1.
Chromosome
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A chromosome is a DNA molecule with part or all of the genetic material of an organism. Prokaryotes usually have one single circular chromosome, whereas most eukaryotes are diploid, chromosomes in eukaryotes are composed of chromatin fiber. Chromatin fiber is made of nucleosomes, a nucleosome is a histone octamer with part of a longer DNA strand attached to and wrapped around it. Chromatin fiber, together with associated proteins is known as chromatin, chromatin is present in most cells, with a few exceptions, for example, red blood cells. Occurring only in the nucleus of cells, chromatin contains the vast majority of DNA, except for a small amount inherited maternally. Chromosomes are normally visible under a microscope only when the cell is undergoing the metaphase of cell division. Before this happens every chromosome is copied once, and the copy is joined to the original by a centromere resulting in an X-shaped structure, the original chromosome and the copy are now called sister chromatids. During metaphase, when a chromosome is in its most condensed state, in this highly condensed form chromosomes are easiest to distinguish and study. In prokaryotic cells, chromatin occurs free-floating in cytoplasm, as these cells lack organelles, the main information-carrying macromolecule is a single piece of coiled double-helix DNA, containing many genes, regulatory elements and other noncoding DNA. The DNA-bound macromolecules are proteins that serve to package the DNA, chromosomes vary widely between different organisms. Some species such as certain bacteria also contain plasmids or other extrachromosomal DNA and these are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer. Chromosomal recombination during meiosis and subsequent sexual reproduction plays a significant role in genetic diversity. In prokaryotes and viruses, the DNA is often densely packed and organized, in the case of archaea, by homologs to eukaryotic histones, small circular genomes called plasmids are often found in bacteria and also in mitochondria and chloroplasts, reflecting their bacterial origins. Some use the term chromosome in a sense, to refer to the individualized portions of chromatin in cells. However, others use the concept in a sense, to refer to the individualized portions of chromatin during cell division. The word chromosome comes from the Greek χρῶμα and σῶμα, describing their strong staining by particular dyes, schleiden, Virchow and Bütschli were among the first scientists who recognized the structures now so familiar to everyone as chromosomes. The term was coined by von Waldeyer-Hartz, referring to the term chromatin, in a series of experiments beginning in the mid-1880s, Theodor Boveri gave the definitive demonstration that chromosomes are the vectors of heredity. His two principles were the continuity of chromosomes and the individuality of chromosomes and it is the second of these principles that was so original
2.
Homology (biology)
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In biology, homology is the existence of shared ancestry between a pair of structures, or genes, in different taxa. Evolutionary biology explains homologous structures adapted to different purposes as the result of descent with modification from a common ancestor, examples include the legs of a centipede, the maxillary palp and labial palp of an insect, and the spinous processes of successive vertebrae in a vertebral column. Sequence homology between protein or DNA sequences is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either an event or a duplication event. Homology among proteins or DNA is inferred from their sequence similarity, significant similarity is strong evidence that two sequences are related by divergent evolution from a common ancestor. Alignments of multiple sequences are used to discover the homologous regions, the word homology, coined in about 1656, derives from the Greek ὁμόλογος homologos from ὁμός homos same and λόγος logos relation. Homology is the relationship between biological structures or sequences that are derived from a common ancestor, for example, many insects possess two pairs of flying wings. In beetles, the first pair of wings has evolved into a pair of hard wing covers, the same major forearm bones are found in fossils of lobe-finned fish such as Eusthenopteron. The opposite of homologous organs are analogous organs which do similar jobs in two taxa that were not present in their last common ancestor but rather evolved separately. For example, the wings of insects and birds evolved independently in widely separated groups, similarly, the wings of a sycamore maple seed and the wings of a bird are analogous but not homologous, as they develop from quite different structures. A structure can be homologous at one level, but only analogous at another, for example, in the pterosaurs, the wing involves both the forelimb and the hindlimb. Analogy is called homoplasy in cladistics, and convergent or parallel evolution in evolutionary biology, specialised terms are used in taxonomic research. Primary homology is that initially conjectured by a researcher based on similar structure or anatomical connections, secondary homology is implied by parsimony analysis, where a character that only occurs once on a tree is taken to be homologous. As implied in this definition, many cladists consider homology to be synonymous with synapomorphy, homologies provide the fundamental basis for all biological classification, although some may be highly counter-intuitive. The homologies between these have been discovered by comparing genes in evolutionary developmental biology, among insects, the stinger of the female honey bee is a modified ovipositor, homologous with ovipositors in other insects such as the Orthoptera, Hemiptera, and those Hymenoptera without stingers. The three small bones in the ear of mammals including humans, the malleus, incus. The malleus and incus develop in the embryo from structures that form jaw bones in lizards, both lines of evidence show that these bones are homologous, sharing a common ancestor. Among the many homologies in mammal reproductive systems, ovaries and testicles are homologous, in many plants, defensive or storage structures are made by modifications of the development of primary leaves, stems, and roots
3.
Embryonic stem cell
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Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage preimplantation embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at time they consist of 50–150 cells. Human ES cells measure approximately 14 μm while mouse ES cells are closer to 8 μm, embryonic stem cells, derived from the blastocyst stage early mammalian embryos, are distinguished by their ability to differentiate into any cell type and by their ability to propagate. Embryonic stem cells properties include having a normal karyotype, maintaining high telomerase activity and these include each of the more than 220 cell types in the adult body. If the pluripotent differentiation potential of stem cells could be harnessed in vitro. This would provide a new treatment approach to a wide variety of conditions where age, disease. Besides the ethical concerns of stem cell therapy, there is a problem of graft-versus-host disease associated with allogeneic stem cell transplantation. However, these associated with histocompatibility may be solved using autologous donor adult stem cells. Stem cell banks or more recently by reprogramming of cells with defined factors. Embryonic stem cells provide hope that it will be possible to overcome the problems of donor tissue shortage and also, other potential uses of embryonic stem cells include investigation of early human development, study of genetic disease and as in vitro systems for toxicology testing. Current research focuses on differentiating ES into a variety of types for eventual use as cell replacement therapies. Some of the types that have or are currently being developed include cardiomyocytes, neurons, hepatocytes, bone marrow cells, islet cells. However, the derivation of such cell types from ESs is not without obstacles, for example, studies are underway to differentiate ES in to tissue specific CMs and to eradicate their immature properties that distinguish them from adult CMs. Studies have shown that cardiomyocytes derived from ES are validated in vitro models to test drug responses, ES derived cardiomyocytes have been shown to respond to pharmacological stimuli and hence can be used to assess cardiotoxicity like Torsades de Pointes. ES-derived hepatocytes are also models that could be used in the preclinical stages of drug discovery. However, the development of hepatocytes from ES has proven to be challenging, therefore, current research is focusing on establishing fully functional ES-derived hepatocytes with stable phase I and II enzyme activity. Researchers have also differentiated ES into dopamine-producing cells with the hope that these neurons could be used in the treatment of Parkinson’s disease, recently, the development of ESC after Somatic Cell Nuclear Transfer of Olfactory ensheathing cells to a healthy Oocyte has been recommended for Neuro-degenerative diseases. ESs have also been differentiated to natural killer cells and bone tissue, studies involving ES are also underway to provide an alternative treatment for diabetes
4.
Human
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Modern humans are the only extant members of Hominina tribe, a branch of the tribe Hominini belonging to the family of great apes. Several of these hominins used fire, occupied much of Eurasia and they began to exhibit evidence of behavioral modernity around 50,000 years ago. In several waves of migration, anatomically modern humans ventured out of Africa, the spread of humans and their large and increasing population has had a profound impact on large areas of the environment and millions of native species worldwide. Humans are uniquely adept at utilizing systems of communication for self-expression and the exchange of ideas. Humans create complex structures composed of many cooperating and competing groups, from families. Social interactions between humans have established a wide variety of values, social norms, and rituals. These human societies subsequently expanded in size, establishing various forms of government, religion, today the global human population is estimated by the United Nations to be near 7.5 billion. In common usage, the word generally refers to the only extant species of the genus Homo—anatomically and behaviorally modern Homo sapiens. In scientific terms, the meanings of hominid and hominin have changed during the recent decades with advances in the discovery, there is also a distinction between anatomically modern humans and Archaic Homo sapiens, the earliest fossil members of the species. The English adjective human is a Middle English loanword from Old French humain, ultimately from Latin hūmānus, the words use as a noun dates to the 16th century. The native English term man can refer to the species generally, the species binomial Homo sapiens was coined by Carl Linnaeus in his 18th century work Systema Naturae. The generic name Homo is a learned 18th century derivation from Latin homō man, the species-name sapiens means wise or sapient. Note that the Latin word homo refers to humans of either gender, the genus Homo evolved and diverged from other hominins in Africa, after the human clade split from the chimpanzee lineage of the hominids branch of the primates. The closest living relatives of humans are chimpanzees and gorillas, with the sequencing of both the human and chimpanzee genome, current estimates of similarity between human and chimpanzee DNA sequences range between 95% and 99%. The gibbons and orangutans were the first groups to split from the leading to the humans. The splitting date between human and chimpanzee lineages is placed around 4–8 million years ago during the late Miocene epoch, during this split, chromosome 2 was formed from two other chromosomes, leaving humans with only 23 pairs of chromosomes, compared to 24 for the other apes. There is little evidence for the divergence of the gorilla, chimpanzee. Each of these species has been argued to be an ancestor of later hominins
5.
Chimpanzee
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Chimpanzees are one of the two species of the genus Pan, the other being the bonobo. Together with gorillas, they are the only exclusively African species of ape that are currently extant. Native to sub-Saharan Africa, both chimpanzees and bonobos are found in the Congo jungle. In addition, P. troglodytes is divided into four subspecies, based on genome sequencing, the two extant Pan species diverged around one million years ago. The most obvious differences are that chimpanzees are somewhat larger, more aggressive and male-dominated, while the bonobos are more gracile, peaceful and their hair is typically black or brown. Males and females differ in size and appearance, both chimps and bonobos are some of the most social great apes, with social bonds occurring among individuals in large communities. Fruit is the most important component of a diet, however, they will also eat vegetation, bark, honey, insects. They can live over 30 years in both the wild and captivity, Chimpanzees and bonobos are equally humanitys closest living relatives. As such, they are among the largest-brained, and most intelligent of primates, they use a variety of sophisticated tools and construct elaborate sleeping nests each night from branches and they have both been extensively studied for their learning abilities. There may even be distinctive cultures within populations, field studies of Pan troglodytes were pioneered by primatologist Jane Goodall. Both Pan species are considered to be endangered as human activities have caused declines in the populations. Threats to wild populations include poaching, habitat destruction. Several conservation and rehabilitation organisations are dedicated to the survival of Pan species in the wild, the first use of the name chimpanze is recorded in The London Magazine in 1738, glossed as meaning mockman in a language of the Angolans. The spelling chimpanzee is found in a 1758 supplement to Chambers Cyclopædia, the colloquialism chimp was most likely coined some time in the late 1870s. The common chimpanzee was named Simia troglodytes by Johann Friedrich Blumenbach in 1776, the species name troglodytes is a reference to the Troglodytae, an African people described by Greco-Roman geographers. Blumenbach first used it in his De generis humani varietate nativa liber in 1776, the genus name Pan was first introduced by Lorenz Oken in 1816. An alternative Theranthropus was suggested by Brookes 1828 and Chimpansee by Voigt 1831, troglodytes was not available, as it had been given as the name of a genus of wren in 1809. The International Commission on Zoological Nomenclature adopted Pan as the official name of the genus in 1895
6.
Pseudogene
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Pseudogenes are segments of DNA that are related to real genes. Pseudogenes have lost at least some functionality, relative to the complete gene, pseudogenes often result from the accumulation of multiple mutations within a gene whose product is not required for the survival of the organism. Although not fully functional, pseudogenes may be functional, similar to kinds of noncoding DNA. The pseudo in pseudogene implies a variation in relative to the parent coding gene. Despite being non-coding, many pseudogenes have important roles in normal physiology, although some pseudogenes do not have introns or promoters, others have some gene-like features such as promoters, CpG islands, and splice sites. They are different from normal due to either a lack of protein-coding ability resulting from a variety of disabling mutations. The term pseudogene was coined in 1977 by Jacq et al, because pseudogenes were initially thought of as the last stop for genomic material that could be removed from the genome, they were often labeled as junk DNA. Nonetheless, pseudogenes contain biological and evolutionary histories within their sequences, pseudogenes are usually characterized by a combination of homology to a known gene and loss of some functionality. That is, although every pseudogene has a DNA sequence that is similar to some functional gene, homology is implied by sequence identity between the DNA sequences of the pseudogene and parent gene. After aligning the two sequences, the percentage of identical base pairs is computed, a high sequence identity means that it is highly likely that these two sequences diverged from a common ancestral sequence, and highly unlikely that these two sequences have evolved independently. Nonfunctionality can manifest itself in many ways, normally, a gene must go through several steps to a fully functional protein, Transcription, pre-mRNA processing, translation, and protein folding are all required parts of this process. If any of these steps fails, then the sequence may be considered nonfunctional, pseudogenes for RNA genes are usually more difficult to discover as they do not need to be translated and thus do not have reading frames. Pseudogenes can complicate molecular genetic studies, for example, amplification of a gene by PCR may simultaneously amplify a pseudogene that shares similar sequences. This is known as PCR bias or amplification bias, similarly, pseudogenes are sometimes annotated as genes in genome sequences. Processed pseudogenes often pose a problem for gene prediction programs, often being misidentified as real genes or exons and it has been proposed that identification of processed pseudogenes can help improve the accuracy of gene prediction methods. Recently 140 human pseudogenes have been shown to be translated, however, the function, if any, of the protein products is unknown. There are four types of pseudogenes, all with distinct mechanisms of origin. The classifications of pseudogenes are as follows, Processed pseudogenes, in higher eukaryotes, particularly mammals, retrotransposition is a fairly common event that has had a huge impact on the composition of the genome
7.
Evidence of common descent
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Additionally, this evidence supports the modern evolutionary synthesis—the current scientific theory that explains how and why life changes over time. Evolutionary biologists document evidence of descent by developing testable predictions, testing hypotheses. Additional genetic information conclusively supports the relatedness of life and has allowed scientists to develop phylogenetic trees and it has also led to the development of molecular clock techniques to date taxon divergence times and to calibrate these with the fossil record. Fossils are important for estimating when various lineages developed in geologic time, vestigial structures and comparisons in embryonic development are largely a contributing factor in anatomical resemblance in concordance with common descent. Since metabolic processes do not leave fossils, research into the evolution of the cellular processes is done largely by comparison of existing organisms physiology. Many lineages diverged at different stages of development, so it is possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor and this is especially obvious in the field of insular biogeography. The development and spread of resistant bacteria provides evidence that evolution due to natural selection is an ongoing process in the natural world. Natural selection is ubiquitous in all research pertaining to evolution, taking note of the fact all of the following examples in each section of the article document the process. Alongside this are observed instances of the separation of populations of species into sets of new species, speciation has been observed directly and indirectly in the lab and in nature. Multiple forms of such have been described and documented as examples for individual modes of speciation, one of the strongest evidences for common descent comes from the study of gene sequences. Comparative sequence analysis examines the relationship between the DNA sequences of different species, producing several lines of evidence that confirm Darwins original hypothesis of common descent. If the hypothesis of common descent is true, then species that share a common ancestor inherited that ancestors DNA sequence, more closely related species have a greater fraction of identical sequence and shared substitutions compared to more distantly related species. The simplest and most powerful evidence is provided by phylogenetic reconstruction, such reconstructions, especially when done using slowly evolving protein sequences, are often quite robust and can be used to reconstruct a great deal of the evolutionary history of modern organisms. These reconstructed phylogenies recapitulate the relationships established through morphological and biochemical studies, while a minority of these elements might later be found to harbor function, in aggregate they demonstrate that identity must be the product of common descent rather than common function. All known extant organisms are based on the biochemical processes, genetic information encoded as nucleic acid, transcribed into RNA. Perhaps most tellingly, the Genetic Code is the same for almost every organism, ATP is used as energy currency by all extant life. A deeper understanding of developmental biology shows that morphology is, in fact. Another noteworthy example is the vertebrate body plan, whose structure is controlled by the homeobox family of genes
8.
University of Edinburgh
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The University of Edinburgh, founded in 1582, is the sixth oldest university in the English-speaking world and one of Scotlands ancient universities. The university is deeply embedded in the fabric of the city of Edinburgh, the University of Edinburgh was ranked 17th and 21st in the world by the 2014–15 and 2015-16 QS rankings. It is now ranked 19th in the according to 2016-17 QS Rankings. It is ranked 16th in the world in arts and humanities by the 2015–16 Times Higher Education Ranking and it is ranked the 23rd most employable university in the world by the 2015 Global Employability University Ranking. It is ranked as the 6th best university in Europe by the U. S. News Best Global Universities Ranking and it is a member of both the Russell Group, and the League of European Research Universities, a consortium of 21 research universities in Europe. It has the third largest endowment of any university in the United Kingdom, after the universities of Cambridge and it continues to have links to the British Royal Family, having had the Duke of Edinburgh as its Chancellor from 1953 to 2010 and Princess Anne since 2011. Edinburgh receives approximately 50,000 applications every year, making it the fourth most popular university in the UK by volume of applicants, after St Andrews, it is the most difficult university to gain admission into in Scotland, and 9th overall in the UK. This was a move at the time, as most universities were established through Papal bulls. Established as the Tounis College, it opened its doors to students in October 1583, instruction began under the charge of another St Andrews graduate Robert Rollock. It was the fourth Scottish university in a period when the more populous. It was renamed King Jamess College in 1617, by the 18th century, the university was a leading centre of the Scottish Enlightenment. The universitys first custom-built building was the Old College, now Edinburgh Law School and its first forte in teaching was anatomy and the developing science of surgery, from which it expanded into many other subjects. From the basement of a nearby house ran the anatomy tunnel corridor and it went under what was then North College Street, and under the university buildings until it reached the universitys anatomy lecture theatre, delivering bodies for dissection. It was from this tunnel the body of William Burke was taken after he had been hanged, towards the end of the 19th century, Old College was becoming overcrowded and Robert Rowand Anderson was commissioned to design new Medical School premises in 1875. The medical school was more or less built to his design and was completed by the addition of the McEwan Hall in the 1880s. The building now known as New College was originally built as a Free Church college in the 1840s and has been the home of divinity at the university since the 1920s. The two oldest schools – law and divinity – are both well-esteemed, with law being based in Old College and divinity in New College on the Mound and they are also represented by the Edinburgh University Sports Union which was founded in 1866. The medical school is renowned throughout the world and it was widely considered the best medical school in the English-speaking world throughout the 18th century and first half of the 19th century
9.
Gene expression
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Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are proteins, but in non-protein coding genes such as transfer RNA or small nuclear RNA genes. The process of gene expression is used by all known life—eukaryotes, prokaryotes, several steps in the gene expression process may be modulated, including the transcription, RNA splicing, translation, and post-translational modification of a protein. Gene regulation gives the cell control over structure and function, and is the basis for differentiation, morphogenesis. In genetics, gene expression is the most fundamental level at which the genotype gives rise to the phenotype, the genetic code stored in DNA is interpreted by gene expression, and the properties of the expression give rise to the organisms phenotype. Such phenotypes are expressed by the synthesis of proteins that control the organisms shape. Regulation of gene expression is critical to an organisms development. A gene is a stretch of DNA that encodes information, genomic DNA consists of two antiparallel and reverse complementary strands, each having 5 and 3 ends. This RNA is complementary to the template 3 →5 DNA strand, therefore, the resulting 5 →3 RNA strand is identical to the coding DNA strand with the exception that thymines are replaced with uracils in the RNA. A coding DNA strand reading ATG is indirectly transcribed through the non-coding strand as AUG in RNA, in prokaryotes, transcription is carried out by a single type of RNA polymerase, which needs a DNA sequence called a Pribnow box as well as a sigma factor to start transcription. RNA polymerase I is responsible for transcription of ribosomal RNA genes, RNA polymerase II transcribes all protein-coding genes but also some non-coding RNAs. Pol II includes a C-terminal domain that is rich in serine residues, when these residues are phosphorylated, the CTD binds to various protein factors that promote transcript maturation and modification. RNA polymerase III transcribes 5S rRNA, transfer RNA genes, transcription ends when the polymerase encounters a sequence called the terminator. These include 5 capping, which is set of reactions that add 7-methylguanosine to the 5 end of pre-mRNA. The m7G cap is then bound by cap binding complex heterodimer, another modification is 3 cleavage and polyadenylation. They occur if polyadenylation signal sequence is present in pre-mRNA, which is usually between protein-coding sequence and terminator, the pre-mRNA is first cleaved and then a series of ~200 adenines are added to form poly tail, which protects the RNA from degradation. Poly tail is bound by multiple poly-binding proteins necessary for mRNA export, a very important modification of eukaryotic pre-mRNA is RNA splicing. The majority of eukaryotic pre-mRNAs consist of alternating segments called exons and introns, in certain cases, some introns or exons can be either removed or retained in mature mRNA
10.
