limit on transfer is due to differing genetic codes and RNA editing between the organelle and the nucleus. The redox control hypothesis states that genes encoding redox reaction proteins are retained in order to effectively couple the need for repair and the synthesis of these proteins. For example, if one of the photosystems is lost from the plastid, the intermediate electron carriers may lose or gain too many electrons, signalling
5330-408: The lysis of the single plastid would likely result in cell death. Consistent with this hypothesis, organisms with multiple plastids show an 80-fold increase in plastid-to-nucleus gene transfer compared with organisms with single plastids. There are many lines of evidence that mitochondria and plastids including chloroplasts arose from bacteria. Primary endosymbiosis involves the engulfment of
5412-525: The mitochondrion; it may possibly also have been able to photosynthesise. The question of when the transition from prokaryotic to eukaryotic form occurred and when the first crown group eukaryotes appeared on earth is unresolved. The oldest known body fossils that can be positively assigned to the Eukaryota are acanthomorphic acritarchs from the 1.631 Gya Deonar Formation of India. These fossils can still be identified as derived post-nuclear eukaryotes with
5494-435: The need for repair of a photosystem. The time delay involved in signalling the nucleus and transporting a cytosolic protein to the organelle results in the production of damaging reactive oxygen species . The final hypothesis states that the assembly of membrane proteins, particularly those involved in redox reactions, requires coordinated synthesis and assembly of subunits; however, translation and protein transport coordination
5576-424: The nuclear membrane also formed the endomembrane system. Prokaryotes do not have a complex internal membrane network like eukaryotes, but they could produce extracellular vesicles from their outer membrane. After the early prokaryote was consumed by a proto-eukaryote, the prokaryote would have continued to produce vesicles that accumulated within the cell. Interaction of internal components of vesicles may have led to
5658-446: The nuclear membrane may explain the adaptive benefit of this gene transfer. Modern eukaryotic cells use the endomembrane system to transport products and wastes in, within, and out of cells. The membrane of nuclear envelope and endomembrane vesicles are composed of similar membrane proteins. These vesicles also share similar membrane proteins with the organelle they originated from or are traveling towards. This suggests that what formed
5740-428: The nucleus are eventually lost. The second is the transfer of genes to the nucleus, while the third is that genes remain in the organelle that was once an organism. The loss of autonomy and integration of the endosymbiont with its host can be primarily attributed to nuclear gene transfer. As organelle genomes have been greatly reduced over evolutionary time, nuclear genes have expanded and become more complex. As
5822-406: The organelle to the nuclear genome. When genetic material from an organelle is incorporated into the nuclear genome, either the organelle or nuclear copy of the gene may be lost from the population. If the organelle copy is lost and this is fixed, or lost through genetic drift, a gene is successfully transferred to the nucleus. If the nuclear copy is lost, horizontal gene transfer can occur again, and
5904-416: The origin of mitochondria and chloroplasts is now widely accepted. Biologists usually distinguish organelles from endosymbionts – whole organisms living inside other organisms – by their reduced genome sizes . As an endosymbiont evolves into an organelle, most of its genes are transferred to the host cell genome . The host cell and organelle therefore need to develop a transport mechanism that enables
5986-449: The outer membrane invaginates and then pinches off to form a vesicle. Some cell products can leave in a vesicle through exocytosis . The nucleus is surrounded by a double membrane known as the nuclear envelope , with nuclear pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum , which is involved in protein transport and maturation. It includes
6068-499: The plastid genome or the mitochondrial genome respectively. While there are numerous examples of mitochondrial descendants ( mitosomes and hydrogenosomes ) that have lost their entire organellar genome, non-photosynthetic plastids tend to retain a small genome. There are two main hypotheses to explain this occurrence: The essential tRNA hypothesis notes that there have been no documented functional plastid-to-nucleus gene transfers of genes encoding RNA products (tRNAs and rRNAs). As
6150-417: The primary cell wall is xyloglucan . Eukaryotes have a life cycle that involves sexual reproduction , alternating between a haploid phase, where only one copy of each chromosome is present in each cell, and a diploid phase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such as eggs and spermatozoa , to form a zygote ; this may grow into
6232-582: The products of its photosynthesis, but once the prey item dies (or is lost) the host returns to a free living state. Obligate secondary endosymbionts become dependent on their organelles and are unable to survive in their absence. A secondary endosymbiosis event involving an ancestral red alga and a heterotrophic eukaryote resulted in the evolution and diversification of several other photosynthetic lineages including Cryptophyta , Haptophyta , Stramenopiles (or Heterokontophyta) , and Alveolata . A possible secondary endosymbiosis has been observed in process in
6314-416: The proto-mitochondrion. The nuclear membrane may have evolved as an adaptive innovation for protecting against nuclear genome DNA damage caused by reactive oxygen species. Substantial transfer of genes from the ancestral proto-mitochondrial genome to the nuclear genome likely occurred during early eukaryotic evolution. The greater protection of the nuclear genome against reactive oxygen species afforded by
6396-457: The return of the protein products needed by the organelle but now manufactured by the cell. Alphaproteobacteria were formerly thought to be the free-living organisms most closely related to mitochondria. Later research indicates that mitochondria are most closely related to Pelagibacterales bacteria, in particular, those in the SAR11 clade. Nitrogen-fixing filamentous cyanobacteria are
6478-498: The rough endoplasmic reticulum, covered in ribosomes which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles ( cisternae ), the Golgi apparatus . Vesicles may be specialized; for instance, lysosomes contain digestive enzymes that break down biomolecules in
6560-410: The several original mitochondrial chromosomes. A unicellular marine alga, Braarudosphaera bigelowii (a coccolithophore , which is a eukaryote), has been found with a cyanobacterium as an endosymbiont. The cyanobacterium forms a nitrogen-fixing structure, dubbed the nitroplast . It divides evenly when the host cell undergoes mitosis, and many of its proteins derive from the host alga, implying that
6642-537: The theory of symbiogenesis (from Greek : σύν syn "together", βίος bios "life", and γένεσις genesis "origin, birth") in his 1905 work, The nature and origins of chromatophores in the plant kingdom , and then elaborated it in his 1910 The Theory of Two Plasms as the Basis of Symbiogenesis, a New Study of the Origins of Organisms . Mereschkowski proposed that complex life-forms had originated by two episodes of symbiogenesis,
6724-498: Was expanded until Ernst Haeckel made it a kingdom encompassing all single-celled eukaryotes, the Protista , in 1866. The eukaryotes thus came to be seen as four kingdoms: The protists were at that time thought to be "primitive forms", and thus an evolutionary grade , united by their primitive unicellular nature. Understanding of the oldest branchings in the tree of life only developed substantially with DNA sequencing , leading to
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