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104-446: The existence of a special structure at the ends of chromosomes was independently proposed in 1938 by Hermann Joseph Muller , studying the fruit fly Drosophila melanogaster , and in 1939 by Barbara McClintock , working with maize. Muller observed that the ends of irradiated fruit fly chromosomes did not present alterations such as deletions or inversions. He hypothesized the presence of a protective cap, which he coined "telomeres", from

208-459: A Unitarian church and considered himself a pantheist ; in high school, he became an atheist . He excelled in the public schools. At 16, he entered Columbia College . From his first semester, he was interested in biology; he became an early convert of the Mendelian - chromosome theory of heredity—and the concept of genetic mutations and natural selection as the basis for evolution . He formed

312-409: A haploid chromosomal composition that is partially replicated, a condition known as merodiploidy . Prokaryotes lack mitochondria and chloroplasts . Instead, processes such as oxidative phosphorylation and photosynthesis take place across the prokaryotic cell membrane . However, prokaryotes do possess some internal structures, such as prokaryotic cytoskeletons . It has been suggested that

416-688: A paraphyletic group, just like dinosaurs without birds. Unlike the above assumption of a fundamental split between prokaryotes and eukaryotes, the most important difference between biota may be the division between Bacteria and the rest (Archaea and Eukaryota). For instance, DNA replication differs fundamentally between the Bacteria and Archaea (including that in eukaryotic nuclei), and it may not be homologous between these two groups. Moreover, ATP synthase , though common (homologous) in all organisms, differs greatly between bacteria (including eukaryotic organelles such as mitochondria and chloroplasts ) and

520-1088: A prokaryotic cytoskeleton that is more primitive than that of the eukaryotes. Besides homologues of actin and tubulin ( MreB and FtsZ ), the helically arranged building-block of the flagellum , flagellin , is one of the most significant cytoskeletal proteins of bacteria, as it provides structural backgrounds of chemotaxis , the basic cell physiological response of bacteria. At least some prokaryotes also contain intracellular structures that can be seen as primitive organelles. Membranous organelles (or intracellular membranes) are known in some groups of prokaryotes, such as vacuoles or membrane systems devoted to special metabolic properties, such as photosynthesis or chemolithotrophy . In addition, some species also contain carbohydrate-enclosed microcompartments, which have distinct physiological roles (e.g. carboxysomes or gas vacuoles). Most prokaryotes are between 1 μm and 10 μm, but they can vary in size from 0.2 μm ( Mycoplasma genitalium ) to 750 μm ( Thiomargarita namibiensis ). Prokaryotic cells have various shapes;

624-401: A taxon to be found nearby the then-unknown Asgard group). For example, histones which usually package DNA in eukaryotic nuclei, have also been found in several archaean groups, giving evidence for homology . This idea might clarify the mysterious predecessor of eukaryotic cells ( eucytes ) which engulfed an alphaproteobacterium forming the first eucyte ( LECA , l ast e ukaryotic c ommon

728-495: A T-loop. G-quadruplexes present an obstacle for enzymes such as DNA-polymerases and are thus thought to be involved in the regulation of replication and transcription. Many organisms have a ribonucleoprotein enzyme called telomerase, which carries out the task of adding repetitive nucleotide sequences to the ends of the DNA. Telomerase "replenishes" the telomere "cap" and requires no ATP. In most multicellular eukaryotic organisms, telomerase

832-505: A T-loop. This loop is analogous to a knot, which stabilizes the telomere, and prevents the telomere ends from being recognized as breakpoints by the DNA repair machinery. Should non-homologous end joining occur at the telomeric ends, chromosomal fusion would result. The T-loop is maintained by several proteins, collectively referred to as the shelterin complex. In humans, the shelterin complex consists of six proteins identified as TRF1 , TRF2 , TIN2 , POT1 , TPP1 , and RAP1 . In many species,

936-1198: A bacterium to bind, take up and recombine donor DNA into its own chromosome, it must first enter a special physiological state called competence . About 40 genes are required in Bacillus subtilis for the development of competence. The length of DNA transferred during B. subtilis transformation can be as much as a third to the whole chromosome. Transformation is a common mode of DNA transfer, and 67 prokaryotic species are thus far known to be naturally competent for transformation. Among archaea, Halobacterium volcanii forms cytoplasmic bridges between cells that appear to be used for transfer of DNA from one cell to another. Another archaeon, Sulfolobus solfataricus , transfers DNA between cells by direct contact. Frols et al. (2008) found that exposure of S. solfataricus to DNA damaging agents induces cellular aggregation, and suggested that cellular aggregation may enhance DNA transfer among cells to provide increased repair of damaged DNA via homologous recombination. While prokaryotes are considered strictly unicellular, most can form stable aggregate communities. When such communities are encased in

1040-734: A biology club and also became a proponent of eugenics ; the connections between biology and society would be his perennial concern. Muller earned a Bachelor of Arts degree in 1910. Muller remained at Columbia (the pre-eminent American zoology program at the time, due to E. B. Wilson and his students) for graduate school. He became interested in the Drosophila genetics work of Thomas Hunt Morgan 's fly lab after undergraduate bottle washers Alfred Sturtevant and Calvin Bridges joined his biology club. In 1911–1912, he studied metabolism at Cornell University , but remained involved with Columbia. He followed

