Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organization. Nucleation is defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. For example, if a volume of water is cooled below 0° C, it will tend to freeze into ice. Volumes of water cooled only a few degrees below 0° C stay free of ice for long periods. At these conditions, nucleation of ice does not occur at all. However, at lower temperatures ice crystals appear after no delay. At these conditions ice nucleation is fast. Nucleation is how first-order phase transitions start, it is the start of the process of forming a new thermodynamic phase. By contrast, new phases at continuous phase transitions start to form immediately. Nucleation is found to be sensitive to impurities in the system; these impurities may be too small to be seen by the naked eye, but still can control the rate of nucleation.
Because of this, it is important to distinguish between heterogeneous nucleation and homogeneous nucleation. Heterogeneous nucleation occurs at nucleation sites on surfaces in the system. Homogeneous nucleation occurs away from a surface. Nucleation is a stochastic process, so in two identical systems nucleation will occur at different times; this behaviour is similar to radioactive decay. A common mechanism is illustrated in the animation to the right; this shows nucleation of a new phase in an existing phase. In the existing phase microscopic fluctuations of the red phase appear and decay continuously, until an unusually large fluctuation of the new red phase is so large it is more favourable for it to grow than to shrink back to nothing; this nucleus of the red phase grows and converts the system to this phase. The standard theory that describes this behaviour for the nucleation of a new thermodynamic phase is called classical nucleation theory. However, the CNT fails in describing experimental results of vapour to liquid nucleation for model substances like Argon by several orders of magnitude.
For nucleation of a new thermodynamic phase, such as the formation of ice in water below 0° C, if the system is not evolving with time and nucleation occurs in one step the probability that nucleation has not occurred should undergo exponential decay as seen in radioactive decay. This is seen for example in the nucleation of ice in supercooled small water droplets; the decay rate of the exponential gives the nucleation rate. Classical nucleation theory is a used approximate theory for estimating these rates, how they vary with variables such as temperature, it predicts that the time you have to wait for nucleation decreases rapidly when supersaturated. It is not just new phases such as crystals that form via nucleation followed by growth; the self-assembly process that forms objects like the amyloid aggregates associated with Alzheimer's disease starts with nucleation. Energy consuming self-organising systems such as the microtubules in cells show nucleation and growth. Heterogeneous nucleation, nucleation with the nucleus at a surface, is much more common than homogeneous nucleation.
For example, in the nucleation of ice from supercooled water droplets, purifying the water to remove all or all impurities results in water droplets that freeze below around - 35 C, whereas water that contains impurities may freeze at - 5 C or warmer. Thus here, we have direct evidence that nucleation of ice on impurities can occur at much higher temperatures than without impurities; this observation that heterogeneous nucleation can occur when the rate of homogeneous nucleation is zero, is understood using classical nucleation theory. This predicts that the nucleation slows exponentially with the height of a free energy barrier ΔG*; this barrier comes from the free energy penalty of forming the surface of the growing nucleus. For homogeneous nucleation the nucleus is approximated by a sphere, but as we can see in the schematic of macroscopic droplets to the right, droplets on surfaces are not complete spheres and so the area of the interface between the droplet and the surrounding fluid is less than a sphere's 4 π r 2.
This reduction in surface area of the nucleus reduces the height of the barrier to nucleation and so speeds nucleation up exponentially. Nucleation can start at the surface of a liquid. For example, computer simulations of gold nanoparticles show that the crystal phase nucleates at the liquid-gold surface. Classical nucleation theory makes a number of assumptions, for example it treats a microscopic nucleus as if it is a macroscopic droplet with a well-defined surface whose free energy is estimated using an equilibrium property: the interfacial tension σ. For a nucleus that may be only of order ten molecules across it is not always clear that we can treat something so small as a volume plus a surface. Nucleation is an inherently out of thermodynamic equilibrium phenomenon so it is not always obvious that its rate can be estimated using equilibrium properties. However, modern computers are powerful enough to calculate exact nucleation rates for simple models; these have been compared with the classical theory, for example for the case of nucleation of the crystal phase in the model of hard spheres.
This is a model of hard spheres in thermal motion, is a simple model of some colloids. For the crystallization of hard spheres the classical theory is a reasonable approximate theory. So for the simple models w
Taylor & Francis
Taylor & Francis Group is an international company originating in England that publishes books and academic journals. It is a division of a United Kingdom-based publisher and conference company; the company was founded in 1852 when William Francis joined Richard Taylor in his publishing business. Taylor founded his company in 1798, their subjects covered agriculture, education, geography, mathematics and social sciences. From 1917 to 1930 Francis' son, Richard Taunton Francis was sole partner in the firm. In 1965 Taylor & Francis began book publishing. In 1988 it acquired Hemisphere Publishing and the company was renamed Taylor & Francis Group to reflect the growing number of imprints. In 1990 Taylor & Francis exited from the printing business to concentrate on publishing. In 1998 Taylor & Francis Group went public on the London Stock Exchange and in the same year the group purchased its academic publishing rival Routledge for £90 million. Acquisitions of other publishers has remained a core part of the group's business strategy.
Taylor & Francis merged with Informa in 2004 to create a new company called T&F Informa, since renamed back to Informa. Following the merger, T&F closed the historic Routledge books office in New Fetter Lane and relocated to its current headquarters in Milton Park, Oxfordshire. Taylor & Francis Group is now the academic publishing arm of Informa and accounted for 30.2% of Group Revenue and 38.1% of Adjusted Profit in 2017. Taylor & Francis publishes more than 2,700 journals, 7,000 new books each year, with a backlist of over 140,000 titles available in print and digital formats, it uses the Routledge imprint for its publishing in humanities, social sciences, behavioural sciences and education and the CRC Press imprint for its publishing in science, technology and mathematics. In 2017, T&F sold assets from its Garland Science imprint to W. W. Norton & Company and ceased to use that brand. Although considered the smallest of the'Big Four' STEM publishers, its Routledge imprint is claimed to be the largest global academic publisher within humanities and social sciences.
The company's journals have been delivered through the Taylor & Francis Online website since June 2011. Prior to that they were provided through the Informaworld website. Taylor & Francis ebooks are now available via the TaylorFrancis website. Taylor & Francis operates a number of Web services for its digital content including Routledge Handbooks Online, the Routledge Performance Archive, Secret Intelligence Files and Routledge Encyclopedia of Modernism. Taylor & Francis offers Open Access publishing options in both its books and journals divisions and through its Cogent Open Access journals imprint. Taylor & Francis is a member of several professional publishing bodies including the Open Access Scholarly Publishers Association, the International Association of Scientific and Medical Publishers, the Association of Learned & Professional Society Publishers and The Publishers Association. In 2017, after collaborating for several years, T&F purchased specialist digital resources company Colwiz.
The group has 1,800 employees located in at least 18 offices worldwide. Its head office is based in Milton Park, Abingdon in the United Kingdom, with other offices in Stockholm, New York, Boca Raton, Kentucky, Kuala Lumpur, Hong Kong, Shanghai, Melbourne, Cape Town and New Delhi; the old Taylor and Francis logo depicts a hand pouring oil into a lit lamp, along with the Latin phrase "alere flammam" - to feed the flame. The modern logo is a stylised oil lamp in a circle. In 2013, the entire board of the Journal of Library Administration resigned in a dispute over author licensing agreements. In 2016 Critical Reviews in Toxicology was accused of being a "broker of junk science" by the Center for Public Integrity. Monsanto was found to have worked with an outside consulting firm to induce the journal to publish a biased review of the health effects of its product "Roundup". In 2017, Taylor & Francis was criticized for getting rid of the editor-in-chief of International Journal of Occupational and Environmental Health, who accepted articles critical of corporate interests.
The company replaced the editor with a corporate consultant without consulting the editorial board. The journal Cogent Social Sciences accepted a hoax article, "The conceptual penis as a social construct", rejected by another Taylor & Francis journal, NORMA: International Journal for Masculinity Studies; when the authors announced the hoax, the article was retracted. In December 2018, the journal Dynamical Systems accepted the paper Saturation of Generalized Partially Hyperbolic Attractors only to have it retracted after publication due to the Iranian nationality of the authors; the European Mathematical Society condemned the retraction and announced that Taylor & Francis had agreed to reverse the decision. Previous instances of Taylor & Francis journals discriminating against Iranian authors were reported in 2013. Taylor & Francis academic journals Munroe, Mary H.. "Taylor & Francis". The Academic Publishing Industry: A Story of Merger and Acquisition. Northern Illinois University Libraries. Archived from the original on 2012-05-04.
Retrieved 2008-06-20. Brock, W. H. & Meadows, A. J.. The Lamp Of Learning: Taylor & Francis And Two Centuries Of Publishing. Taylor & Francis. Official website Taylor & Francis online journals and reference works Taylor & Francis eBooks Informa Divisions - Academic Publishing
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles 0.2 µm, or 1/100th the size of a cloud droplet on which water vapor condenses. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny liquid particles called CCNs; when no CCNs are present, water vapour can be supercooled at about −13°C for 5–6 hours before droplets spontaneously form. In above freezing temperatures the air would have to be supersaturated to around 400% before the droplets could form; the concept of cloud condensation nuclei is used in cloud seeding, that tries to encourage rainfall by seeding the air with condensation nuclei. It has further been suggested that creating such nuclei could be used for marine cloud brightening, a climate engineering technique. A typical raindrop is about 2 mm in diameter, a typical cloud droplet is on the order of 0.02 mm, a typical cloud condensation nucleus is on the order of 0.0001 mm or 0.1 µm or greater in diameter.
