Bronze is the most popular metal for cast metal sculptures. It can be used for statues, singly or in groups and small statuettes and figurines, as well as bronze elements to be fitted to other objects such as furniture, it is gilded to give gilt-bronze or ormolu. Common bronze alloys have the unusual and desirable property of expanding just before they set, thus filling the finest details of a mould; as the bronze cools, it shrinks a little, making it easier to separate from the mould. Their strength and ductility is an advantage when figures in action are to be created when compared to various ceramic or stone materials; these qualities allow the creation of extended figures, as in Jeté, or figures that have small cross sections in their support, such as the equestrian statue of Richard the Lionheart. But the value of the bronze for uses other than making statues is disadvantageous to the preservation of sculptures; as as 2007 several life sized bronze sculptures by John Waddell were stolen due to the value of the metal after the work has been melted.
There are many different bronze alloys, the term is now tending to be regarded by museums as too imprecise, replaced in descriptions by "copper alloy" for older objects. Modern bronze is 88% copper and 12% tin. Alpha bronze consists of the alpha solid solution of tin in copper. Alpha bronze alloys of 4 -- 5 % tin are used to make a number of mechanical applications. Historical bronzes are variable in composition, as most metalworkers used whatever scrap was on hand; the proportions of this mixture may suggest. The Benin Bronzes are brass, the Romanesque Baptismal font at St Bartholomew's Church, Liège is described as both bronze and brass. In the Bronze Age, two forms of bronze were used: "classic bronze", about 10% tin, was used in casting. Bladed weapons were cast from classic bronze, while helmets and armour were hammered from mild bronze. According to one definition, modern "statuary bronze" is 10 % tin; the great civilizations of the old world worked in bronze for art, from the time of the introduction of the alloy for tools and edged weapons.
Dancing Girl from Mohenjodaro, belonging to the Harappan civilization and dating back to c. 2500 BCE, is the first known bronze statue. The Greeks were the first to scale the figures up to life size. Few examples exist in good condition. Far more Roman bronze statues have survived; the ancient Chinese knew both lost-wax casting and section mould casting, during the Shang dynasty created large numbers of Chinese ritual bronzes, ritual vessels covered with complex decoration, which were buried in sets of up to 200 pieces in the tombs of royalty and the nobility. Over the long creative period of Egyptian dynastic art, small lost-wax bronze figurines were made in large numbers. Sri Lankan Sinhalese bronze statue of Buddhist Alakothiveshwara Tara Devi statue, now in England, is an excellent example of Bronze statues. From the ninth through the thirteenth century the Chola dynasty in South India represented the pinnacle of bronze casting in India. Making bronzes is skilled work, a number of distinct casting processes may be employed, including lost-wax casting, sand casting and centrifugal casting.
The term "bronze" is applied to metal sculptures made by electrotyping, although these sculptures are pure copper and their fabrication does not involve metal casting. In lost-wax or investment casting, the artist starts with a full-sized model of the sculpture, most a non-drying oil-based clay such as Plasticine model for smaller sculptures or for sculptures to be developed over an extended period, water-based clay for larger sculptures or for sculptures for which it is desired to capture a gestural quality - one that transmits the motion of the sculptor in addition to that of the subject. A mould is made from the clay pattern, either as a piece mould from plaster, or using flexible gel or similar rubber-like materials stabilized by a plaster jacket of several pieces. A plaster master will be made from this mould for further refinement; such a plaster is a means of preserving the artwork until a patron may be found to finance a bronze casting, either from the original moulds or from a new mould made from the refined plaster positive.