Transcription (biology)
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Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA by the enzyme RNA polymerase. Both DNA and RNA are nucleic acids, which use base pairs of nucleotides as a complementary language, during transcription, a DNA sequence is read by an RNA polymerase, which produces a complementary, antiparallel RNA strand called a primary transcript. Transcription proceeds in the general steps, RNA polymerase, together with one or more general transcription factors. RNA polymerase creates a bubble, which separates the two strands of the DNA helix. This is done by breaking the bonds between complementary DNA nucleotides. RNA sugar-phosphate backbone forms with assistance from RNA polymerase to form an RNA strand, hydrogen bonds of the RNA–DNA helix break, freeing the newly synthesized RNA strand. If the cell has a nucleus, the RNA may be further processed and this may include polyadenylation, capping, and splicing. The RNA may remain in the nucleus or exit to the cytoplasm through the pore complex. The stretch of DNA transcribed into an RNA molecule is called a transcription unit, if the gene encodes a protein, the transcription produces messenger RNA, the mRNA, in turn, serves as a template for the proteins synthesis through translation. Alternatively, the gene may encode for either non-coding RNA, ribosomal RNA, transfer RNA. Overall, RNA helps synthesize, regulate, and process proteins, in virology, the term may also be used when referring to mRNA synthesis from an RNA molecule. For instance, the genome of a negative-sense single-stranded RNA virus may be template for a positive-sense single-stranded RNA and this is because the positive-sense strand contains the information needed to translate the viral proteins for viral replication afterwards. This process is catalyzed by a viral RNA replicase, the regulatory sequence before the coding sequence is called the five prime untranslated region, the sequence after the coding sequence is called the three prime untranslated region. As opposed to DNA replication, transcription results in an RNA complement that includes the nucleotide uracil in all instances where thymine would have occurred in a DNA complement, only one of the two DNA strands serve as a template for transcription. The antisense strand of DNA is read by RNA polymerase from the 3 end to the 5 end during transcription. The complementary RNA is created in the direction, in the 5 →3 direction. This directionality is because RNA polymerase can only add nucleotides to the 3 end of the growing mRNA chain and this use of only the 3 →5 DNA strand eliminates the need for the Okazaki fragments that are seen in DNA replication. This also removes the need for an RNA primer to initiate RNA synthesis, the non-template strand of DNA is called the coding strand, because its sequence is the same as the newly created RNA transcript
11.
