Eukaryotes are organisms whose cells have a sealed nucleus in the membrane, unlike Prokaryotes (Bacteria and other Archaea). Eukaryotes belong to the Eukaryotes or Eukarya domain. Their name comes from the Greek language ?? ( eu , "good" or "right") and ?????? ( karyon , "bean" or "kernel"). Eukaryotic cells also contain other membrane-bound organelles such as mitochondria and Golgi apparatus, and in addition, some plant and algae cells contain chloroplasts. Unlike archaea and unicellular bacteria, eukaryotes can also be multicellular and include organisms that comprise many types of cells that make up various types of tissues.
Eukaryotes can reproduce asexually through mitosis and sexuality through meiotic 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 haploid cells. It acts as a genital cell (gametes). Each gamete has only one set of chromosomes, each unique mixture of the parent pair of chromosomes produced from genetic recombination during meiosis.
The Eukaryota domain seems monophyletic, and is one of the domains of life in a three domain system. The other two domains, Bacteria and Archaea, are prokaryotes and do not have the above features. Eukaryotes represent a small minority of all living things. However, due to their generally larger size, their collective biomass worldwide is estimated to be equivalent to prokaryotes. Eukaryotes evolved about 1.6-2.1 billion years ago, during the proterozoic eon.
Video Eukaryote
Histori
In 1905 and 1910, Russian biologist Konstantin Mereschkowski (1855-1921) argued that plastids reduce cyanobacteria in symbiosis with a non-photosynthetic (heterotrophic) host formed by a symbiosis between the amoeba host and bacteria-like cells that make up the nucleus. Plants have inherited photosynthesis from cyanobacteria.
The concept of eukaryotes has been attributed to the French biologist Edouard Chatton (1883-1947). The term prokaryotes and eukaryotes is more definitively reintroduced by Canadian microbiologist Roger Stanier and the Dutch-American microbiologist CB van Niel in 1962. In his work in 1938 Titres et Travaux Scientifiques Chatton proposed two terms, summoning prokaryotic bacteria and organisms with nuclei in their eukaryotic cell cells. But he mentioned this in only one paragraph, and the idea was effectively ignored until Chatton's remark was rediscovered by Stanier and van Niel.
In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in eukaryotic cells in his paper, at the origin of mitotic cells. In 1970, Carl Woese explored microbial phylogenetics, studying variations. in 16S RNA ribosome. It helps to reveal the origin of eukaryotes and symbiogenesis of two important eukaryotic organelles, mitochondria and chloroplasts. In 1977, Woese and George Fox introduced the "third life form", which they called Archaebacteria; in 1990, Woese, Otto Kandler and Mark L. Wheeler renamed this Archaea.
In 1979, G. W. Gould and G. J. Dring suggested that eukaryotic cell nuclei stem from the ability of Gram-positive bacteria to form endospores. In 1987 and later papers, Thomas Cavalier-Smith suggested otherwise that the nuclear membrane and endoplasmic reticulum were first formed by moistening the prokaryotic plasma membrane. In the 1990s, several other biologists proposed an endosymbiotic origin for the nucleus, effectively reviving the Mereschkowsky theory.
Maps Eukaryote
Cell features
Eukaryotic cells are usually much larger than prokaryotic cells that have a volume about 10,000 times larger than prokaryotic cells. They have a variety of internal membrane-bound structures, called organelles, and cytoskeleton consisting of microtubules, microfilaments, and medium filaments, which play an important role in defining the organization and the cell shape. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by microtubule spindles during nuclear division.
Internal membranes
The eukaryotic cells include various structures bound to the membrane, collectively referred to as the endomembrane system. Simple compartments, called vesicles and vacuoles, can be formed by growing another membrane. Many cells ingest food and other materials through the process of endocytosis, where the outer membrane evolved and then pinching to form vesicles. Perhaps most of the membrane-bound organelles eventually come from those vesicles. Alternatively, some products produced by cells can leave vesicles through exocytosis.
The nucleus is surrounded by a double membrane (commonly referred to as nuclear membrane or nuclear envelope), with pores that allow the material to move in and out. Various extensions of nuclear membrane-like and tube-like membranes form the endoplasmic reticulum, which is involved in protein transport and maturation. These include a rough endoplasmic reticulum in which the ribosome is attached to synthesize proteins, which enter the interior or lumen space. Furthermore, they generally enter the vesicles, which move from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in the flattened vesicles (cisternae), Golgi apparatus.
Vesicles can be devoted to a variety of purposes. For example, lysosomes contain digestive enzymes that break down most of the biomolecules in the cytoplasm. Peroxisomes are used to break down peroxides, which are toxic. Many protozoa have a contractile vacuole, which collects and secretes excess water, and extrusomes, which release materials used to deflect predators or capture prey. In higher plants, much of the cell volume is taken up by a central vacuole, which contains mostly water and primarily maintains its osmotic pressure.
Mitochondria and plastids
Mitochondria are organelles found in almost all eukaryotes that provide energy to cells by converting sugars into ATP. They have two adjacent membranes (each layer-phospholipid), the inside being folded into an invagination called cristae, in which aerobic respiration occurs. Mitochondria contain their own DNA. They are now commonly held to have been developed from endosymbiotic prokaryotes, probably proteobacteria. Protozoa and microbes that have no mitochondria, such as Pelomyxa amoebozoan and metamonads such as Giardia and Trichomonas , have usually been found to contain organelles derived from mitochondria, such as hydrogenosomes and myosomes. , and thus may lose secondary mitochondria. They gain energy by enzymatic action on nutrients absorbed from the environment. Metamonad Monocercomonoides is also obtained, with lateral gene transfer, a cytosolic sulfur mobilization system that provides the group of iron and sulfur required for protein synthesis. Normal sulfur-sulfur group sulfur pathways have disappeared secondary.
Plants and various groups of algae also have plastids. Plastids have their own DNA and are developed from endosymbionts, in this case cyanobacteria. They usually take the form of chloroplasts, which like cyanobacteria contain chlorophyll and produce organic compounds (such as glucose) through photosynthesis. Others are involved in food storage. Although plastids may have a single origin, not all groups containing plastids are closely related. In contrast, some eukaryotes have acquired it from others through secondary endosimbiosis or consumption. The collection and storage of photosynthetic cells and chloroplasts occurs in many types of modern eukaryotic organisms and is known as kleptoplasty.
The origin of endosymbiotics has also been proposed for the nucleus, and for eukaryotic flagella.
Cytoskeletal Structure
Many eukaryotes have long slender motile cytoplasm projection, called flagella, or similar structures called cilia. Flagella and cilia are sometimes referred to as undulipodia, and various are involved in movement, eating, and sensation. They are mostly composed of tubulin. This is entirely different from the prokaryotic flagellae. They are supported by microtubule bundles arising from centrioles, which are typically arranged as nine twins around two singlets. Flagella may also have hairs, or mastigonemes, and scales that connect internal membranes and rods. Their interiors are continuous with cell cytoplasm.
The mycophilic structure comprising actin and actinic binding proteins, for example, β-actinin, fimbrin, filamins are present in layers and submembranous cortex bundles. Microtubule motor proteins, for example, dynein or kinesin and actin, for example, myosins provide the dynamic character of the network.
Centrioles are often present even in cells and groups that do not have flagella, but conifers and flowering plants do not have. They generally occur in groups that give rise to various microtubule roots. It forms a major component of the cytoskeletal structure, and often converges over several cell divisions, with one flagella retained from the parent and the other coming from it. Centriol produces spindles during nuclear division.
The importance of cytoskeletal structures is underlined in the determination of cellular forms, and they are an important component of migratory responses such as chemotaxis and chemokinesis. Some protists have a variety of other organelles that are supported by microtubules. These include radiolaria and heliozoa, which produce axopodia used in flotation or to capture prey, and haptophytes, which have a unique flagellum-like organelle called haptonema.
Cell wall
Plant cells and algae, fungi and most chromalveolates have cell walls, layers outside the cell membrane, providing cells with structural support, protection, and filtering mechanisms. The cell wall also prevents over-expansion when water enters the cell.
The main polysaccharides that make up the main cell wall of a land plant are cellulose, hemicellulose, and pectin. Cellulosic microfibrils are connected via the hemicellulosic tether to form cellulose-hemicellulose tissue, embedded in a pectin matrix. The most common hemicellulose in primary cell wall is xyloglucan.
Difference between eukaryotic cells
There are many types of eukaryotic cells, although animals and plants are the best known eukaryotes, and thus provide an excellent starting point for understanding eukaryotic structures. Mushrooms and many protists have some substantial differences.
Animal cell
All animals are eukaryotic. The animal cells differ from other eukaryote cells, especially plants, because they do not have cell walls and chloroplasts and have smaller vacuoles. Due to the lack of cell walls, animal cells can adopt various forms. Phagocytic cells can even ingest other structures.
Plant cell
Plant cells are very different from those of other eukaryotic organisms. Their distinctive features are:
- A large central vacuole (enclosed by membrane, tonoplast), which maintains cell turgor and controls the movement of molecules between the cytosol and the sap
- The main cell wall containing cellulose, hemicellulose and pectin, is deposited by protoplasts on the outside of the cell membrane; this contrasts with the cell wall of the fungus, which contains chitin, and the prokaryotic envelope cell, where the peptidoglycan is the principal structural molecule
- Plasmodesmata, the pores in the cell wall connecting adjacent cells and allowing plant cells to communicate with adjacent cells. Animals have different pause crossing systems but functionally function among adjacent cells.
- Plastids, especially chlorophyll-containing chloroplasts, pigments that give green color to plants and allow them to photosynthesize
- Bryophytes and seedless vascular plants have only flagella and centriol in sperm cells. The cyclic sperm and Ginkgo are large and complex cells that swim with hundreds to thousands of flagellae.
- Conifers (Pinophyta) and flowering plants (Angiospermae) do not have flagella and centrioles present in animal cells.
Mushroom cells
The fungal cells are most similar to animal cells, with the following exceptions:
- Cell wall containing chitin
- Less definition between cells; The higher mushroom hyphae has a porous partition called septa, which allows the passage of cytoplasm, organelle, and, occasionally, nuclei. The primitive fungi have little or no septa, so each organism is essentially a gigantic multinuclear supercell; This fungus is described as coenocytic.
- Only the most primitive mushrooms, chytrid, have flagella.
Other eukaryotic cells
Some groups of eukaryotes have unique organelles, such as cyanelles (unusual chloroplasts) of glaucophytes, haptonema from haptophytes, or ejectosomes of cryptomonads. Other structures, such as pseudopodia, are found in various groups of eukaryotes in various forms, such as amozozoan lobose or reticuloid foraminiferans.
Reproduction
Cell division usually occurs asexually by mitosis, a process that allows each daughter's nucleus to receive one copy of each chromosome. Most eukaryotes also have life cycles involving sexual reproduction, alternating between the haploid phases, where only one copy of each chromosome exists in every cell and diploid phase, where two copies of each chromosome exist in each cell. The diploid phase is formed by the fusion of two haploid gametes to form a zygote, which can divide by mitosis or undergo chromosome reduction with meiosis. There are many variations in this pattern. Animals do not have a multicellular haploid phase, but each plant generation may consist of haploid and diploid multicellular phases.
Eukaryotes have a smaller surface area ratio than prokaryotes, and thus have lower metabolic rates and longer generation times.
The evolution of sexual reproduction can be a primordial and fundamental characteristic of eukaryotes. Based on phylogenetic analysis, Dacks and Roger propose that facultative sex is present in the same progenitor of all eukaryotes. A set of core 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 deviate from the beginning of the eukaryotic evolutionary tree, it is concluded that the meiotic genes of the nucleus, and hence sex, are likely present in the same progenitor of all eukaryotes. Eukaryotic species once considered asexual, such as the parasitic protozoa of the genus Leishmania , have been shown to have a sexual cycle. Also, evidence now suggests that amoeba, previously regarded as asexual, are essentially sexual and that the majority of current asexual groups may have emerged recently and independently.
Classification
In ancient times, two lineages of animals and plants were recognized. They were given the rank of the Royal taxonomy by Linnaeus. Although he incorporated mushrooms with crops with several objections, it was later realized that they were very different and guaranteed separate kingdoms, compositions not quite clear until the 1980s. Various single cell eukaryotes were originally placed with plants or animals when they became known. In 1830, German biologist Georg A. Goldfuss coined the word protozoa to refer to organisms such as ciliates, and this group expanded to include all single-celled eukaryotes, and given their own empire, Protista, by Ernst Haeckel in 1866. Thus, eukaryotes consist of four kingdoms:
- Royal Protista
- Royal Plantae
- Great Mushrooms
- Animalia Kingdom
Protists are understood as "primitive forms", and thus represent the value of evolution, united by their primitive unicellular nature. The separation of deep divisions in the tree of life only really begins with DNA sequencing, which leads to the domain system rather than the kingdom as the top rank advanced by Carl Woese, unifying all eukaryotes under the eukaryotes domain. At the same time, working on the protist tree is intensified, and still active take place today. Several alternative classifications have been forwarded, although there is no consensus on the ground.
Eukaryotes is a clade that is usually considered a brother of Heimdallarchaeota in the Asgard group at Archaea. Basal groupings are Opimoda, Diphoda, Discoba, and Loukozoa. Eukaryotic roots are usually rated close to or even in Discoba.
Classification produced in 2005 for the Protista International Society, which reflects the consensus of time, divides eukaryotes into six 'supergroups' considered monophyletic. However, in the same year (2005), doubts were expressed whether some of these monophyletic supergroups, notably Chromalveolata, and reviews in 2006 noted a lack of evidence for some of the six supposed suspected supergroups. The revised classification in 2012 recognizes five supergroups.
There is also a small group of eukaryotes whose positions are uncertain or appear to be outside a large group - in particular, Haptophyta, Cryptophyta, Centrohelida, Telonemia, Picozoa, Apusomonadida, Ancyromonadida, Breviatea, and Collodictyon genus. Overall, it seems that, although progress has been made, there is still a very significant uncertainty in the evolutionary history and eukaryotes classification. As Roger & amp; Simpson said in 2009 "with the pace of change in our understanding of the eukaryotic life tree, we must proceed with caution."
In an article published in Nature Microbiology in April 2016 the authors, "reinforced once again that the life we ​​see around us - plants, animals, humans and so-called eukaryotes - represents a small percentage of the world's biodiversity. "They classify eukaryotes" based on their legacy information systems as opposed to lipids or other cellular structures. " Jillian F. Banfield of the University of California, Berkeley and his fellow scientists used a super computer to generate diagrams from new life trees based on DNA from 3000 species including 2,072 known species and 1,011 newly reported microbes, whose DNA they have collected from an environment diverse. Because the capacity of sequencing DNA becomes easier, Banfield and the team can perform metagenomic sequencing - "to sequence the entire community of organisms at once and select individual groups based on their own genes."
Phylogeny
The rRNA trees constructed during the 1980s and 1990s left most eukaryotes in an unsolved "crown" group (technically not a real crown), usually divided by the shape of mitochondrial cristae; see the eukaryotes crown. Some groups that lack mitochondria branch off separately, and therefore absence is believed to be primitive; but these are now regarded as long branch pull artifacts, and they are known to have lost them secondary.
In 2011, there was widespread agreement that Rhizaria belonged to Stramenopiles and Alveolata, in a clause dubbed the supergroup SAR, so Rhizaria was not one of the main groups of eukaryotes; also that the Amoebozoa and Opisthokonta are monophyletic and clade-shaped, often called uniqueonts. Beyond this, there seems to be no consensus.
It is estimated that there may be 75 different lineages from eukaryotes. Most of these lineages are protists.
The known size of the eukaryotic genome varies from 8.2 megabases (Mb) in Babesia bovis to 112,000-220,050 Mb in dinoflagellate Prorocentrum micans , indicating that the ancestral eukaryotic genome has experienced considerable variation during its evolution. The last common ancestor of all eukaryotes is believed to have become a phagotrophic protista with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex (meiosis and syngamy), dormant cysts with chitin and/or cellulose and peroxisome cell walls. Then endosymbiosis causes the spread of plastids in some lineages.
Five supergroups
A global eukaryotes tree of a consensus of phylogenetic evidence (especially, phylogenetic), rare genomic signatures, and morphological characteristics are presented in Adl et al. 2012 and Burki 2014/2016 with Cryptophyta and picozoa have appeared in Archaeplastida. Similar inclusions of Glaucophyta, Cryptista (and also, remarkably, Haptista) have also been made.
In some analyzes, the Hacrobia group (Haptophyta Cryptophyta) is placed next to the Archaeplastida, but in the other it lodges inside the Archaeplastida. However, several recent studies have concluded that Haptophyta and Cryptophyta do not form monophyletic groups. The former can be a group of brothers for the SAR group, the last group with Archaeplastida (plants in the broadest sense).
The division of eukaryotes into two main clade, bikonts (Archaeplastida SAR Excavata) and uniqueonts (Amoebozoa Opisthokonta), derived from ancestral biflagellar organisms and ancestral uniflagellar organisms, respectively, have been suggested previously. A 2012 study yielded a somewhat similar division, although it noted that the terms "uniqueonts" and "bikont" were not used in the original sense.
A highly convergent and congruent set of trees appears in Derelle et al (2015), Ren et al (2016), Yang et al (2017) and Cavalier-Smith (2015) including supplementary information, resulting in more conservative and consolidated trees. This is combined with some results from Cavalier-Smith for Basal Opimoda. The main controversies left are roots, and the exact position of Rhodophyta and Rhizaria bikonts, Haptista, Cryptista, Picozoa and Telonemia, many of which may be eukaryotes eukaryotes. Archeaplastida develops possible Chloroplasts by the endosymbiosis of an ancestor associated with the currently existing cyanobacterium, Gloeomargarita lithophora.
Cavalier-Smith Tree
Thomas Cavalier-Smith 2010, 2013, 2014, 2017, and 2018 put the eukaryotic tree roots between the Excavata (with ventral groove supported by microtubule roots) and unbroken Eugo Eugenepa, and monophyletic Chromista, correlated with one endosymlycotic event catching red - algae. He et al specifically supports the eukaryotic rooting tree between the monopiletic Discoba (Discicristata Jakobida) and the Amorphea-Diaphoretickes clade.
The origin of eukaryotes
Fossils
The origin of eukaryotic cells is a milestone in the evolution of life, because eukaryotes encompass all the complex cells and almost all multicellular organisms. The timing of this series of events is difficult to determine; Knoll (2006) states that they grew about 1.6 to 2.1 billion years ago. Some acritarchs are known from at least 1.65 billion years ago, and the possibility of Grypania algae has been found as far back as 2.1 billion years ago. Geosiphon -as a fossilized fungus Diskagma has been found in paleosol 2.2 billion years.
Organized living structures have been found in the black flakes of the Frenchvillian Palaeoproterozoic B Formation in Gabon, 2.1 billion years old. Eukaryotic life can evolve at that time. Clear fossils associated with modern groups began to emerge about 1.2 billion years ago, in the form of red algae, although recent work shows the existence of fossil filament algae in the Vindhya basin that probably dates from 1.6 to 1.7 billion years ago then.
Biomarkers show that at least the eukaryotes of stems appear earlier. The presence of steranes in Australian flakes indicates that eukaryotes present in these rocks are dated to 2.7 billion years, although it is suggested they may be derived from sample contamination.
Whenever their origins, eukaryotes may not become the dominant ecologist until much later; Massive uptick in zinc sea sediment composition 800 million years ago has been associated with a large population increase of eukaryotes, which preferentially consume and incorporate zinc relative to prokaryotes.
Relationship with Archaea
Nuclear DNA and eukaryotic genetic machinery are more similar to Archaea than Bacteria, leading to controversial suggestions that eukaryotes should be grouped with Archaea in the Neomura clade. In other respects, such as membrane composition, eukaryotes are similar to Bacteria. Three main explanations for this have been put forward:
- Eukaryotes are produced from a complete blend of two or more cells, in which the cytoplasm is formed from eubacterium, and the nucleus of the archaeon, from the virus, or from the pre-cell.
