Colugos are arboreal gliding mammals found in Southeast Asia, the closest relative of the primates. Just two extant species make up the entire family order Dermoptera, they are the most capable gliders of all gliding mammals, using flaps of extra skin between their legs to glide from higher to lower locations. They are known as cobegos or flying lemurs, but they are not true lemurs close relatives. Colugos are tree-dwelling mammals, they weigh 1 to 2 kg. They have long, slender front and rear limbs, a medium-length tail, a light build; the head is small, with large, front-focused eyes for excellent binocular vision, small rounded ears. Colugos are proficient gliders, they can travel as far as 70 m from one tree to another without losing much altitude, with a Malayan colugo individual having travelled 150 m in one glide. Of all the gliding mammals, colugos have the most extensive adaptation for flight, they have a large membrane of skin which extends between their paired limbs and gives them the ability to glide significant distances between trees.
This gliding membrane, or patagium, runs from the shoulder blades to the fore paws, from the tip of the rear-most fingers to the tip of the toes, from the hind legs to the tip of the tail. The spaces between the colugo's fingers and toes are webbed; as a result, colugos were once considered to be close relatives of bats. Today, they are considered to be the closest living relatives of primates. Colugos are unskilled climbers, they progress up trees in a series of slow hops, gripping onto the bark with their small, sharp claws. Colugos spend most of the day hanging inconspicuously under branches. At night, colugos spend most of their time up in the trees foraging, with gliding being used to either find another foraging tree or to find possible mates and protect territory. Colugos are shy, solitary animals found in the tropical forests of Southeast Asia. Little is known about their behavior, they are herbivorous and eat leaves, flowers and fruit. They have well-developed stomachs and long intestines capable of extracting nutrients from leaves and other fibrous material.
The incisor teeth of colugos are distinctive. The incisors are analogous in appearance and function to the incisor suite in strepsirrhines, used for grooming; the second upper incisors have another unique feature among mammals. The dental formula of colugos is: 184.108.40.206.1.2.3 Although they are placental mammals, colugos raise their young in a manner similar to marsupials. Newborn colugos weigh only 35 g, they spend the first six months of life clinging to their mother's belly. The mother colugo curls her tail and folds her patagium into a warm, quasi pouch to protect and transport her young; the young do not reach maturity until they are three years old. In captivity they live up to 15 years. Both species are threatened by habitat destruction, the Philippine flying lemur was once classified by the IUCN as vulnerable. In 1996, the IUCN declared the species vulnerable owing to destruction of lowland forests and hunting, it still faces the same threats. In addition to the ongoing clearing of its rainforest habitat, it is hunted for its fur.
It is a favorite prey item for the gravely endangered Philippine eagle: some studies suggest colugos account for 90% of the eagle's diet. Order Dermoptera Family Cynocephalidae Cynocephalus Philippine flying lemur, Cynocephalus volans Galeopterus Sunda flying lemur, Galeopterus variegatus †Dermotherium †Dermotherium major †Dermotherium chimaeraThe Mixodectidae and Plagiomenidae appear to be fossil Dermoptera. Although other Paleogene mammals have been interpreted as related to dermopterans, the evidence for this association is uncertain and many of the fossils are no longer interpreted as being gliding mammals. At present, the fossil record of definitive dermopterans is limited to two species of the Eocene and Oligocene cynocephalid genus Dermotherium. Recent molecular phylogenetic studies have demonstrated that colugos emerged as a basal Primatomorpha clade, a basal Euarchontoglires clade. Treeshrews emerged in an unnamed sister clade of the Primatomorpha; the names Colugidae and Galeopteridae are synonyms for Cynocephalidae.
Colugo, Galeolemur, Galeopithecus and Pleuropterus are synonyms for Cynocephalus. Flying lemurs are the closest relatives of primates Philippine eagle hunting Colugos
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia; the seas were warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families: the Leporidae and the Ochotonidae. The name of the order is derived from the Ancient Greek lagos + morphē. There are about eighty-seven extant species of lagomorph, including about twenty-nine species of pika, twenty-eight species of rabbit and cottontail, thirty species of hare. Lagomorphs share a common ancestor with rodents, together forming the clade Glires. Like the ancestors of most modern mammalian groups, this most recent common ancestor lived after the last great extinction event, the K–Pg extinction 66 million years ago that drove all dinosaurs except birds to extinction. Early lagomorphs arose in Asia and spread across the northern hemisphere. Rodents came to dominate more environmental niches, lagomorphs seem to have been in decline. Other names used for this order, now considered synonymous, include: Duplicidentata - Illiger, 1811; the extinct family Prolagidae is represented by a single species, the Sardinian pika Prolagus sardus, fossils of which are known from Sardinia and nearby small islands.
It may have survived until about 1774. The evolutionary history of the lagomorphs is still not well understood; until it was agreed that Eurymylus, which lived in eastern Asia and dates back to the late Paleocene or early Eocene, was an ancestor of the lagomorphs. More recent examination of the fossil evidence suggests that the lagomorphs may have instead descended from Anagaloidea known as "mimotonids", while Eurymylus was more related to rodents; the leporids first appeared in the late Eocene and spread throughout the Northern Hemisphere. The pikas appeared somewhat in the Oligocene of eastern Asia. Lagomorphs were more diverse in the past than in the present, with around 75 genera and over 230 species represented in the fossil record and many more species in a single biome; this is evidence. Recent finds suggest an Indian origin for the clade, having evolved in isolation when India was an island continent in the Paleocene. Lagomorphs are similar to other mammals in that they all have hair, four limbs, mammary glands and are endothermic.
They differ in. Although lagomorphs are more related to rodents than any other mammals, the two orders still have some major differences. Lagomorphs differ from rodents. Lagomorphs are strictly herbivorous, unlike rodents, many of which will eat both meat and vegetable matter, they resemble rodents, however, in that their incisor teeth grow continuously throughout their lives, thus necessitating constant chewing on fibrous food to prevent the teeth from growing too long. To the rodents and some mammalian insectivores, they have a smooth-surfaced cerebrum. Rabbits and hares move by jumping, pushing off with their strong hind legs and using their forelimbs to soften the impact on landing. Pikas lack certain skeletal modifications present in leporids, such as a arched skull, an upright posture of the head, strong hind limbs and pelvic girdle, long limbs. Pikas have a short nasal region and lack a supraorbital foramen, while leporids have prominent supraorbital foramina and nasal regions. Pikas known as conies, are represented by the family Ochotonidae and are small mammals native to mountainous regions of western North America, Central Asia.
They are about 15 cm long and have greyish-brown, silky fur, small rounded ears, no tail. Their four legs are nearly equal in length; some species live in scree, making their homes in the crevices between broken rocks, while others construct burrows in upland areas. The rock-dwelling species are long-lived and solitary, have one or two litters of a small number of young each year and have stable populations; the burrowing species, in contrast, are short-lived and have multiple large litters during the year. These species tend to have large swings in population size; the gestation period of the pika is around one month long, the newborns are altricial– they require parental care. The social behaviour of the two groups differs: the rock dwellers aggressively maintain scent-marked territories, while the burrowers live in family groups, interact vocally with each other and defend a mutual territory. Pikas are active early and late in the day during hot weather, they feed on all sorts of plant material.
As they do not hibernate, they make "haypiles" of dried vegetation which they collect and carry back to their homes to store for use during winter. Hares, members of genus Lepus of family Leporidae, are medium size mammals native to Europe, Asia and North America. North American jackrabbits are hares. Species vary in size from 40 to 70 cm in length and have long powerful back legs, ears up to 20 cm in length. Although greyish-brown, some species turn white in winter, they are solitary animals and several litters of young are born during the year in a form, a hollow in the ground amongst dense vegetation. The young are born furred and active, they are preyed upon by large mammalian birds of prey. Rabbits, members of family Leporidae outside Lepus, are gen
Black-tailed prairie dog
The black-tailed prairie dog is a rodent of the family Sciuridae found in the Great Plains of North America from about the United States-Canada border to the United States-Mexico border. Unlike some other prairie dogs, these animals do not hibernate; the black-tailed prairie dog can be seen above ground in midwinter. A black-tailed prairie dog town in Texas was reported to cover 25,000 sq mi and included 400,000,000 individuals. Prior to habitat destruction, this species may have been the most abundant prairie dog in central North America; this species was one of two described by the Lewis and Clark Expedition in the journals and diaries of their expedition. Black-tailed prairie dogs are tan in color, with lighter-colored bellies, they may have color variation in their pelt, such as dark fur on their back in black and brown tones. Their tails have black tips. Adults can weigh from 1.5 to 3.0 lb, males are heavier than females. Body length is from 14 to 17 in, with a 3-to-4 in tail; the black-tailed have black long claws used for digging.
The body of the black-tailed prairie dog is compact, the ears are small and close to the head. The historic range of the black-tailed prairie dog was from southern Saskatchewan to Chihuahua and included portions of Montana, North Dakota, South Dakota, Colorado, Kansas, Texas and New Mexico; as of 2007, black-tailed prairie dogs occur across most excluding Arizona. Black-tailed prairie dogs are diurnal. Above-ground activity is reduced when rain or snow is falling and during days when the temperature exceeds 100 °F. During the winter months, black-tailed prairie dogs do not hibernate, they continue to leave the burrow to forage, but will enter a state of torpor at night to conserve energy. Torpor is categorized by a drop in metabolism, heart rate and respiration similar to hibernation, but is involuntary and shorter in duration. On average, black-tailed prairie dogs will lose twenty percent of their body weight during the fall winter season when they go through bouts of torpor; as winter progressed, the amount of time spent in torpor increases.
Between different colonies the overall time spent in torpor varies, independent of prairie dog body mass. This may be due to weather during the previous growing season; as black-tailed prairie dogs receive most of their water from their diet, in years with poor rainfall, the black footed prairie dogs spend more time in torpor. Black-tailed prairie dogs are native to grassland habitats in North America, they inhabit shortgrass prairie, mixed-grass prairie, sagebrush steppe, desert grassland. Habitat preferences for the black-tailed prairie dog are influenced by vegetative cover type, soil type, amount of rainfall, their foraging and burrowing activities influence environmental heterogeneity, nutrient cycling, landscape architecture, plant succession in grassland habitats. Black-tailed prairie dogs inhabit grasslands, including short- and mixed-grass prairie, sagebrush steppe, desert grasslands. Shortgrass prairies dominated by buffalo grass, blue grama, western wheatgrass, mixed-grass prairies that have been grazed by native and non-native herbivores are their preferred habitat.
Slopes of 2% to 5% and vegetation heights between 3 and 5 in are optimal for detecting predators and facilitating communication. In the Great Plains region, black-tailed prairie dog colonies occur near rivers and creeks. Of 86 colonies located in Mellette County, South Dakota, 30 were located on benches or terraces adjacent to a creek or floodplain, 30 occurred in rolling hills with a slope more than 5°, 20 were in flat areas, six were in badland areas; the slopes of playa lakes in the Texas Panhandle and surrounding regions are used as habitat for the black-tailed prairie dog. Colonies in Phillips County, were associated with reservoirs, cattle salting grounds, other areas affected by humans. Black-tailed prairie dogs tolerate "high degrees" of disturbance over long periods of time. New colonies are created on rangeland in "good" to "excellent" condition. Black-tailed prairie dogs may colonize grazed sites, but do not specialize in colonizing overgrazed areas. Overgrazing may occur subsequent to their colonization.
Black-tailed prairie dogs were associated with areas intensively grazed by livestock and/or areas where topsoil had been disturbed by human activities in sagebrush-grassland habitat on the Charles M. Russell National Wildlife Refuge and Fort Belknap Agency, Montana. Roads and cattle trails were found in 150 of 154 black-tailed prairie dog colonies, colonies were located closer to livestock water developments and homestead sites than randomly located points. Black-tailed prairie dog distribution is not limited by soil type, but by indirect effects of soil texture on moisture and vegetation. Colonies occur in many types of soil, including deep, alluvial soils with medium to fine textures, gravel. Soil not prone to collapsing or flooding is preferred. Though they do not select specific types of soil to dig burrows, silty loam clay soils are best for tunnel construction. Surface soil textures in colonies near Fort Collins, varied from sandy loam to sandy clay loam in the top 6 in, with a sandy clay loam subsoil.
In northern latitudes, colonies occur on south aspects due to
The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, named after Cambria, the Latinised name for Wales, where rocks from this age were first studied; the Precambrian accounts for 88% of the Earth's geologic time. The Precambrian is an informal unit of geologic time, subdivided into three eons of the geologic time scale, it spans from the formation of Earth about 4.6 billion years ago to the beginning of the Cambrian Period, about 541 million years ago, when hard-shelled creatures first appeared in abundance. Little is known about the Precambrian, despite it making up seven-eighths of the Earth's history, what is known has been discovered from the 1960s onwards; the Precambrian fossil record is poorer than that of the succeeding Phanerozoic, fossils from the Precambrian are of limited biostratigraphic use. This is because many Precambrian rocks have been metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain buried beneath Phanerozoic strata.
It is thought that the Earth coalesced from material in orbit around the Sun at 4,543 Ma, may have been struck by a large planetesimal shortly after it formed, splitting off material that formed the Moon. A stable crust was in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma; the term "Precambrian" is recognized by the International Commission on Stratigraphy as the only "supereon" in geologic time. "Precambrian" is still used by geologists and paleontologists for general discussions not requiring the more specific eon names. As of 2010, the United States Geological Survey considers the term informal, lacking a stratigraphic rank. A specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area.
However, there is evidence. There is a solid record of bacterial life throughout the remainder of the Precambrian. Excluding a few contested reports of much older forms from North America and India, the first complex multicellular life forms seem to have appeared at 1500 Ma, in the Mesoproterozoic era of the Proterozoic eon. Fossil evidence from the Ediacaran period of such complex life comes from the Lantian formation, at least 580 million years ago. A diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma; these are referred to as Vendian biota. Hard-shelled creatures appeared toward the end of that time span, marking the beginning of the Phanerozoic eon. By the middle of the following Cambrian period, a diverse fauna is recorded in the Burgess Shale, including some which may represent stem groups of modern taxa; the increase in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life. While land seems to have been devoid of plants and animals and other microbes formed prokaryotic mats that covered terrestrial areas.
Tracks from an animal with leg like appendages have been found in what was mud 551 million years ago. Evidence of the details of plate motions and other tectonic activity in the Precambrian has been poorly preserved, it is believed that small proto-continents existed prior to 4280 Ma, that most of the Earth's landmasses collected into a single supercontinent around 1130 Ma. The supercontinent, known as Rodinia, broke up around 750 Ma. A number of glacial periods have been identified going as far back as the Huronian epoch 2400–2100 Ma. One of the best studied is the Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth"; the atmosphere of the early Earth is not well understood. Most geologists believe it was composed of nitrogen, carbon dioxide, other inert gases, was lacking in free oxygen. There is, evidence that an oxygen-rich atmosphere existed since the early Archean. At present, it is still believed that molecular oxygen was not a significant fraction of Earth's atmosphere until after photosynthetic life forms evolved and began to produce it in large quantities as a byproduct of their metabolism.