The number of cloud condensation nuclei in the air can be measured and ranges between around 100 to 1000 per cubic centimetre. The total mass of CCNs injected into the atmosphere has been estimated at 2x1012 kg over a year's time. There are many different types of atmospheric particulates that can act as CCN; the particles may be composed of dust or clay, soot or black carbon from grassland or forest fires, sea salt from ocean wave spray, soot from factory smokestacks or internal combustion engines, sulfate from volcanic activity, phytoplankton or the oxidation of sulfur dioxide and secondary organic matter formed by the oxidation of volatile organic compounds. The ability of these different types of particles to form cloud droplets varies according to their size and their exact composition, as the hygroscopic properties of these different constituents are different. Sulfate and sea salt, for instance absorb water whereas soot, organic carbon and mineral particles do not; this is made more complicated by the fact that many of the chemical species may be mixed within the particles.
Additionally, while some particles do not make good CCN, they do act as ice nuclei in colder parts of the atmosphere. The number and type of CCNs can affect the precipitation amount and radiative properties of clouds as well as the amount and hence have an influence on climate change. There is speculation that solar variation may affect cloud properties via CCNs, hence affect climate. Sulfate aerosol act as CCNs; these sulfate aerosols form from the dimethyl sulfide produced by phytoplankton in the open ocean. Large algal blooms in ocean surface waters occur in a wide range of latitudes and contribute considerable DMS into the atmosphere to act as nuclei; the idea that an increase in global temperature would increase phytoplankton activity and therefore CCN numbers was seen as a possible natural phenomenon that would counteract climate change. An increase of phytoplankton has been observed by scientists in certain areas but the causes are unclear. A counter-hypothesis is advanced in The Revenge of the book by James Lovelock.
Warming oceans are to become stratified, with most ocean nutrients trapped in the cold bottom layers while most of the light needed for photosynthesis in the warm top layer. Under this scenario, deprived of nutrients, marine phytoplankton would decline, as would sulfate cloud condensation nuclei, the high albedo associated with low clouds; this is known as the CLAW hypothesis but no conclusive evidence to support this has yet been reported. Bergeron process Evapotranspiration Global dimming Seed crystal Water cycle www.grida.no
In cell biology, the nucleus is a membrane-bound organelle found in eukaryotic cells. Eukaryotes have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, a few others including osteoclasts have many; the cell nucleus contains all of the cell's genome, except for a small fraction of mitochondrial DNA, organized as multiple long linear DNA molecules in a complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are structured in such a way to promote cell function; the nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, the control center of the cell. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, the nuclear matrix, a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole.
Because the nuclear envelope is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope. The pores cross both nuclear membranes, providing a channel through which larger molecules must be transported by carrier proteins while allowing free movement of small molecules and ions. Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes. Although the interior of the nucleus does not contain any membrane-bound subcompartments, its contents are not uniform, a number of nuclear bodies exist, made up of unique proteins, RNA molecules, particular parts of the chromosomes; the best-known of these is the nucleolus, involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA; the nucleus was the first organelle to be discovered. What is most the oldest preserved drawing dates back to the early microscopist Antonie van Leeuwenhoek.
He observed the nucleus, in the red blood cells of salmon. Unlike mammalian red blood cells, those of other vertebrates still contain nuclei; the nucleus was described by Franz Bauer in 1804 and in more detail in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of London. Brown was studying orchids under the microscope when he observed an opaque area, which he called the "areola" or "nucleus", in the cells of the flower's outer layer, he did not suggest a potential function. In 1838, Matthias Schleiden proposed that the nucleus plays a role in generating cells, thus he introduced the name "cytoblast", he believed that he had observed new cells assembling around "cytoblasts". Franz Meyen was a strong opponent of this view, having described cells multiplying by division and believing that many cells would have no nuclei; the idea that cells can be generated de novo, by the "cytoblast" or otherwise, contradicted work by Robert Remak and Rudolf Virchow who decisively propagated the new paradigm that cells are generated by cells.
The function of the nucleus remained unclear. Between 1877 and 1878, Oscar Hertwig published several studies on the fertilization of sea urchin eggs, showing that the nucleus of the sperm enters the oocyte and fuses with its nucleus; this was the first time. This was in contradiction to Ernst Haeckel's theory that the complete phylogeny of a species would be repeated during embryonic development, including generation of the first nucleated cell from a "monerula", a structureless mass of primordial mucus. Therefore, the necessity of the sperm nucleus for fertilization was discussed for quite some time. However, Hertwig confirmed his observation in other animal groups, including amphibians and molluscs. Eduard Strasburger produced the same results for plants in 1884; this paved the way to assign the nucleus an important role in heredity. In 1873, August Weismann postulated the equivalence of the maternal and paternal germ cells for heredity; the function of the nucleus as carrier of genetic information became clear only after mitosis was discovered and the Mendelian rules were rediscovered at the beginning of the 20th century.
The nucleus is the largest organelle in animal cells. In mammalian cells, the average diameter of the nucleus is 6 micrometres, which occupies about 10% of the total cell volume; the contents of the nucleus are held in the nucleoplasm similar to the cytoplasm in the rest of the cell. The fluid component of this is termed the nucleosol, similar to the cytosol in the cytoplasm. In most types of granulocyte, a white blood cell, the nucleus is lobated and can be bi-lobed, tri-lobed or multi-lobed; the nuclear envelope, otherwise known as nuclear membrane, consists of two cellular membranes, an inner and an outer membrane, arranged parallel to one another and separated by 10 to 50 nanometres. The nuclear envelope encloses the nucleus and separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing between the nucleoplasm and the cytoplasm; the outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum, is studded with ribosomes.
The space between the membranes is called the perinuclear space and is continuous with the RER lumen. Nuclear pores, which provide aqueous cha
The nucleus is the solid, central part of a comet, popularly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock and frozen gases; when heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04. This is blacker than coal, may be caused by a covering of dust. Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons produced from photoionization of water molecules by solar radiation, not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.
On 30 July 2015, scientists reported that the Philae spacecraft, that landed on comet 67P/Churyumov-Gerasimenko in November 2014, detected at least 16 organic compounds, of which four were detected for the first time on a comet. Comets, or their precursors, formed in the outer Solar System millions of years before planet formation. How and when comets formed is debated, with distinct implications for Solar System formation and geology. Three-dimensional computer simulations indicate the major structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals; the favored creation mechanism is that of the nebular hypothesis, which states that comets are a remnant of the original planetesimal "building blocks" from which the planets grew. Astronomers think that comets originate in both the scattered disk. Most cometary nuclei are thought to be no more than about 16 kilometers across; the largest comets that have come inside the orbit of Saturn are C/2002 VQ94, Hale–Bopp, 29P, 109P/Swift–Tuttle, 28P/Neujmin.
The potato-shaped nucleus of Halley's comet contains equal amounts of dust. During a flyby in September 2001, the Deep Space 1 spacecraft observed the nucleus of Comet Borrelly and found it to be about half the size of the nucleus of Halley's Comet. Borrelly's nucleus was potato-shaped and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight; the nucleus of comet Hale–Bopp was estimated to be 60 ± 20 km in diameter. Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas; the nucleus of P/2007 R5 is only 100–200 meters in diameter. The largest centaurs are estimated to be 250 km to 300 km in diameter. Three of the largest would include 10199 Chariklo, 2060 Chiron, the lost 1995 SN55. Known comets have been estimated to have an average density of 0.6 g/cm3. Below is a list of comets that have had estimated sizes and masses. About 80% of the Halley's Comet nucleus is water ice, frozen carbon monoxide makes up another 15%.
Much of the remainder is frozen carbon dioxide and ammonia. Scientists think; the nucleus of Halley's Comet is an dark black. Scientists think that the surface of the comet, most other comets, is covered with a black crust of dust and rock that covers most of the ice; these comets release gas only when holes in this crust rotate toward the Sun, exposing the interior ice to the warming sunlight. The composition of water vapor from Churyumov–Gerasimenko comet, as determined by the Rosetta mission, is different from that found on Earth; the ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water. This makes it unlikely. On 67P/Churyumov–Gerasimenko comet, some of the resulting water vapour may escape from the nucleus, but 80% of it recondenses in layers beneath the surface; this observation implies that the thin ice-rich layers exposed close to the surface may be a consequence of cometary activity and evolution, that global layering does not occur early in the comet's formation history.
Measurements carried out by the Philae lander on 67P/Churyumov–Gerasimenko comet, indicate that the dust layer could be as much as 20 cm thick. Beneath, hard ice, or a mixture of ice and dust. Porosity appears to increase toward the center of the comet. While most scientists thought that all the evidence indicated that the structure of nuclei of comets is processed rubble piles of smaller ice planetesimals of a previous generation, the Rosetta mission dispelled the idea that comets are "rubble piles" of disparate material; the nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. Splitting comets include 3D/Biela in 1846, Shoemaker–Levy 9 in 1992, 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to internal gas pressure, or impact. Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet.
Numerical integrations have shown that both comets h
Nucleic acids are the biopolymers, or small biomolecules, essential to all known forms of life. The term nucleic acid is the overall name for DNA and RNA, they are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. If the sugar is a compound ribose, the polymer is RNA. Nucleic acids are the most important of all biomolecules, they are found in abundance in all living things, where they function to create and encode and store information in the nucleus of every living cell of every life-form organism on Earth. In turn, they function to transmit and express that information inside and outside the cell nucleus—to the interior operations of the cell and to the next generation of each living organism; the encoded information is contained and conveyed via the nucleic acid sequence, which provides the'ladder-step' ordering of nucleotides within the molecules of RNA and DNA. Strings of nucleotides are bonded to form helical backbones—typically, one for RNA, two for DNA—and assembled into chains of base-pairs selected from the five primary, or canonical, which are: adenine, guanine and uracil.
Using amino acids and the process known as protein synthesis, the specific sequencing in DNA of these nucleobase-pairs enables storing and transmitting coded instructions as genes. In RNA, base-pair sequencing provides for manufacturing new proteins that determine the frames and parts and most chemical processes of all life forms. Nuclein were discovered by Friedrich Miescher in 1869. In the early 1880s Albrecht Kossel further purifies the substance and discovers its acidic properties, he also identifies the nucleobases. In 1889 Richard Altmann creates the term nucleic acid In 1938 Astbury and Bell published the first X-ray diffraction pattern of DNA. In 1953 Watson and Crick determined the structure of DNA. Experimental studies of nucleic acids constitute a major part of modern biological and medical research, form a foundation for genome and forensic science, the biotechnology and pharmaceutical industries; the term nucleic acid is the overall name for DNA and RNA, members of a family of biopolymers, is synonymous with polynucleotide.
Nucleic acids were named for their initial discovery within the nucleus, for the presence of phosphate groups. Although first discovered within the nucleus of eukaryotic cells, nucleic acids are now known to be found in all life forms including within bacteria, mitochondria, chloroplasts and viroids.. All living cells contain both DNA and RNA, while viruses contain either DNA or RNA, but not both; the basic component of biological nucleic acids is the nucleotide, each of which contains a pentose sugar, a phosphate group, a nucleobase. Nucleic acids are generated within the laboratory, through the use of enzymes and by solid-phase chemical synthesis; the chemical methods enable the generation of altered nucleic acids that are not found in nature, for example peptide nucleic acids. Nucleic acids are very large molecules. Indeed, DNA molecules are the largest individual molecules known. Well-studied biological nucleic acid molecules range in size from 21 nucleotides to large chromosomes. In most cases occurring DNA molecules are double-stranded and RNA molecules are single-stranded.
There are numerous exceptions, however—some viruses have genomes made of double-stranded RNA and other viruses have single-stranded DNA genomes, and, in some circumstances, nucleic acid structures with three or four strands can form. Nucleic acids are linear polymers of nucleotides; each nucleotide consists of three components: a purine or pyrimidine nucleobase, a pentose sugar, a phosphate group. The substructure consisting of a nucleobase plus sugar is termed a nucleoside. Nucleic acid types differ in the structure of the sugar in their nucleotides–DNA contains 2'-deoxyribose while RNA contains ribose; the nucleobases found in the two nucleic acid types are different: adenine and guanine are found in both RNA and DNA, while thymine occurs in DNA and uracil occurs in RNA. The sugars and phosphates in nucleic acids are connected to each other in an alternating chain through phosphodiester linkages. In conventional nomenclature, the carbons to which the phosphate groups attach are the 3'-end and the 5'-end carbons of the sugar.
This gives nucleic acids directionality, the ends of nucleic acid molecules are referred to as 5'-end and 3'-end. The nucleobases are joined to the sugars via an N-glycosidic linkage involving a nucleobase ring nitrogen and the 1' carbon of the pentose sugar ring. Non-standard nucleosides are found in both RNA and DNA and arise from modification of the standard nucleosides within the DNA molecule or the primary RNA transcript. Transfer RNA molecules contain a large number of modified nucleosides. Double-stranded nucleic acids are made up of complementary sequences, in which extensive Watson-Crick base pairing results in a repeated and quite uniform double-helical three-dimensional structure. In contrast, single-stranded