Once a production mould is obtained, a wax is cast from the mould. For a hollow sculpture, a core is cast into the void, is retained in its proper location by pins of the same metal used for casting. One or more wax sprues are added to conduct the molten metal into the sculptures - directing the liquid metal from a pouring
Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products. Sophisticated programs of gene expression are observed in biology, for example to trigger developmental pathways, respond to environmental stimuli, or adapt to new food sources. Any step of gene expression can be modulated, from transcriptional initiation, to RNA processing, to the post-translational modification of a protein. One gene regulator controls another, so on, in a gene regulatory network. Gene regulation is essential for viruses and eukaryotes as it increases the versatility and adaptability of an organism by allowing the cell to express protein when needed. Although as early as 1951, Barbara McClintock showed interaction between two genetic loci and Dissociator, in the color formation of maize seeds, the first discovery of a gene regulation system is considered to be the identification in 1961 of the lac operon, discovered by François Jacob and Jacques Monod, in which some enzymes involved in lactose metabolism are expressed by E. coli only in the presence of lactose and absence of glucose.
In multicellular organisms, gene regulation drives cellular differentiation and morphogenesis in the embryo, leading to the creation of different cell types that possess different gene expression profiles from the same genome sequence. Although this does not explain how gene regulation originated, evolutionary biologists include it as a partial explanation of how evolution works at a molecular level, it is central to the science of evolutionary developmental biology. Any step of gene expression may be modulated, from the DNA-RNA transcription step to post-translational modification of a protein; the following is a list of stages where gene expression is regulated, the most extensively utilised point is Transcription Initiation: Chromatin domains Transcription Post-transcriptional modification RNA transport Translation mRNA degradation In eukaryotes, the accessibility of large regions of DNA can depend on its chromatin structure, which can be altered as a result of histone modifications directed by DNA methylation, ncRNA, or DNA-binding protein.
Hence these modifications may down regulate the expression of a gene. Some of these modifications that regulate gene expression are inheritable and are referred to as epigenetic regulation. Transcription of DNA is dictated by its structure. In general, the density of its packing is indicative of the frequency of transcription. Octameric protein complexes called nucleosomes are responsible for the amount of supercoiling of DNA, these complexes can be temporarily modified by processes such as phosphorylation or more permanently modified by processes such as methylation; such modifications are considered to be responsible for more or less permanent changes in gene expression levels. Methylation of DNA is a common method of gene silencing. DNA is methylated by methyltransferase enzymes on cytosine nucleotides in a CpG dinucleotide sequence. Analysis of the pattern of methylation in a given region of DNA can be achieved through a method called bisulfite mapping. Methylated cytosine residues are unchanged by the treatment, whereas unmethylated ones are changed to uracil.
The differences are analyzed by DNA sequencing or by methods developed to quantify SNPs, such as Pyrosequencing or MassArray, measuring the relative amounts of C/T at the CG dinucleotide. Abnormal methylation patterns are thought to be involved in oncogenesis. Histone acetylation is an important process in transcription. Histone acetyltransferase enzymes such as CREB-binding protein dissociate the DNA from the histone complex, allowing transcription to proceed. DNA methylation and histone deacetylation work together in gene silencing; the combination of the two seems to be a signal for DNA to be packed more densely, lowering gene expression. Regulation of transcription thus controls how much RNA is created. Transcription of a gene by RNA polymerase can be regulated by several mechanisms. Specificity factors alter the specificity of RNA polymerase for a given promoter or set of promoters, making it more or less to bind to them. Repressors bind to the Operator, coding sequences on the DNA strand that are close to or overlapping the promoter region, impeding RNA polymerase's progress along the strand, thus impeding the expression of the gene.
The image to the right demonstrates regulation by a repressor in the lac operon. General transcription factors position RNA polymerase at the start of a protein-coding sequence and release the polymerase to transcribe the mRNA. Activators enhance the interaction between RNA polymerase and a particular promoter, encouraging the expression of the gene. Activators do this by increasing the attraction of RNA polymerase for the promoter, through interactions with subunits of the RNA polymerase or indirectly by changing the structure of the DNA. Enhancers are sites on the DNA helix that are bound by activators in order to loop the DNA bringing a specific promoter to the initiation complex. Enhancers are much more common in eukaryotes than prokaryotes. Silencers are regions of DNA sequences that, when bound by particular transcription factors, can silence expression of the gene. In vertebrates, the majority of gene promoters contain a CpG island with numerous CpG sites; when many of a gene's promoter CpG sites are methylated the gene becomes silenced.
Colorectal cancers have 3 to 6 driver mutat
Eurypterus is an extinct genus of eurypterid, a group of organisms called "sea scorpions". The genus lived during the Silurian period, from around 432 to 418 million years ago. Eurypterus is by far the most well-studied and well-known eurypterid and its fossil specimens represent more than 90% of all known eurypterid specimens. There are fifteen species belonging to the genus Eurypterus, the most common of, Eurypterus remipes, the first eurypterid fossil discovered and the state fossil of New York. Members of Eurypterus averaged at about 13 to 23 cm in length, but the largest individual discovered was 1.3 m long. They all possessed a large paddle they used for swimming, they were generalist species likely to engage in predation or scavenging. The first fossil of Eurypterus was found in 1818 by a fossil collector, it was recovered from the Bertie Formation of New York. Mitchill interpreted the appendages on the carapace as barbels arising from the mouth, he identified the fossil as a catfish of the genus Silurus.
It was only after seven years, in 1825, that the American zoologist James Ellsworth De Kay identified the fossil as an arthropod. He named it established the genus Eurypterus in the process; the name means "wide wing" or "broad paddle", referring to the swimming legs, from Greek εὐρύς and πτερόν. However, De Kay thought. Soon after, Eurypterus lacustris was discovered in New York in 1835 by the paleontologist Richard Harlan. Another species was discovered in Estonia in 1858 by Jan Nieszkowski, he considered it to be of the same species as the first discovery. These specimens from Estonia are of extraordinary quality, retaining the actual cuticle of their exoskeletons. In 1898, the Swedish paleontologist Gerhard Holm separated these fossils from the bedrock with acids. Holm was able to examine the perfectly preserved fragments under a microscope, his remarkable study led to the modern breakthrough on eurypterid morphology. More fossils were recovered in great abundance in New York in the 19th century, elsewhere in eastern Eurasia and North America.
Today, Eurypterus remains one of the most found and best known eurypterid genera, comprising more than 95% of all known eurypterid fossils. The genus Eurypterus belongs to the family Eurypteridae, they are classified under the superfamily Eurypteroidea, suborder Eurypterina, order Eurypterida, the subphylum Chelicerata. Until eurypterids were thought to belong to the class Merostomata along with order Xiphosura, it is now believed that eurypterids are a sister group to Arachnida, closer to scorpions and spiders than to horseshoe crabs. Eurypterus is the most common; as a consequence, nearly every remotely similar eurypterid in the 19th century was classified under the genus. The genus was split into several genera as the science of taxonomy developed. In 1958, several species distinguishable by closer placed eyes and spines on their swimming legs were split off into the separate genus Erieopterus by Erik Kjellesvig-Waering. Another split was proposed by Lief Størmer in 1973 when he reclassified some Eurypterus to Baltoeurypterus based on the size of some of the last segments of their swimming legs.
O. Erik Tetlie in 2006 deemed these differences too insignificant to justify a separate genus, he merged Baltoeurypterus back into Eurypterus. It is now believed that the minor variations described by Størmer are the differences found in adults and juveniles within a species; the largest arthropods to have existed were eurypterids. The largest known species reached up to 2.5 m about the size of a crocodile. Species of Eurypterus, were much smaller. E. remipes are between 13 to 20 cm in length. E. lacustris average at larger sizes at 15 to 23 cm in length. The largest specimen of E. remipes found was 1.3 m long on display at the Paleontological Research Institution of New York. Eurypterus fossils occur in similar sizes in a given area; this may be a result of the fossils being'sorted' into windrows as they were being deposited in shallow waters by storms and wave action. The Eurypterus body is broadly divided into two parts: the opisthosoma; the prosoma is the forward part of the body, it is composed of six segments fused together to form the head and the thorax.
It contains the semicircular to subrectangular platelike carapace. On the dorsal side of the latter are two large crescent-shaped compound eyes, they possessed two smaller light-sensitive simple eyes near the center of the carapace on a small elevation. