The dictyostelids are a group of cellular slime molds, or social amoebae. When food is available dictyostelids behave as individual amoebae, which feed and divide normally. However, when the food supply is exhausted, they aggregate to form a multicellular assembly, called a pseudoplasmodium, grex, or slug; the grex has a definite anterior and posterior, responds to light and temperature gradients, has the ability to migrate. Under the correct circumstances the grex matures forming a sorocarp with a stalk supporting one or more sori; these spores are inactive cells protected by resistant cell walls, become new amoebae once food is available. In Acytostelium, the sorocarp is supported by a stalk composed of cellulose, but in other dictyostelids the stalk is composed of cells, sometimes taking up the majority of the original amoebae. With a few exceptions, these cells die during stalk formation, there is a definite correspondence between parts of the grex and parts of the fruiting body. Aggregation of amoebae takes place in converging streams.
The amoebae move using filose pseudopods, are attracted to chemicals produced by other amoebae. In Dictyostelium, aggregation is signalled by cAMP. In the species Dictyostelium purpureum, the grouping is by kinship, not just by proximity. Dictyostelium has been used as a model organism in molecular biology and genetics, is studied as an example of cell communication and programmed cell death, it is an interesting example of the evolution of cooperation and cheating. A large body of research data concerning D. discoideum is available on-line at DictyBase. The mechanism behind the aggregation of the amoebae relies on cyclic adenosine monophosphate as a signal molecule. One cell, the founder of the colony, begins to secrete cAMP in response to stress. Others detect this signal, respond in two ways: The amoeba moves towards the signal; the amoeba secretes more cAMP to boost the signal. The effect of this is to relay the signal throughout the nearby population of amoebae and cause inward movement to the area of highest cAMP concentration.
Within an individual cell, the mechanism is as follows: cAMP reception at the cell membrane activates a G-protein G protein stimulates Adenylate cyclase cAMP diffuses out of cell into medium Internal cAMP inactivates the external cAMP receptor. A different g-protein stimulates Phospholipase C IP3 induces calcium ion release Calcium ions act on the cytoskeleton to induce the extension of pseudopodia; because the internal cAMP concentration inactivates the receptor for external cAMP, an individual cell shows oscillatory behaviour. This behaviour produces beautiful spirals seen in converging colonies and is reminiscent of the Belousov-Zhabotinsky reaction and two-dimensional cyclic cellular automata; the entire genome of Dictyostelium discoideum was published in Nature in 2005 by geneticist Ludwig Eichinger and coworkers. The haploid genome contains 12,500 genes on 6 chromosomes. For comparison, the diploid human genome has 20,000-25,000 genes on 23 chromosome pairs. There is a high level of the nucleotides adenosine and thymidine leading to a codon usage that favors more adenosines and thymidines in the third position.
Tandem repeats of trinucleotides are abundant in Dictyostelium, which in humans cause Trinucleotide repeat disorders. Sexual development can occur when amoeboid cells are starved for their bacterial food supply and dark humid conditions are present. Both heterothallic and homothallic strains of Dictostelium can undergo mating. Heterothallic sexual development has been most extensively studied in D. discoideum, homothallic sexual development has been most well studied in D. mucoroides. Heterothallic matings are initiated by fusion of haploid cells from two strains of opposite mating type; this contrasts with homothallic strains. Mating is initiated by gametogenesis that produces small, motile gametes that fuse to form a small binucleate cell; the volume of the binucleate cell increases to produce a giant binuclear cell. As growth proceeds, the nuclei swell, fuse forming a true diploid zygote giant cell; as this is occurring, amoebae have been undergoing cAMP-induced chemotaxis towards the giant cell surface.
This forms a cellular aggregate and at the center of the aggregate the zygote giant cell ingests the surrounding amoebae. Phagocytosis is followed by digestion of the ingested amoebae. Next the zygote forms a macrocyst characterized by a surrounding extracellular cellulose sheath. After the macrocyst is formed it ordinarily remains dormant for a period before germination can occur. Within the macrocyst the diploid zygote undergoes meiosis followed by successive mitotic divisions; when the macrocyst germinates it releases many haploid amoeboid cells. The first dictyostelid to be described was Dictyostelium mucoroides in 1869 by Oskar Brefeld. First discovered in a North Carolina forest in 1935, Dictyostelium discoideum was at first classified under'lower fungi.' and in subsequent years into the kingdoms Protoctista and Tubulomitochondrae. By the 1990s, most scientists accepted the current classification. Amoebozoa are now considered by most to form a separate kingdom-level clade, being more related to both animals and fungi than to plants.