1144-635: A convention for drafting a world constitution . As a result, for the first time in human history, a World Constituent Assembly convened to draft and adopt a Constitution for the Federation of Earth . Muller had a daughter, Helen J. Muller, now a professor emerita at the University of New Mexico , who has a daughter, Mala Htun , also a professor at the University of New Mexico. His son, David E. Muller , professor emeritus of mathematics and computer science at

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1248-544: A debate with the perennial genetics gadfly Richard Goldschmidt over the existence of the gene, for which little direct physical evidence existed at the time. When Muller returned to the United States in 1940, he took an untenured research position at Amherst College , in the department of Otto C. Glaser . After the U.S. entry into World War II , his position was extended indefinitely and expanded to include teaching. His Drosophila work in this period focused on measuring

1352-573: A group of scientists organized at Geron Corporation by Geron's founder Michael D. West , that tied telomere shortening with the Hayflick limit. The cloning of the catalytic component of telomerase enabled experiments to test whether the expression of telomerase at levels sufficient to prevent telomere shortening was capable of immortalizing human cells. Telomerase was demonstrated in a 1998 publication in Science to be capable of extending cell lifespan, and now

1456-399: A higher metabolic rate , a higher growth rate, and as a consequence, a shorter generation time than eukaryotes. There is increasing evidence that the roots of the eukaryotes are to be found in (or at least next to) the archaean Asgard group, perhaps Heimdallarchaeota (an idea which is a modern version of the 1984 eocyte hypothesis , eocytes being an old synonym for Thermoproteota ,

1560-491: A major influence on telomere shortening in vivo . There is a multitude of ways in which oxidative stress, mediated by reactive oxygen species (ROS), can lead to DNA damage; however, it is yet unclear whether the elevated rate in telomeres is brought about by their inherent susceptibility or a diminished activity of DNA repair systems in these regions. Despite widespread agreement of the findings, widespread flaws regarding measurement and sampling have been pointed out; for example,

1664-531: A media sensation after he delivered a paper entitled "The Problem of Genetic Modification" at the Fifth International Congress of Genetics in Berlin ; it would make him one of the better-known public intellectuals of the early 20th century. By 1928, others had replicated his dramatic results, expanding them to other model organisms , such as wasps and maize . In the following years, he began publicizing

1768-521: A ncestor) according to endosymbiotic theory . There might have been some additional support by viruses, called viral eukaryogenesis . The non-bacterial group comprising archaea and eukaryota was called Neomura by Thomas Cavalier-Smith in 2002. However, in a cladistic view, Eukaryota are Archaea in the same sense as birds are dinosaurs because they evolved from the Maniraptora dinosaur group. In contrast, archaea without eukaryota appear to be

1872-659: A nucleus. Both eukaryotes and prokaryotes contain large RNA / protein structures called ribosomes , which produce protein , but the ribosomes of prokaryotes are smaller than those of eukaryotes. Mitochondria and chloroplasts , two organelles found in many eukaryotic cells, contain ribosomes similar in size and makeup to those found in prokaryotes. This is one of many pieces of evidence that mitochondria and chloroplasts are descended from free-living bacteria. The endosymbiotic theory holds that early eukaryotic cells took in primitive prokaryotic cells by phagocytosis and adapted themselves to incorporate their structures, leading to

1976-690: A similar group of selfish individuals (see inclusive fitness and Hamilton's rule ). Should these instances of prokaryotic sociality prove to be the rule rather than the exception, it would have serious implications for the way we view prokaryotes in general, and the way we deal with them in medicine. Bacterial biofilms may be 100 times more resistant to antibiotics than free-living unicells and may be nearly impossible to remove from surfaces once they have colonized them. Other aspects of bacterial cooperation—such as bacterial conjugation and quorum-sensing-mediated pathogenicity , present additional challenges to researchers and medical professionals seeking to treat

2080-473: A single founder (in the way that animals and plants are founded by single cells), which presents a number of theoretical issues. Most explanations of co-operation and the evolution of multicellularity have focused on high relatedness between members of a group (or colony, or whole organism). If a copy of a gene is present in all members of a group, behaviors that promote cooperation between members may permit those members to have (on average) greater fitness than

2184-679: A stabilizing polymer matrix ("slime"), they may be called " biofilms ". Cells in biofilms often show distinct patterns of gene expression (phenotypic differentiation) in time and space. Also, as with multicellular eukaryotes, these changes in expression often appear to result from cell-to-cell signaling , a phenomenon known as quorum sensing . Biofilms may be highly heterogeneous and structurally complex and may attach to solid surfaces, or exist at liquid-air interfaces, or potentially even liquid-liquid interfaces. Bacterial biofilms are often made up of microcolonies (approximately dome-shaped masses of bacteria and matrix) separated by "voids" through which

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2288-700: A suspected species and tissue dependency of oxidative damage to telomeres is said to be insufficiently accounted for. Population-based studies have indicated an interaction between anti-oxidant intake and telomere length. In the Long Island Breast Cancer Study Project (LIBCSP), authors found a moderate increase in breast cancer risk among women with the shortest telomeres and lower dietary intake of beta carotene, vitamin C or E. These results suggest that cancer risk due to telomere shortening may interact with other mechanisms of DNA damage, specifically oxidative stress. Although telomeres shorten during

2392-435: A third domain: Eukaryota . Prokaryotes evolved before eukaryotes, and lack nuclei, mitochondria , and most of the other distinct organelles that characterize the eukaryotic cell. It was once thought that prokaryotic cellular components were unenclosed within the cytoplasm except for an outer cell membrane , but bacterial microcompartments , which are thought to be quasi-organelles enclosed in protein shells (such as