- Eukaryotes developed from Archaea, and gained their eubacterial characteristics through endosymbiosis from proto-mitochondria of eubacterial origin.
- Eukaryotes and Archaea are developed separately from modified eubacterium.
Alternative proposals include:
- The cronite hypothesis postulates that primitive eukaryotic cells are formed by endosymbiosis from both archaea and bacteria by a third cell type, called chronocyte.
- The universal universal ancestor (UCA) of the tree of life today is a complex organism that survived mass extinction events rather than early stages in the evolution of life. Certain eukaryotes and akaryote (Bacteria and Archaea) evolved through reductive reductions, resulting in the similarity of results from the original feature differential retention.
Assuming no other groups are involved, there are three possible phylogeny for Bacteria, Archaea and Eukaryotes where each is monophyletic. It is labeled 1 to 3 in the table below. The eocyte hypothesis is a modification of hypothesis 2 in which Archaea is paraphyletic. (Tables and names for hypotheses are based on Harish and Kurland, 2017.)
In recent years, most researchers liked either three domains (3D) or the eocyte hypothesis. The rRNA analysis supports the eocyte scenario, apparently with Eukaryotic roots in Excavata. Cladograms that support the eocyte hypothesis, positioning eukaryotes in Archaea, based on phylogenetic analysis of Asgard's arch, are:
In this scenario, Asgard groups are seen as TACK's taxi brothers, consisting of Crenarchaeota (formerly eocytes), Thaumarchaeota, and others.
By 2017, there is a significant boost to this scenario, arguing that eukaryotes do not appear in Archaea. Cunha et al. produces an analysis that supports three domains (3D) or Woese hypotheses (2 in the table above) and rejects the eocyte hypothesis (4 above). Harish and Kurland find strong support for the previous two kingdoms (2D) or the Mayr hypothesis (1 in the above table), based on an analysis of the protein domain coding circuit. They reject the eocyte hypothesis as the most likely. A possible interpretation of their analysis is that the universal common ancestor (UCA) of the tree of life today is a complex organism that survives evolutionary obstacles, not the simpler organisms that emerged at the beginning of life's history.
Endomembrane and mitochondrial systems
The origins of endomembrane and mitochondrial systems are also unclear. The phagotrophic hypothesis proposes that eukaryotic type membranes that do not have cell walls originate first, with the development of endocytosis, whereas mitochondria are obtained by swallowing as endosymbionts. The syntrophic hypothesis suggests that proto-eukaryotes rely on proto-mitochondrion for food, and eventually grow around it. Here the membrane originates after swallowing the mitochondria, thanks in part to the mitochondrial genes (the hydrogen hypothesis is one particular version).
In a study using the genome to build supertrees, Pisani et al. (2007) states that, together with evidence that there are never eukaryotes without mitochondrion, eukaryotes evolved from a syntrophy between archaea that is closely related to Thermoplasmatales and -proteobacterium, possibly symbiotic driven by sulfur or hydrogen. Mitochondria and their genomes are the remains of endosymbiont -proteobacteria.
Hypothesis
Various hypotheses have been proposed about how eukaryotic cells become present. This hypothesis can be classified into two distinct classes - autogenous models and chimeric models.
Autogenous model
The autogenous model proposes that proto-eukaryotic cells containing the nucleus exist first, and then acquire the mitochondria. According to this model, large prokaryotes develop invagination in the plasma membrane to obtain sufficient surface area to serve the cytoplasmic volume. When invagination is distinguished in function, some become separate compartments - causing the endomembranous system, including the endoplasmic reticulum, golgi apparatus, nuclear membrane, and single membranous structures such as lysosomes. The proposed mitochondria are derived from endosymbiosis from aerobic proteobacterium, and it is assumed that all eukaryotic lineages that do not get mitochondria become extinct. Chloroplasts arise from other endosymbiotic events involving cyanobacteria. Since all eukaryotes have mitochondria, but not all have chloroplasts, serial endosymbiosis theory suggests that mitochondria arise first.