This radical shift from a chemically inert to an oxidizing atmosphere caused an ecological crisis, sometimes called the oxygen catastrophe. At first, oxygen would have combined with other elements in Earth's crust iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides. A terminology has evolved covering the early years of the Earth's existence, as radiometric dating has allowed real dates to be assigned to specific formations and features; the Precambrian is divided into
The Holocene is the current geological epoch. It began 11,650 cal years before present, after the last glacial period, which concluded with the Holocene glacial retreat; the Holocene and the preceding Pleistocene together form the Quaternary period. The Holocene has been identified with the current warm period, known as MIS 1, it is considered by some to be an interglacial period within the Pleistocene Epoch. The Holocene has seen the growth and impacts of the human species worldwide, including all its written history, development of major civilizations, overall significant transition toward urban living in the present. Human impacts on modern-era Earth and its ecosystems may be considered of global significance for future evolution of living species, including synchronous lithospheric evidence, or more hydrospheric and atmospheric evidence of human impacts. In July 2018, the International Union of Geological Sciences split the Holocene epoch into three distinct subsections, Greenlandian and Meghalayan, as proposed by International Commission on Stratigraphy.
The boundary stratotype of Meghalayan is a speleothem in Mawmluh cave in India, the global auxiliary stratotype is an ice core from Mount Logan in Canada. The name Holocene comes from the Ancient Greek words ὅλος and καινός, meaning "entirely recent", it is accepted by the International Commission on Stratigraphy that the Holocene started 11,650 cal years BP. The Subcommission on Quaternary Stratigraphy quotes Gibbard and van Kolfschoten in Gradstein Ogg and Smith in stating the term'Recent' as an alternative to Holocene is invalid and should not be used and observe that the term Flandrian, derived from marine transgression sediments on the Flanders coast of Belgium has been used as a synonym for Holocene by authors who consider the last 10,000 years should have the same stage-status as previous interglacial events and thus be included in the Pleistocene; the International Commission on Stratigraphy, considers the Holocene an epoch following the Pleistocene and the last glacial period. Local names for the last glacial period include the Wisconsinan in North America, the Weichselian in Europe, the Devensian in Britain, the Llanquihue in Chile and the Otiran in New Zealand.
The Holocene can be subdivided into five time intervals, or chronozones, based on climatic fluctuations: Preboreal, Atlantic and Subatlantic. Note: "ka" means "kilo-annum" Before Present, i.e. 1,000 years before 1950 The Blytt–Sernander classification of climatic periods defined by plant remains in peat mosses, is being explored. Geologists working in different regions are studying sea levels, peat bogs and ice core samples by a variety of methods, with a view toward further verifying and refining the Blytt–Sernander sequence, they find a general correspondence across Eurasia and North America, though the method was once thought to be of no interest. The scheme was defined for Northern Europe, but the climate changes were claimed to occur more widely; the periods of the scheme include a few of the final pre-Holocene oscillations of the last glacial period and classify climates of more recent prehistory. Paleontologists have not defined any faunal stages for the Holocene. If subdivision is necessary, periods of human technological development, such as the Mesolithic and Bronze Age, are used.
However, the time periods referenced by these terms vary with the emergence of those technologies in different parts of the world. Climatically, the Holocene may be divided evenly into the Neoglacial periods. According to some scholars, a third division, the Anthropocene, has now begun; the International Commission on Stratigraphy Subcommission on Quaternary Stratigraphy’s working group on the'Anthropocene' note this term is used to denote the present time interval in which many geologically significant conditions and processes have been profoundly altered by human activities. The'Anthropocene' is not a formally defined geological unit. Continental motions due to plate tectonics are less than a kilometre over a span of only 10,000 years. However, ice melt caused world sea levels to rise about 35 m in the early part of the Holocene. In addition, many areas above about 40 degrees north latitude had been depressed by the weight of the Pleistocene glaciers and rose as much as 180 m due to post-glacial rebound over the late Pleistocene and Holocene, are still rising today.