2496-490: A trip intended as a limited sabbatical stretched into an eight-year, five-country journey. In Berlin, he met two physicists who would later be significant to the biology community: Niels Bohr and Max Delbrück . The Nazi movement was precipitating the rapid emigration of scientific talent from Germany, and Muller was particularly opposed to the politics of National Socialism. The FBI was investigating Muller because of his involvement with The Spark , so he chose instead to go to

2600-404: Is evidence on Mars of fossil or living prokaryotes. However, this possibility remains the subject of considerable debate and skepticism. The division between prokaryotes and eukaryotes is usually considered the most important distinction or difference among organisms. The distinction is that eukaryotic cells have a "true" nucleus containing their DNA , whereas prokaryotic cells do not have

2704-664: Is a single-cell organism whose cell lacks a nucleus and other membrane -bound organelles . The word prokaryote comes from the Ancient Greek πρό ( pró ), meaning 'before', and κάρυον ( káruon ), meaning 'nut' or 'kernel'. In the two-empire system arising from the work of Édouard Chatton , prokaryotes were classified within the empire Prokaryota . However in the three-domain system , based upon molecular analysis , prokaryotes are divided into two domains : Bacteria (formerly Eubacteria) and Archaea (formerly Archaebacteria). Organisms with nuclei are placed in

2808-400: Is active only in germ cells , some types of stem cells such as embryonic stem cells , and certain white blood cells . Telomerase can be reactivated and telomeres reset back to an embryonic state by somatic cell nuclear transfer . The steady shortening of telomeres with each replication in somatic (body) cells may have a role in senescence and in the prevention of cancer . This is because

2912-497: Is dominated by cross-sectional and correlational studies, which makes causal interpretation problematic. A 2020 review argued that the relationship between psychosocial stress and telomere length appears strongest for stress experienced in utero or early life. The phenomenon of limited cellular division was first observed by Leonard Hayflick , and is now referred to as the Hayflick limit . Significant discoveries were subsequently made by

3016-452: Is essential for telomere maintenance and capping. Multiple proteins binding single- and double-stranded telomere DNA have been identified. These function in both telomere maintenance and capping. Telomeres form large loop structures called telomere loops, or T-loops. Here, the single-stranded DNA curls around in a long circle, stabilized by telomere-binding proteins . At the very end of the T-loop,

3120-641: Is the Terminal Restriction Fragment (TRF) southern blot. There is a Web-based Analyser of the Length of Telomeres ( WALTER ), software processing the TRF pictures. A Real-Time PCR assay for telomere length involves determining the Telomere-to-Single Copy Gene (T/S) ratio, which is demonstrated to be proportional to the average telomere length in a cell. Tools have also been developed to estimate

3224-470: Is used to quantify the length of telomeres in human white blood cells. A semi-automated method for measuring the average length of telomeres with Flow FISH was published in Nature Protocols in 2006. While multiple companies offer telomere length measurement services, the utility of these measurements for widespread clinical or personal use has been questioned. Nobel Prize winner Elizabeth Blackburn , who

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3328-406: Is variable among taxa and linked to the method used for estimating telomere length. In contrast, the available information shows no sex differences in telomere length across vertebrates. Phylogeny and life history traits such as body size or the pace of life can also affect telomere dynamics. For example, it has been described across species of birds and mammals. In 2019, a meta-analysis confirmed that

3432-524: Is well-recognized as capable of immortalizing human somatic cells. Two studies on long-lived seabirds demonstrate that the role of telomeres is far from being understood. In 2003, scientists observed that the telomeres of Leach's storm-petrel ( Oceanodroma leucorhoa ) seem to lengthen with chronological age, the first observed instance of such behaviour of telomeres. A study reported that telomere length of different mammalian species correlates inversely rather than directly with lifespan, and concluded that

3536-1185: The American Academy of Arts and Sciences in 1942 and the American Philosophical Society in 1947 Muller was awarded the Linnean Society of London 's Darwin-Wallace Medal in 1958 and the Kimber Genetics Award of the U.S. National Academy of Sciences, of which he was a member, in 1955. He served as president of the American Humanist Association from 1956 to 1958. The American Mathematical Society selected him as its Gibbs Lecturer for 1958. He retired in 1964. The Drosophila basic units of inheritance, their chromosomal arms , are named " Muller elements " in Muller's honor. he died in 1967. H. J. Muller and science fiction writer Ursula K. Le Guin were first cousins once removed ; his father (Hermann J. Muller Sr.) and her father's mother (Johanna Muller Kroeber) were siblings,

3640-600: The Third International Eugenics Congress , Muller gave a speech and stated, "eugenics might yet perfect the human race, but only in a society consciously organized for the common good". In 1926, a series of major breakthroughs began. In November, Muller carried out two experiments with varied doses of X-rays, the second of which used the crossing over suppressor stock ("ClB") he had found in 1919. A clear, quantitative connection between radiation and lethal mutations quickly emerged. Muller's discovery created

3744-545: The USSR , and he helped edit and distribute an illegal leftist student newspaper, The Spark . It was a difficult period for Muller both scientifically and personally; his marriage was falling apart, and he was increasingly dissatisfied with his life in Texas. Meanwhile, the waning of the eugenics movement, ironically hastened by his own work pointing to the previously ignored connections between environment and genetics, meant that his ideas on