Dictyostelium shares many molecular features with the human host of Legionella. The cytoskeletal composition of D. discoideum is similar to that of mammalian cells as are the processes driven by these components, such as phagocytos
A trophozoite is the activated, feeding stage in the life cycle of certain protozoan parasites such as in the malaria-causing Plasmodium falciparum and those of the Giardia group.. Trophozoite and cyst stages are shown in the life cycle of Balantidium coli the causative agent of balantidiasis. In the apicomplexan life cycle the trophozoite undergoes schizogony and develops into a schizont which contains merozoites; the trophozoite life stage of Giardia proliferates in the small intestine. Trophozoites develop during the course of the infection into cysts, the infectious life stage
Amoebozoa is a major taxonomic group containing about 2,400 described species of amoeboid protists possessing blunt, lobose pseudopods and tubular mitochondrial cristae. In most classification schemes, Amoebozoa is ranked as a phylum within either the kingdom Protista or the kingdom Protozoa. In the classification favored by the International Society of Protistologists, it is retained as an unranked "supergroup" within Eukaryota. Molecular genetic analysis supports Amoebozoa as a monophyletic clade. Most phylogenetic trees identify it as the sister group to Opisthokonta, another major clade which contains both fungi and animals as well as some 300 species of unicellular protists. Amoebozoa and Opisthokonta are sometimes grouped together in a high-level taxon, variously named Unikonta, Amorphea or Opimoda. Amoebozoa includes many of the best-known amoeboid organisms, such as Chaos, Entamoeba and the genus Amoeba itself. Species of Amoebozoa may be either shelled, or naked, cells may possess flagella.
Free-living species are common in both salt and freshwater as well as soil and leaf litter. Some live as parasites or symbiotes of other organisms, some are known to cause disease in humans and other organisms. While the majority of amoebozoan species are unicellular, the group includes several varieties of slime molds, which have a macroscopic, multicellular stage of life during which individual amoeboid cells aggregate to produce spores. Amoebozoa vary in size; some are only 10 -- 20 μm in diameter. The well-known species Amoeba proteus, which may reach 800 μm in length, is studied in schools and laboratories as a representative cell or model organism because of its convenient size. Multinucleate amoebae like Chaos and Pelomyxa may be several millimetres in length, some multicellular amoebozoa, such as the "dog vomit" slime mold Fuligo septica, can cover an area of several square meters. Amoebozoa is a large and diverse group; the amoebozoan cell is divided into a granular central mass, called endoplasm, a clear outer layer, called ectoplasm.
During locomotion, the endoplasm flows forwards and the ectoplasm runs backwards along the outside of the cell. In motion, many amoebozoans have a defined anterior and posterior and may assume a "monopodial" form, with the entire cell functioning as a single pseudopod. Large pseudopods may produce numerous clear projections called subpseudopodia, which are extended to a certain length and retracted, either for the purpose of locomotion or food intake. A cell may form multiple indeterminate pseudopodia, through which the entire contents of the cell flow in the direction of locomotion; these are more or less tubular and are filled with granular endoplasm. The cell mass flows into a leading pseudopod, the others retract, unless the organism changes direction. While most amoebozoans are "naked," like the familiar Amoeba and Chaos, or covered with a loose coat of minute scales, like Cochliopodium and Korotnevella, members of the order Arcellinida form rigid shells, or tests, equipped with a single aperture through which the pseudopods emerge.
Arcellinid tests may be secreted from organic materials, as in Arcella, or built up from collected particles cemented together, as in Difflugia. In all amoebozoa, the primary mode of nutrition is phagocytosis, in which the cell surrounds potential food particles with its pseudopods, sealing them into vacuoles within which they may be digested and absorbed; some amoebozoans have a posterior bulb called a uroid, which may serve to accumulate waste, periodically detaching from the rest of the cell. When food is scarce, most species can form cysts, which may be carried aerially and introduce them to new environments. In slime moulds, these structures are called spores, form on stalked structures called fruiting bodies or sporangia; the majority of Amoebozoa lack flagella and more do not form microtubule-supported structures except during mitosis. However, flagella do occur among the Archamoebae, many slime moulds produce biflagellate gametes; the flagellum is anchored by a cone of microtubules, suggesting a close relationship to the opisthokonts.
The mitochondria in amoebozoan cells characteristically have branching tubular cristae. However, among the Archamoebae, which are adapted to anoxic or microaerophilic habitats, mitochondria have been lost, it appears that the members of Amoebozoa form a sister group to animals and fungi, diverging from this lineage after it had split from the other groups, as illustrated below in a simplified diagram: Strong similarities between Amoebozoa and Opisthokonts lead to the hypothesis that they form a distinct clade. Thomas Cavalier-Smith proposed the name "unikonts" for this branch, whose members were believed to have been descended from a common ancestor possessing a single emergent flagellum rooted in one basal body. However, while the close relationship between Amoebozoa and Opisthokonta is robustly supported, recent work has shown that the hypothesis of a uniciliate ancestor is false. In their Revised Classification of Eukaryotes, Adl et al. proposed Amorphea as a more suitable name for a clade of the same composition, a sister group to the Diaphoretickes.
More recent work places the members of Amorphea together with the malawimonids and collodictyonids in a proposed clade called Opimoda, which comprises one of two major lineages diverging at the root of the eukaryote tree of life. Traditionally all amoebozoa with lobose pseudopods were grouped together in the class L
Commensalism is a long-term biological interaction in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit from each other, where one is harmed while the other is unaffected, parasitism, where one benefits while the other is harmed; the commensal may obtain nutrients, support, or locomotion from the host species, unaffected. The commensal relation is between a larger host and a smaller commensal. Both remora and pilot fish feed on the leftovers of their hosts' meals. Numerous birds perch on bodies of large mammal herbivores or feed on the insects turned up by grazing mammals; the word "commensalism" is derived from the word "commensal", meaning "eating at the same table" in human social interaction, which in turn comes through French from the Medieval Latin commensalis, meaning "sharing a table", from the prefix com-, meaning "together", mensa, meaning "table" or "meal".
Commensality, at Oxford and Cambridge Universities, refers to professors eating at the same table as students. Pierre-Joseph van Beneden introduced the term "commensalism" in 1876; the commensal pathway was traveled by animals that fed on refuse around human habitats or by animals that preyed on other animals drawn to human camps. Those animals established a commensal relationship with humans in which the animals benefited but the humans received little benefit or harm; those animals that were most capable of taking advantage of the resources associated with human camps would have been the ‘tamer’ individuals: less aggressive, with shorter fight-or-flight distances. These animals developed closer social or economic bonds with humans and led to a domestic relationship; the leap from a synanthropic population to a domestic one could only have taken place after the animals had progressed from anthropophily to habituation, to commensalism and partnership, at which point the establishment of a reciprocal relationship between animal and human would have laid the foundation for domestication, including captivity and human-controlled the breeding.
From this perspective, animal domestication is a coevolutionary process in which a population responds to selective pressure while adapting to a novel niche that includes another species with evolving behaviors. Commensal pathway animals include dogs, cats and pigs; the dog was the first domesticated animal, was domesticated and established across Eurasia before the end of the Pleistocene, well before the cultivation of crops or the domestication of other animals. The dog is hypothesised to be a classic example of a domestic animal that traveled a commensal pathway into domestication. Archaeological evidence, such as the Bonn-Oberkassel dog dating to ~14,000BP, supports the hypothesis that dog domestication preceded the emergence of agriculture and began close to the Last Glacial Maximum when hunter-gatherers preyed on megafauna; the wolves more drawn to human camps were the less-aggressive, subdominant pack members with lowered flight response, higher stress thresholds, less wary around humans, therefore better candidates for domestication.
Proto-dogs might have taken advantage of carcasses left on site by early hunters, assisted in the capture of prey, or provided defense from large competing predators at kills. However, the extent to which proto-domestic wolves could have become dependent on this way of life prior to domestication and without human provisioning is unclear and debated. In contrast, cats may have become dependent on a commensal lifestyle before being domesticated by preying on other commensal animals, such as rats and mice, without any human provisioning. Debate over the extent to which some wolves were commensal with humans prior to domestication stems from debate over the level of human intentionality in the domestication process, which remains untested; the earliest sign of domestication in dogs was the neotonization of skull morphology and the shortening of snout length that results in tooth crowding, reduction in tooth size, a reduction in the number of teeth, attributed to the strong selection for reduced aggression.
This process may have begun during the initial commensal stage of dog domestication before humans began to be active partners in the process. A mitochondrial, Y-chromosome assessment of two wolf populations in North America combined with satellite telemetry data revealed significant genetic and morphological differences between one population that migrated with and preyed upon caribou and another territorial ecotype population that remained in a boreal coniferous forest. Although these two populations spend a period of the year in the same place, though there was evidence of gene flow between them, the difference in prey–habitat specialization has been sufficient to maintain genetic and coloration divergence. A different study has identified the remains of a population of extinct Pleistocene Beringian wolves with unique mitochondrial signatures; the skull shape, tooth wear, isotopic signatures suggested these remains were derived from a population of specialist megafauna hunters and scavengers that became extinct while less specialized wolf ecotypes survived.
Analogous to the modern wolf ecotype that has evolved to track and prey upon caribou, a Pleistocene wolf population could have begun following mobile hunter-gatherers, thus acquiring genetic and ph
A base pair is a unit consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA and RNA. Dictated by specific hydrogen bonding patterns, Watson–Crick base pairs allow the DNA helix to maintain a regular helical structure, subtly dependent on its nucleotide sequence; the complementary nature of this based-paired structure provides a redundant copy of the genetic information encoded within each strand of DNA. The regular structure and data redundancy provided by the DNA double helix make DNA well suited to the storage of genetic information, while base-pairing between DNA and incoming nucleotides provides the mechanism through which DNA polymerase replicates DNA and RNA polymerase transcribes DNA into RNA. Many DNA-binding proteins can recognize specific base-pairing patterns that identify particular regulatory regions of genes. Intramolecular base pairs can occur within single-stranded nucleic acids.
This is important in RNA molecules, where Watson–Crick base pairs permit the formation of short double-stranded helices, a wide variety of non-Watson–Crick interactions allow RNAs to fold into a vast range of specific three-dimensional structures. In addition, base-pairing between transfer RNA and messenger RNA forms the basis for the molecular recognition events that result in the nucleotide sequence of mRNA becoming translated into the amino acid sequence of proteins via the genetic code; the size of an individual gene or an organism's entire genome is measured in base pairs because DNA is double-stranded. Hence, the number of total base pairs is equal to the number of nucleotides in one of the strands; the haploid human genome is estimated to be about 3.2 billion bases long and to contain 20,000–25,000 distinct protein-coding genes. A kilobase is a unit of measurement in molecular biology equal to 1000 base pairs of DNA or RNA; the total amount of related DNA base pairs on Earth is estimated at 5.0×1037 and weighs 50 billion tonnes.
In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC. Hydrogen bonding is the chemical interaction. Appropriate geometrical correspondence of hydrogen bond donors and acceptors allows only the "right" pairs to form stably. DNA with high GC-content is more stable than DNA with low GC-content. But, contrary to popular belief, the hydrogen bonds do not stabilize the DNA significantly; the larger nucleobases and guanine, are members of a class of double-ringed chemical structures called purines. Purines are complementary only with pyrimidines: pyrimidine-pyrimidine pairings are energetically unfavorable because the molecules are too far apart for hydrogen bonding to be established. Purine-pyrimidine base-pairing of AT or GC or UA results in proper duplex structure; the only other purine-pyrimidine pairings would be AC and GT and UG. The GU pairing, with two hydrogen bonds, does occur often in RNA. Paired DNA and RNA molecules are comparatively stable at room temperature, but the two nucleotide strands will separate above a melting point, determined by the length of the molecules, the extent of mispairing, the GC content.
Higher GC content results in higher melting temperatures. On the converse, regions of a genome that need to separate — for example, the promoter regions for often-transcribed genes — are comparatively GC-poor. GC content and melting temperature must be taken into account when designing primers for PCR reactions; the following DNA sequences illustrate pair double-stranded patterns. By convention, the top strand is written from the 5' end to the 3' end. A base-paired DNA sequence: ATCGATTGAGCTCTAGCG TAGCTAACTCGAGATCGCThe corresponding RNA sequence, in which uracil is substituted for thymine in the RNA strand: AUCGAUUGAGCUCUAGCG UAGCUAACUCGAGAUCGC Chemical analogs of nucleotides can take the place of proper nucleotides and establish non-canonical base-pairing, leading to errors in DNA replication and DNA transcription; this is due to their isosteric chemistry. One common mutagenic base analog is 5-bromouracil, which resembles thymine but can base-pair to guanine in its enol form. Other chemicals, known as DNA intercalators, fit into the gap between adjacent bases on a single strand and induce frameshift mutations by "masquerading" as a base, causing the DNA replication machinery to skip or insert additional nucleotides at the intercalated site.