The sea level rise and temporary land depression allowed temporary marine incursions into areas that are now far from the sea. Holocene marine fossils are known, from Vermont and Michigan. Other than higher-latitude temporary marine incursions associated with glacial depression, Holocene fossils are found in lakebed and cave deposits. Holocene marine deposits along low-latitude coastlines are rare because the rise in sea levels during the period exceeds any tectonic uplift of non-glacial origin. Post-glacial rebound in the Scandinavia region resulted in the formation of the Baltic Sea; the region continues to rise, still causing weak earthquakes across Northern Europe. The equivalent event in North America was the rebound of Hudson Bay, as it shrank from its larger, immediate post-glacial Tyrrell Sea phase, to near its present boundaries. Climate has been stable over the Holocene. Ice core
Deoxyribonucleic acid is a molecule composed of two chains that coil around each other to form a double helix carrying the genetic instructions used in the growth, development and reproduction of all known organisms and many viruses. DNA and ribonucleic acid are nucleic acids; the two DNA strands are known as polynucleotides as they are composed of simpler monomeric units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases, a sugar called deoxyribose, a phosphate group; the nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules, with hydrogen bonds to make double-stranded DNA; the complementary nitrogenous bases are divided into two groups and purines. In DNA, the pyrimidines are cytosine. Both strands of double-stranded DNA store the same biological information.
This information is replicated as and when the two strands separate. A large part of DNA is non-coding, meaning that these sections do not serve as patterns for protein sequences; the two strands of DNA are thus antiparallel. Attached to each sugar is one of four types of nucleobases, it is the sequence of these four nucleobases along the backbone. RNA strands are created using DNA strands as a template in a process called transcription. Under the genetic code, these RNA strands specify the sequence of amino acids within proteins in a process called translation. Within eukaryotic cells, DNA is organized into long structures called chromosomes. Before typical cell division, these chromosomes are duplicated in the process of DNA replication, providing a complete set of chromosomes for each daughter cell. Eukaryotic organisms store most of their DNA inside the cell nucleus as nuclear DNA, some in the mitochondria as mitochondrial DNA, or in chloroplasts as chloroplast DNA. In contrast, prokaryotes store their DNA only in circular chromosomes.
Within eukaryotic chromosomes, chromatin proteins, such as histones and organize DNA. These compacting structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed. DNA was first isolated by Friedrich Miescher in 1869, its molecular structure was first identified by Francis Crick and James Watson at the Cavendish Laboratory within the University of Cambridge in 1953, whose model-building efforts were guided by X-ray diffraction data acquired by Raymond Gosling, a post-graduate student of Rosalind Franklin. DNA is used by researchers as a molecular tool to explore physical laws and theories, such as the ergodic theorem and the theory of elasticity; the unique material properties of DNA have made it an attractive molecule for material scientists and engineers interested in micro- and nano-fabrication. Among notable advances in this field are DNA origami and DNA-based hybrid materials. DNA is a long polymer made from repeating units called nucleotides.
The structure of DNA is dynamic along its length, being capable of coiling into tight loops and other shapes. In all species it is composed of two helical chains, bound to each other by hydrogen bonds. Both chains are coiled around the same axis, have the same pitch of 34 angstroms; the pair of chains has a radius of 10 angstroms. According to another study, when measured in a different solution, the DNA chain measured 22 to 26 angstroms wide, one nucleotide unit measured 3.3 Å long. Although each individual nucleotide is small, a DNA polymer can be large and contain hundreds of millions, such as in chromosome 1. Chromosome 1 is the largest human chromosome with 220 million base pairs, would be 85 mm long if straightened. DNA does not exist as a single strand, but instead as a pair of strands that are held together; these two long strands coil in the shape of a double helix. The nucleotide contains both a segment of the backbone of a nucleobase. A nucleobase linked to a sugar is called a nucleoside, a base linked to a sugar and to one or more phosphate groups is called a nucleotide.
A biopolymer comprising multiple linked nucleotides is called a polynucleotide. The backbone of the DNA strand is made from alternating sugar residues; the sugar in DNA is 2-deoxyribose, a pentose sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings; these are known as the 3′-end, 5′-end carbons, the prime symbol being used to distinguish these carbon atoms from those of the base to which the deoxyribose forms a glycosidic bond. When imagining DNA, each phosphoryl is considered to "belong" to the nucleotide whose 5′ carbon forms a bond therewith. Any DNA strand therefore has one end at which there is a phosphoryl attached to the 5′ carbon of a ribose and another end a