Chimeric Model
The Chimeric model claims that two prokaryotic cells exist initially - an archaeon and a bacterium. These cells undergo a process of incorporation, either by physical fusion or by endosimbiosis, thus leading to the formation of eukaryotic cells. In this chimeric model, several studies further claim that mitochondria originates from bacterial ancestors while others emphasize the role of the endosymbiotic process behind the origin of mitochondria.
Based on the process of symbiotic symbiosis, the hypothesis can be categorized as - serial endosymbiotic theory (SET), hydrogen hypothesis (most symbiotic processes in which hydrogen transfer occurs between different species), and hypothesis syntrophy.
According to serial endosymbiotic theory (championed by Lynn Margulis), the union between anaerobic motile bacteria (such as Spirochaeta) and thermoacidophilic crenarchaeon (such as sulfidogenic thermoplasm) spawns eukaryotes today. This union forms a motile organism capable of living in acidic waters and sulfur that already exist. Oxygen is known to cause toxicity in organisms that do not have the required metabolic machinery. Thus, archaeons provide bacteria with highly beneficial reduced environments (sulfur and sulphate are reduced to sulphides). In microaerophilic conditions, oxygen is reduced to water thus creating a mutually beneficial platform. Bacteria on the other hand, contributes to the necessary fermentation products and electron acceptor along with motility features to the arkeon thus gaining motility for swimming organisms. From a consortium of bacterial DNA and archaea derived from the nuclear genome of eukaryotic cells. Spirochetes give rise to features of eukaryotic cell motile. Endosymbiotic unification of the ancestors alpha-proteobacteria and cyanobacteria, causing the origin of mitochondria and plastids respectively. For example, Thiodendron has been known to originate by ectosimbiotic processes based on similar synthetic sulfur present between two types of bacteria - Desulphobacter and Spirochaeta . However, such relationships based on motile symbiosis have never been observed practically. There is also no evidence of archaeans and spirochetes that adapt to a strong acid-based environment.
In the hydrogen hypothesis, the symbiotic relationship of anaerobic and autotrophic methanogenic archaeon (host) with alpha-proteobacterium (symbionts) gives rise to eukaryotes. The host uses hydrogen (H 2 ) and carbon dioxide (CO 2 ) to produce methane while the symbionts, capable of aerobic respiration, excrete H 2 and CO 2 as a by-product of the anaerobic fermentation process. The host's methanogenic environments work as sinks for H 2 , which results in increased bacterial fermentation. Endosymbiotic gene transfer (EGT) acts as a catalyst for hosts to obtain the metabolism of symbionar carbohydrates and to alter heterotrophic in nature. Furthermore, the ability of the methane formation of the host is lost. Thus, the origin of the heterotrophic organelle (symbiont) is identical with the origin of the eukaryotic lineage. In this hypothesis, the presence of H 2 represents a selective force that faked eukaryotes from prokaryotes.
The syntrophy hypothesis was developed in contrast to the hydrogen hypothesis and proposed the existence of two symbiotic events. According to this theory, the origin of eukaryotic cells is based on a metabolic symbiosis (syntrophy) between methanogenic archaeon and delta-proteobacterium. This syntrophic symbiosis was initially facilitated by H 2 transfers between different species under the anaerobic environment. In the early stages, alpha-proteobacterium becomes a member of this integration, and later develops into mitochondria. Gene transfer from the delta-proteobacterium to the archaeon causes the methanogenic archaeon to develop into the nucleus. Archaeon is a genetic tool, while delta-proteobacterium contributes to cytoplasmic features. This theory combines two selective forces at the time of nuclear evolution - (a) the presence of metabolic partitions to avoid the harmful effects of co-existence of anabolic and catabolic cellular pathways, and (b) the prevention of abnormal protein biosynthesis as very broad. the spread of introns in the archaeal gene after obtaining the lost mitochondria and methanogenesis.
See also
- The evolution of sexual reproduction
- List of eukaryotic sequence genomes
- Parikaryote
- Prokaryote
- Thaumarchaeota
- Vault (organelle)
References
This article incorporates public domain material from the NCBI document "Science Primer".
External links
- Eukaryotes (Tree of Life website)
- Eukaryotes in the Encyclopedia of Life
Source of the article : Wikipedia
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