Evolutionary biology
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Evolutionary biology is the subfield of biology that studies the evolutionary processes that produced the diversity of life on Earth, starting from a single origin of life. These processes include natural selection, common descent, and speciation, the study of evolution is the central unifying concept in biology. Biology can be divided in various ways, one way is by the level of biological organisation, from molecular to cell, organism to population. Another way is by taxonomic group, with such as zoology, ornithology. A third way is by approach, such as biology, theoretical biology, experimental evolution. These alternative ways of dividing up the subject can be combined with evolutionary biology to create subfields like evolutionary ecology, Evolutionary biology, as an academic discipline in its own right, emerged during the period of the modern evolutionary synthesis in the 1930s and 1940s. It was not until the 1980s that many universities had departments of evolutionary biology, in the United States, many universities have created departments of molecular and cell biology or ecology and evolutionary biology, in place of the older departments of botany and zoology. Palaeontology is often grouped with earth science, microbiology too is becoming an evolutionary discipline, now that microbial physiology and genomics are better understood. The quick generation time of bacteria and viruses such as makes it possible to explore evolutionary questions. Many biologists have contributed to our current understanding of evolution, ford established an empirical research programme. Ronald Fisher, Sewall Wright and J. S. Haldane created a theoretical framework. Ernst Mayr in systematics, George Gaylord Simpson in palaeontology and G. Ledyard Stebbins in botany helped to form the modern synthesis, james Crow, Richard Lewontin, Dan Hartl, Marcus Feldman, and Brian Charlesworth trained a generation of evolutionary biologists. Current research in evolutionary biology covers diverse topics and incorporates ideas from diverse areas, such as molecular genetics, first, some fields of evolutionary research try to explain phenomena that were poorly accounted for in the modern evolutionary synthesis. These include speciation, the evolution of reproduction, the evolution of cooperation, the evolution of ageing. Second, biologists ask the most straightforward evolutionary question, what happened and this includes fields such as palaeobiology, as well as systematics and phylogenetics. Third, the evolutionary synthesis was devised at a time when nobody understood the molecular basis of genes. Today, evolutionary biologists try to determine the architecture of interesting evolutionary phenomena such as adaptation and speciation. They try to reconcile the high heritability seen in studies with the difficulty in finding which genes are responsible for this heritability using genome-wide association studies
12.
Histone
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In biology, histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the protein components of chromatin, acting as spools around which DNA winds. Without histones, the unwound DNA in chromosomes would be very long, five major families of histones exist, H1/H5, H2A, H2B, H3, and H4. Histones H2A, H2B, H3 and H4 are known as the core histones, the core histones all exist as dimers, which are similar in that they all possess the histone fold domain, three alpha helices linked by two loops. It is this structure that allows for interaction between distinct dimers, particularly in a head-tail fashion. The resulting four distinct dimers then come together to form one octameric nucleosome core, around 146 base pairs of DNA wrap around this core particle 1.65 times in a left-handed super-helical turn to give a particle of around 100 Angstroms across. The linker histone H1 binds the nucleosome at the entry and exit sites of the DNA, thus locking the DNA into place, the most basic such formation is the 10 nm fiber or beads on a string conformation. This involves the wrapping of DNA around nucleosomes with approximately 50 base pairs of DNA separating each pair of nucleosomes, higher-order structures include the 30 nm fiber and 100 nm fiber, these being the structures found in normal cells. During mitosis and meiosis, the chromosomes are assembled through interactions between nucleosomes and other regulatory proteins. In animals, genes encoding canonical histones are typically clustered along the chromosome, lack introns, genes encoding histone variants are usually not clustered, have introns and their mRNAs are regulated with polyA tails. Complex multicellular organisms typically have a number of histone variants providing a variety of different functions. Recent data are accumulating about the roles of diverse histone variants highlighting the links between variants and the delicate regulation of organism development. Histone variants from different organisms, their classification and variant specific features can be found in HistoneDB2.0 - Variants database. The following is a list of human proteins, The nucleosome core is formed of two H2A-H2B dimers and a H3-H4 tetramer, forming two nearly symmetrical halves by tertiary structure. The H2A-H2B dimers and H3-H4 tetramer also show pseudodyad symmetry, the 4 core histones are relatively similar in structure and are highly conserved through evolution, all featuring a helix turn helix turn helix motif. They also share the feature of long tails on one end of the amino acid structure - this being the location of post-translational modification, despite the differences in their topology, these three folds share a homologous helix-strand-helix motif. Using an electron paramagnetic resonance spin-labeling technique, British researchers measured the distances between the spools around which eukaryotic cells wind their DNA and they determined the spacings range from 59 to 70 Å. Histones are subject to post translational modification by enzymes primarily on their N-terminal tails, such modifications include methylation, citrullination, acetylation, phosphorylation, SUMOylation, ubiquitination, and ADP-ribosylation
. PMID 22949105.