3848-560: The University of Illinois and at New Mexico State University , died in 2008 in Las Cruces, New Mexico . David's mother was Jessie Jacobs Muller Offermann (1890–1954), Hermann's first wife. Helen's mother was Dorothea Kantorowicz Muller (1909–1986), Hermann's second wife, who came to the U.S. in 1940 as a German Jewish refugee. He had a brief affair with Milly Bennett . Prokaryote A prokaryote ( / p r oʊ ˈ k ær i oʊ t , - ə t / ; less commonly spelled procaryote )

3952-468: The drosophilists as the first genetic maps emerged from Morgan's experiments, and joined Morgan's group in 1912 (after two years of informal participation). In the fly group, Muller's contributions were primarily theoretical—explanations for experimental results and ideas and predictions for new experiments. In the emerging collaborative culture of the drosophilists, however, credit was assigned based on results rather than ideas; Muller felt cheated when he

4056-555: The encapsulin protein cages ), have been discovered, along with other prokaryotic organelles . While being unicellular, some prokaryotes, such as cyanobacteria , may form colonies held together by biofilms , and large colonies can create multilayered microbial mats . Others, such as myxobacteria , have multicellular stages in their life cycles . Prokaryotes are asexual , reproducing via binary fission without any fusion of gametes , although horizontal gene transfer may take place. Molecular studies have provided insight into

4160-553: The 1958 petition to the United Nations, calling for an end to nuclear weapons testing, which was initiated by the Nobel Prize-winning chemist Linus Pauling . Muller's opinions on the effect of radiation on mutagenesis were used by Rachel Carson in her book Silent Spring , however, his opinions have been criticized by some scientists; geneticist James F. Crow called Muller's view "alarmist" and wrote that it created in

4264-480: The 3'-end of the template (corresponding to the potential 5'-end of the lagging-strand). Originally it was believed that the last primer would sit at the very end of the template, thus, once removed, the DNA-polymerase that substitutes primers with DNA (DNA-Pol δ in eukaryotes) would be unable to synthesize the "replacement DNA" from the 5'-end of the lagging strand so that the template nucleotides previously paired to

Telomere - Misplaced Pages Continue

4368-454: The Greek telos (end) and meros (part). In the early 1970s, Soviet theorist Alexey Olovnikov first recognized that chromosomes could not completely replicate their ends; this is known as the "end replication problem". Building on this, and accommodating Leonard Hayflick 's idea of limited somatic cell division, Olovnikov suggested that DNA sequences are lost every time a cell replicates until

4472-558: The Russian genetics community did what they could to oppose Trofim Lysenko and his Larmarckian evolutionary theory, but Muller was soon forced to leave the Soviet Union after Stalin read a translation of his eugenics book and was "displeased by it, and...ordered an attack prepared against it." By this time, Muller had already asked for a leave of absence. News of the Lysenko trials had reached

4576-468: The Soviet Union (an environment better suited to his political beliefs). In 1933, Muller and his wife reconciled, and their son David E. Muller and she moved with Hermann to Leningrad . There, at the Institute of Genetics, he imported the basic equipment for a Drosophila lab—including the flies—and set up shop. The institute was moved to Moscow in 1934, and Muller and his wife were divorced in 1935. In

4680-440: The U.S. entry into World War I , convinced Muller to return to Columbia to teach and to expand his experimental program. At Columbia, Muller and his collaborator and longtime friend Edgar Altenburg continued the investigation of lethal mutations. The primary method for detecting such mutations was to measure the sex ratios of the offspring of female flies. They predicted the ratio would vary from 1:1 due to recessive mutations on

4784-538: The USSR, Muller supervised a large and productive lab, and organized work on medical genetics. Most of his work involved further explorations of genetics and radiation. There he completed his eugenics book, Out of the Night , the main ideas of which dated to 1910. By 1936, however, Joseph Stalin 's repressive policies and the rise of Lysenkoism was making the USSR an increasingly problematic place to live and work. Muller and many of

4888-649: The United States, and his son David was being raised there, after his divorce. In the official declaration by the Institute, biological determinism was rejected: "The development of society is subject not to biological laws but to higher social laws. Attempts to spread to humanity the laws of the animal kingdom are an attempt to lower the human being to the level of beasts." Muller, with about 250 strains of Drosophila , moved to University of Edinburgh in September 1937, after brief stays in Madrid and Paris . In 1938, with war on

4992-405: The University of Texas from 1920 until 1932. Soon after returning to Texas, he married mathematics professor Jessie Marie Jacobs , whom he had courted previously. In his early years at Texas, Muller's Drosophila work was slow going; the data from his mutation rate studies were difficult to interpret. In 1923, he began using radium and X-rays , but the relationship between radiation and mutation

5096-514: The X chromosome, which would be expressed only in males (which lacked the functional allele on a second X chromosome). Muller found a strong temperature dependence in mutation rate, leading him to believe that spontaneous mutation was the dominant mode (and to initially discount the role of external factors such as ionizing radiation or chemical agents). In 1920, Muller and Altenburg coauthored a seminal paper in Genetics on "modifier genes" that determine

5200-473: The activity of the Eugenics Society". Muller told the assembled that eugenic ideals could no longer be achieved, because the capitalistic system produces the wrong motives of individual action, and he disdained the natures of the dominant class, and the type of society they were creating. In September 1932, Muller moved to Berlin to work with the Russian expatriate geneticist Nikolay Timofeeff-Ressovsky ;