Most intercalators are known or suspected carcinogens. Examples include ethidium acridine. An unnatural base pair is a designed subunit of DNA, created in a laboratory and does not occur in nature. DNA sequences have been described which use newly created nucleobases to form a third base pair, in addition to the two ba
In cell biology, the nucleus is a membrane-bound organelle found in eukaryotic cells. Eukaryotes have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, a few others including osteoclasts have many; the cell nucleus contains all of the cell's genome, except for a small fraction of mitochondrial DNA, organized as multiple long linear DNA molecules in a complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are structured in such a way to promote cell function; the nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, the control center of the cell. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, the nuclear matrix, a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole.
Because the nuclear envelope is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope. The pores cross both nuclear membranes, providing a channel through which larger molecules must be transported by carrier proteins while allowing free movement of small molecules and ions. Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes. Although the interior of the nucleus does not contain any membrane-bound subcompartments, its contents are not uniform, a number of nuclear bodies exist, made up of unique proteins, RNA molecules, particular parts of the chromosomes; the best-known of these is the nucleolus, involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA; the nucleus was the first organelle to be discovered. What is most the oldest preserved drawing dates back to the early microscopist Antonie van Leeuwenhoek.
He observed the nucleus, in the red blood cells of salmon. Unlike mammalian red blood cells, those of other vertebrates still contain nuclei; the nucleus was described by Franz Bauer in 1804 and in more detail in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of London. Brown was studying orchids under the microscope when he observed an opaque area, which he called the "areola" or "nucleus", in the cells of the flower's outer layer, he did not suggest a potential function. In 1838, Matthias Schleiden proposed that the nucleus plays a role in generating cells, thus he introduced the name "cytoblast", he believed that he had observed new cells assembling around "cytoblasts". Franz Meyen was a strong opponent of this view, having described cells multiplying by division and believing that many cells would have no nuclei; the idea that cells can be generated de novo, by the "cytoblast" or otherwise, contradicted work by Robert Remak and Rudolf Virchow who decisively propagated the new paradigm that cells are generated by cells.
The function of the nucleus remained unclear. Between 1877 and 1878, Oscar Hertwig published several studies on the fertilization of sea urchin eggs, showing that the nucleus of the sperm enters the oocyte and fuses with its nucleus; this was the first time. This was in contradiction to Ernst Haeckel's theory that the complete phylogeny of a species would be repeated during embryonic development, including generation of the first nucleated cell from a "monerula", a structureless mass of primordial mucus. Therefore, the necessity of the sperm nucleus for fertilization was discussed for quite some time. However, Hertwig confirmed his observation in other animal groups, including amphibians and molluscs. Eduard Strasburger produced the same results for plants in 1884; this paved the way to assign the nucleus an important role in heredity. In 1873, August Weismann postulated the equivalence of the maternal and paternal germ cells for heredity; the function of the nucleus as carrier of genetic information became clear only after mitosis was discovered and the Mendelian rules were rediscovered at the beginning of the 20th century.
The nucleus is the largest organelle in animal cells. In mammalian cells, the average diameter of the nucleus is 6 micrometres, which occupies about 10% of the total cell volume; the contents of the nucleus are held in the nucleoplasm similar to the cytoplasm in the rest of the cell. The fluid component of this is termed the nucleosol, similar to the cytosol in the cytoplasm. In most types of granulocyte, a white blood cell, the nucleus is lobated and can be bi-lobed, tri-lobed or multi-lobed; the nuclear envelope, otherwise known as nuclear membrane, consists of two cellular membranes, an inner and an outer membrane, arranged parallel to one another and separated by 10 to 50 nanometres. The nuclear envelope encloses the nucleus and separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing between the nucleoplasm and the cytoplasm; the outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum, is studded with ribosomes.