5304-545: The adoption of the linear no-threshold model of radiation on cancer risks. The Nobel Prize, in the wake of the atomic bombings of Hiroshima and Nagasaki , focused public attention on a subject Muller had been publicizing for two decades - the dangers of radiation. In 1952, nuclear fallout became a public issue; since Operation Crossroads , more and more evidence had been leaking out about radiation sickness and death caused by nuclear testing . Muller and many other scientists pursued an array of political activities to defuse

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5408-608: The archaea/eukaryote nucleus group. The last common antecessor of all life (called LUCA , l ast u niversal c ommon a ncestor) should have possessed an early version of this protein complex. As ATP synthase is obligate membrane bound, this supports the assumption that LUCA was a cellular organism. The RNA world hypothesis might clarify this scenario, as LUCA might have been a ribocyte (also called ribocell) lacking DNA, but with an RNA genome built by ribosomes as primordial self-replicating entities . A Peptide-RNA world (also called RNP world) hypothesis has been proposed based on

5512-478: The associated diseases. Prokaryotes have diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct prokaryotic types. For example, in addition to using photosynthesis or organic compounds for energy, as eukaryotes do, prokaryotes may obtain energy from inorganic compounds such as hydrogen sulfide . This enables prokaryotes to thrive in harsh environments as cold as

5616-450: The bacterial phylum Planctomycetota has a membrane around the nucleoid and contains other membrane-bound cellular structures. However, further investigation revealed that Planctomycetota cells are not compartmentalized or nucleated and, like other bacterial membrane systems, are interconnected. Prokaryotic cells are usually much smaller than eukaryotic cells. Therefore, prokaryotes have a larger surface-area-to-volume ratio , giving them

5720-421: The biofilm—has led some to speculate that this may constitute a circulatory system and many researchers have started calling prokaryotic communities multicellular (for example ). Differential cell expression, collective behavior, signaling, programmed cell death , and (in some cases) discrete biological dispersal events all seem to point in this direction. However, these colonies are seldom if ever founded by

5824-490: The bodies of other organisms, including humans. Prokaryotes have high populations in the soil - including the rhizosphere and rhizosheath . Soil prokaryotes are still heavily undercharacterized despite their easy proximity to humans and their tremendous economic importance to agriculture . In 1977, Carl Woese proposed dividing prokaryotes into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of

5928-413: The children of Nicholas Müller, who immigrated to the United States in 1848, and at that time dropped the umlaut from his name. Another cousin was Herbert J. Muller , whose grandfather Otto was another son of Nicholas and a sibling of Hermann Sr. and Johanna. In a recent retrospective article about Muller's contribution, James Haber wrote as follows: Drosophila geneticist, Hermann Muller, envisioned

6032-913: The contribution of telomere length to lifespan remains controversial. There is little evidence that, in humans, telomere length is a significant biomarker of normal aging with respect to important cognitive and physical abilities. Experimentally verified and predicted telomere sequence motifs from more than 9000 species are collected in research community curated database TeloBase . Some of the experimentally verified telomere nucleotide sequences are also listed in Telomerase Database website (see nucleic acid notation for letter representations). Preliminary research indicates that disease risk in aging may be associated with telomere shortening, senescent cells , or SASP ( senescence-associated secretory phenotype ). Several techniques are currently employed to assess average telomere length in eukaryotic cells. One method

6136-454: The current set of prokaryotic species may have evolved from more complex eukaryotic ancestors through a process of simplification. Others have argued that the three domains of life arose simultaneously, from a set of varied cells that formed a single gene pool. This controversy was summarized in 2005: There is no consensus among biologists concerning the position of the eukaryotes in the overall scheme of cell evolution. Current opinions on

6240-669: The dramatic discontinuous alterations in Oenothera lamarckiana that were the basis of Hugo de Vries 's theory of mutationism : "balanced lethals" allowed the accumulation of recessive mutations, and rare crossing over events resulted in the sudden expression of these hidden traits. In other words, de Vries's experiments were explainable by the Mendelian-chromosome theory. Muller's work was increasingly focused on mutation rate and lethal mutations . In 1918, Morgan, short-handed because many of his students and assistants were drafted for

6344-427: The end replication problem and therefore do not age. Olovnikov suggested that in germline cells, cells of vegetatively propagated organisms, and immortal cell populations such as most cancer cell lines, an enzyme might be activated to prevent the shortening of DNA termini with each cell division. In 1975–1977, Elizabeth Blackburn , working as a postdoctoral fellow at Yale University with Joseph G. Gall , discovered

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6448-406: The eukaryotic chromosomes in structure and function. The known structures of bacterial telomeres take the form of proteins bound to the ends of linear chromosomes, or hairpin loops of single-stranded DNA at the ends of the linear chromosomes. At the very 3'-end of the telomere there is a 300 base pair overhang which can invade the double-stranded portion of the telomere forming a structure known as

6552-530: The evolution and interrelationships of the three domains of life. The division between prokaryotes and eukaryotes reflects the existence of two very different levels of cellular organization; only eukaryotic cells have an enveloped nucleus that contains its chromosomal DNA , and other characteristic membrane-bound organelles including mitochondria. Distinctive types of prokaryotes include extremophiles and methanogens ; these are common in some extreme environments. The distinction between prokaryotes and eukaryotes

6656-545: The exposure to stressors (e.g. pathogen infection, competition, reproductive effort and high activity level) was associated with shorter telomeres across different animal taxa. Studies on ectotherms , and other non-mammalian organisms, show that there is no single universal model of telomere erosion; rather, there is wide variation in relevant dynamics across Metazoa , and even within smaller taxonomic groups these patterns appear diverse. Hermann Joseph Muller Hermann Joseph Muller (December 21, 1890 – April 5, 1967)

6760-480: The formation of the Earth's crust. Eukaryotes only appear in the fossil record later, and may have formed from endosymbiosis of multiple prokaryote ancestors. The oldest known fossil eukaryotes are about 1.7 billion years old. However, some genetic evidence suggests eukaryotes appeared as early as 3 billion years ago. While Earth is the only place in the universe where life is known to exist, some have suggested that there

6864-909: The four basic shapes of bacteria are: The archaeon Haloquadratum has flat square-shaped cells. Bacteria and archaea reproduce through asexual reproduction, usually by binary fission . Genetic exchange and recombination still occur, but this is a form of horizontal gene transfer and is not a replicative process, simply involving the transference of DNA between two cells, as in bacterial conjugation . DNA transfer between prokaryotic cells occurs in bacteria and archaea, although it has been mainly studied in bacteria. In bacteria, gene transfer occurs by three processes. These are (1) bacterial virus ( bacteriophage )-mediated transduction , (2) plasmid -mediated conjugation , and (3) natural transformation . Transduction of bacterial genes by bacteriophage appears to reflect an occasional error during intracellular assembly of virus particles, rather than an adaptation of

6968-415: The fundamental principles that such a molecule must have: to be auto-assembling and to be mutable but then again stable. He followed his prescient review of these properties with a remarkable prediction: learning about the hereditary material and its properties would not come from studying Drosophila, but from studying bacteria and their bacteriophages. He was one of the signatories of the agreement to convene

7072-536: The future of human evolution had reduced impact in the public sphere. Muller's speech before the Third International Eugenics Conference in New York has been credited with marking the end of Galtonism , and perhaps even eugenics itself, as a popular movement in the sciences. H. Bentley Glass , a contemporary observer and Ph.D. student of Muller's, would say Muller's speech "just about finished

7176-532: The horizon, he began looking for a permanent position back in the United States. He also began courting Dorothea "Thea" Kantorowicz, a German refugee; they were married in May 1939. The Seventh International Congress on Genetics was held in Edinburgh later that year; Muller wrote a "Geneticists' Manifesto" in response to the question: "How could the world's population be improved most effectively genetically?" He also engaged in

7280-432: The host bacteria. The transfer of bacterial DNA is under the control of the bacteriophage's genes rather than bacterial genes. Conjugation in the well-studied E. coli system is controlled by plasmid genes, and is an adaptation for distributing copies of a plasmid from one bacterial host to another. Infrequently during this process, a plasmid may integrate into the host bacterial chromosome, and subsequently transfer part of

7384-537: The host bacterial DNA to another bacterium. Plasmid mediated transfer of host bacterial DNA (conjugation) also appears to be an accidental process rather than a bacterial adaptation. Natural bacterial transformation involves the transfer of DNA from one bacterium to another through the intervening medium. Unlike transduction and conjugation, transformation is clearly a bacterial adaptation for DNA transfer, because it depends on numerous bacterial gene products that specifically interact to perform this complex process. For

7488-426: The idea that oligopeptides may have been built together with primordial nucleic acids at the same time, which also supports the concept of a ribocyte as LUCA. The feature of DNA as the material base of the genome might have then been adopted separately in bacteria and in archaea (and later eukaryote nuclei), presumably by help of some viruses (possibly retroviruses as they could reverse transcribe RNA to DNA). As

7592-534: The last primer would not be replicated. It has since been questioned whether the last lagging strand primer is placed exactly at the 3'-end of the template and it was demonstrated that it is rather synthesized at a distance of about 70–100 nucleotides which is consistent with the finding that DNA in cultured human cell is shortened by 50–100 base pairs per cell division . If coding sequences are degraded in this process, potentially vital genetic code would be lost. Telomeres are non-coding, repetitive sequences located at

7696-410: The length of telomere from whole genome sequencing (WGS) experiments. Amongst these are TelSeq, Telomerecat and telomereHunter. Length estimation from WGS typically works by differentiating telomere sequencing reads and then inferring the length of telomere that produced that number of reads. These methods have been shown to correlate with preexisting methods of estimation such as PCR and TRF. Flow-FISH

7800-590: The lifetime of an individual, it is telomere shortening-rate rather than telomere length that is associated with the lifespan of a species. Critically short telomeres trigger a DNA damage response and cellular senescence . Mice have much longer telomeres, but a greatly accelerated telomere shortening-rate and greatly reduced lifespan compared to humans and elephants. Telomere shortening is associated with aging, mortality, and aging-related diseases in experimental animals. Although many factors can affect human lifespan, such as smoking, diet, and exercise, as persons approach

7904-469: The likely dangers of radiation exposure in humans (such as physicians who frequently operate X-ray equipment or shoe sellers who radiated their customers' feet). His lab grew quickly, but it shrank again following the onset of the Great Depression . Especially after the stock market crash, Muller was increasingly pessimistic about the prospects of capitalism . Some of his visiting lab members were from