The space between the membranes is called the perinuclear space and is continuous with the RER lumen. Nuclear pores, which provide aqueous cha
The mitochondrion is a double-membrane-bound organelle found in most eukaryotic organisms. Some cells in some multicellular organisms may, lack them. A number of unicellular organisms, such as microsporidia and diplomonads, have reduced or transformed their mitochondria into other structures. To date, only one eukaryote, Monocercomonoides, is known to have lost its mitochondria; the word mitochondrion comes from the Greek μίτος, mitos, "thread", χονδρίον, chondrion, "granule" or "grain-like". Mitochondria generate most of the cell's supply of adenosine triphosphate, used as a source of chemical energy. A mitochondrion was thus termed the powerhouse of the cell. Mitochondria are between 0.75 and 3 μm in diameter but vary in size and structure. Unless stained, they are not visible. In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, cell death, as well as maintaining control of the cell cycle and cell growth. Mitochondrial biogenesis is in turn temporally coordinated with these cellular processes.
Mitochondria have been implicated in several human diseases, including mitochondrial disorders, cardiac dysfunction, heart failure and autism. The number of mitochondria in a cell can vary by organism and cell type. For instance, red blood cells have no mitochondria, whereas liver cells can have more than 2000; the organelle is composed of compartments. These compartments or regions include the outer membrane, the intermembrane space, the inner membrane, the cristae and matrix. Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome that shows substantial similarity to bacterial genomes. Mitochondrial proteins vary depending on the species. In humans, 615 distinct types of protein have been identified from cardiac mitochondria, whereas in rats, 940 proteins have been reported; the mitochondrial proteome is thought to be dynamically regulated. The first observations of intracellular structures that represented mitochondria were published in the 1840s.
Richard Altmann, in 1890, established them as cell organelles and called them "bioblasts". The term "mitochondria" was coined by Carl Benda in 1898. Leonor Michaelis discovered that Janus green can be used as a supravital stain for mitochondria in 1900. In 1904, Friedrich Meves, made the first recorded observation of mitochondria in plants in cells of the white waterlily, Nymphaea alba and in 1908, along with Claudius Regaud, suggested that they contain proteins and lipids. Benjamin F. Kingsbury, in 1912, first related them with cell respiration, but exclusively based on morphological observations. In 1913, particles from extracts of guinea-pig liver were linked to respiration by Otto Heinrich Warburg, which he called "grana". Warburg and Heinrich Otto Wieland, who had postulated a similar particle mechanism, disagreed on the chemical nature of the respiration, it was not until 1925, when David Keilin discovered cytochromes, that the respiratory chain was described. In 1939, experiments using minced muscle cells demonstrated that cellular respiration using one oxygen atom can form two adenosine triphosphate molecules, and, in 1941, the concept of the phosphate bonds of ATP being a form of energy in cellular metabolism was developed by Fritz Albert Lipmann.
In the following years, the mechanism behind cellular respiration was further elaborated, although its link to the mitochondria was not known. The introduction of tissue fractionation by Albert Claude allowed mitochondria to be isolated from other cell fractions and biochemical analysis to be conducted on them alone. In 1946, he concluded that cytochrome oxidase and other enzymes responsible for the respiratory chain were isolated to the mitochondria. Eugene Kennedy and Albert Lehninger discovered in 1948 that mitochondria are the site of oxidative phosphorylation in eukaryotes. Over time, the fractionation method was further developed, improving the quality of the mitochondria isolated, other elements of cell respiration were determined to occur in the mitochondria; the first high-resolution electron micrographs appeared in 1952, replacing the Janus Green stains as the preferred way of visualising the mitochondria. This led to a more detailed analysis of the structure of the mitochondria, including confirmation that they were surrounded by a membrane.
It showed a second membrane inside the mitochondria that folded up in ridges dividing up the inner chamber and that the size and shape of the mitochondria varied from cell to cell. The popular term "powerhouse of the cell" was coined by Philip Siekevitz in 1957. In 1967, it was discovered. In 1968, methods were developed for mapping the mitochondrial genes, with the genetic and physical map of yeast mitochondrial DNA being completed in 1976. There are two hypotheses about the origin of mitochondria: autogenous; the endosymbiotic hypothesis suggests that mitochondria were prokaryotic cells, capable of implementing oxidative mechanisms that were not possible for eukaryotic cells. In the autogenous hypothesis, mitochondria were born by splitting off a portion of DNA from the nucleus of the eukaryotic cell at the time of divergence with the prokaryotes. Since mitochondria have many features in common with bacteria, the endosymbiotic hypothesis is more accepted. A mitochondrion contains DNA, which i