8008-588: The loss reaches a critical level, at which point cell division ends. According to his theory of marginotomy, DNA sequences at the ends of telomeres are represented by tandem repeats, which create a buffer that determines the number of divisions that a certain cell clone can undergo. Furthermore, it was predicted that a specialized DNA polymerase (originally called a tandem-DNA-polymerase) could extend telomeres in immortal tissues such as germ line, cancer cells and stem cells. It also followed from this hypothesis that organisms with circular genome, such as bacteria, do not have

8112-424: The major differences in the structure and genetics between the two groups of organisms. Archaea were originally thought to be extremophiles, living only in inhospitable conditions such as extremes of temperature , pH , and radiation but have since been found in all types of habitats . The resulting arrangement of Eukaryota (also called "Eucarya"), Bacteria, and Archaea is called the three-domain system , replacing

8216-406: The medium (e.g., water) may flow easily. The microcolonies may join together above the substratum to form a continuous layer, closing the network of channels separating microcolonies. This structural complexity—combined with observations that oxygen limitation (a ubiquitous challenge for anything growing in size beyond the scale of diffusion) is at least partially eased by movement of medium throughout

8320-571: The mitochondria and chloroplasts. The genome in a prokaryote is held within a DNA/protein complex in the cytosol called the nucleoid , which lacks a nuclear envelope . The complex contains a single, cyclic, double-stranded molecule of stable chromosomal DNA, in contrast to the multiple linear, compact, highly organized chromosomes found in eukaryotic cells. In addition, many important genes of prokaryotes are stored in separate circular DNA structures called plasmids . Like eukaryotes, prokaryotes may partially duplicate genetic material, and can have

8424-403: The nucleus, that eukaryotes arose without endosymbiosis, and that eukaryotes arose through a symbiotic event entailing a simultaneous endosymbiotic origin of the flagellum and the nucleus, in addition to many other models, which have been reviewed and summarized elsewhere. The oldest known fossilized prokaryotes were laid down approximately 3.5 billion years ago, only about 1 billion years after

8528-408: The origin and position of eukaryotes span a broad spectrum including the views that eukaryotes arose first in evolution and that prokaryotes descend from them, that eukaryotes arose contemporaneously with eubacteria and archaebacteria and hence represent a primary line of descent of equal age and rank as the prokaryotes, that eukaryotes arose through a symbiotic event entailing an endosymbiotic origin of

8632-411: The parent strands. This is a consequence of its unidirectional mode of DNA synthesis: it can only attach new nucleotides to an existing 3'-end (that is, synthesis progresses 5'-3') and thus it requires a primer to initiate replication. On the leading strand (oriented 5'-3' within the replication fork), DNA-polymerase continuously replicates from the point of initiation all the way to the strand's end with

8736-419: The physical and physiological nature of the gene, was becoming a central topic in biology, and X-ray mutagenesis was a key to many recent advances, among them George Beadle and Edward Tatum 's work on Neurospora that established in 1941 the one gene-one enzyme hypothesis . In Muller's Nobel Prize lecture, he argued that no threshold dose of radiation existed that did not produce mutagenesis , which led to

8840-445: The primer (made of RNA ) then being excised and substituted by DNA. The lagging strand, however, is oriented 3'-5' with respect to the replication fork so continuous replication by DNA-polymerase is impossible, which necessitates discontinuous replication involving the repeated synthesis of primers further 5' of the site of initiation (see lagging strand replication ). The last primer to be involved in lagging-strand replication sits near

8944-411: The public "an irrational fear of low-level radiation relative to other risks". It has been argued that Muller's opinion was not supported by studies on the survivors of the atomic bombings , or by research on mice, and that he ignored another study that contradicted the linear no-threshold model he supported, thereby affecting the formulation of policy that favored this model. Muller was elected to

9048-460: The rate of spontaneous (as opposed to radiation-induced) mutations. Muller's publication rate decreased greatly in this period, from a combination of lack of lab workers and experimentally challenging projects. However, he also worked as an adviser in the Manhattan Project (though he did not know that was what it was), as well as a study of the mutational effects of radar . Muller's appointment

9152-494: The sequence repeats are enriched in guanine , e.g. TTAGGG in vertebrates , which allows the formation of G-quadruplexes , a special conformation of DNA involving non-Watson-Crick base pairing. There are different subtypes depending on the involvement of single- or double-stranded DNA, among other things. There is evidence for the 3'-overhang in ciliates (that possess telomere repeats similar to those found in vertebrates ) to form such G-quadruplexes that accommodate it, rather than

9256-434: The single-stranded telomere DNA is held onto a region of double-stranded DNA by the telomere strand disrupting the double-helical DNA, and base pairing to one of the two strands. This triple-stranded structure is called a displacement loop or D-loop. Apart from the end replication problem, in vitro studies have shown that telomeres accumulate damage due to oxidative stress and that oxidative stress-mediated DNA damage has

9360-408: The size of mutant Drosophila wings. In 1919, Muller made the important discovery of a mutant (later found to be a chromosomal inversion ) that appeared to suppress crossing over, which opened up new avenues in mutation-rate studies. However, his appointment at Columbia was not continued; he accepted an offer from the University of Texas and left Columbia after the summer of 1920. Muller taught at

9464-427: The snow surface of Antarctica , studied in cryobiology , or as hot as undersea hydrothermal vents and land-based hot springs . Prokaryotes live in nearly all environments on Earth. Some archaea and bacteria are extremophiles , thriving in harsh conditions, such as high temperatures ( thermophiles ) or high salinity ( halophiles ). Many archaea grow as plankton in the oceans. Symbiotic prokaryotes live in or on

9568-557: The telomeres act as a sort of time-delay "fuse", eventually running out after a certain number of cell divisions and resulting in the eventual loss of vital genetic information from the cell's chromosome with future divisions. Telomere length varies greatly between species, from approximately 300 base pairs in yeast to many kilobases in humans, and usually is composed of arrays of guanine -rich, six- to eight-base-pair-long repeats. Eukaryotic telomeres normally terminate with 3′ single-stranded-DNA overhang ranging from 75 to 300 bases, which

9672-681: The termini of linear chromosomes to act as buffers for those coding sequences further behind. They "cap" the end-sequences and are progressively degraded in the process of DNA replication. The "end replication problem" is exclusive to linear chromosomes as circular chromosomes do not have ends lying without reach of DNA-polymerases. Most prokaryotes , relying on circular chromosomes, accordingly do not possess telomeres. A small fraction of bacterial chromosomes (such as those in Streptomyces , Agrobacterium , and Borrelia ), however, are linear and possess telomeres, which are very different from those of

9776-526: The threat of nuclear war . With the Castle Bravo fallout controversy in 1954, the issue became even more urgent. In 1955, Muller was one of 11 prominent intellectuals to sign the Russell–Einstein Manifesto , the upshot of which was the first 1957 Pugwash Conference on Science and World Affairs , which addressed the control of nuclear weapons. He was a signatory (with many other scientists) of

9880-426: The traditional two-empire system . According to the phylogenetic analysis of Hug (2016), the relationships could be the following: A widespread current model of the evolution of the first living organisms is that these were some form of prokaryotes, which may have evolved out of protocells , while the eukaryotes evolved later in the history of life. Some authors have questioned this conclusion, arguing that

9984-461: The unusual nature of telomeres, with their simple repeated DNA sequences composing chromosome ends. Blackburn, Carol Greider , and Jack Szostak were awarded the 2009 Nobel Prize in Physiology or Medicine for the discovery of how chromosomes are protected by telomeres and the enzyme telomerase . During DNA replication, DNA polymerase cannot replicate the sequences present at the 3' ends of

10088-433: The upper limit of human life expectancy , longer telomeres may be associated with lifespan. Meta-analyses found that increased perceived psychological stress was associated with a small decrease in telomere length—but that these associations attenuate to no significant association when accounting for publication bias . The literature concerning telomeres as integrative biomarkers of exposure to stress and adversity

10192-447: Was a third-generation American whose father's ancestors were originally Catholic and came to the United States from Koblenz . His mother's family was of mixed Jewish (descended from Spanish and Portuguese Jews ) and Anglican background, and had come from Britain. Among his first cousins was Alfred Kroeber (Kroeber was Ursula Le Guin 's father) and first cousins once removed was Herbert J. Muller . As an adolescent, Muller attended

10296-513: Was an American geneticist who was awarded the 1946 Nobel Prize in Physiology or Medicine , "for the discovery that mutations can be induced by X-rays". Muller warned of long-term dangers of radioactive fallout from nuclear war and nuclear testing , which resulted in greater public scrutiny of these practices. Muller was born in New York City , the son of Frances (Lyons) and Hermann Joseph Muller Sr., an artisan who worked with metals. Muller

10400-563: Was co-founder of one company, promoted the clinical utility of telomere length measures. During the last two decades, eco-evolutionary studies have investigated the relevance of life-history traits and environmental conditions on telomeres of wildlife. Most of these studies have been conducted in endotherms , i.e. birds and mammals. They have provided evidence for the inheritance of telomere length; however, heritability estimates vary greatly within and among species. Age and telomere length often negatively correlate in vertebrates, but this decline

10504-432: Was difficult to measure because such radiation also sterilized the flies. In this period, he also became involved with eugenics and human genetics. He carried out a study of twins separated at birth that seemed to indicate a strong hereditary component of I.Q. Muller was critical of the new directions of the eugenics movement (such as anti-immigration), but was hopeful about the prospects for positive eugenics. In 1932, at

10608-557: Was ended after the 1944–1945 academic year, and despite difficulties stemming from his socialist political activities, he found a position as professor of zoology at Indiana University . Here, he lived in a Dutch Colonial Revival house in Bloomington 's Vinegar Hill neighborhood. In 1946, Muller was awarded the Nobel Prize in Physiology or Medicine , "for the discovery that mutations can be induced by X-rays". Genetics, and especially

10712-520: Was firmly established by the microbiologists Roger Stanier and C. B. van Niel in their 1962 paper The concept of a bacterium (though spelled procaryote and eucaryote there). That paper cites Édouard Chatton 's 1937 book Titres et Travaux Scientifiques for using those terms and recognizing the distinction. One reason for this classification was so that what was then often called blue-green algae (now called cyanobacteria ) would not be classified as plants but grouped with bacteria. Prokaryotes have

10816-434: Was left out of major publications. In 1914, Julian Huxley offered Muller a position at the recently founded William Marsh Rice Institute, now Rice University ; he hurried to complete his Doctor of philosophy degree and moved to Houston for the beginning of the 1915–1916 academic year (his degree was issued in 1916). At Rice, Muller taught biology and continued Drosophila lab work. In 1918, he proposed an explanation for

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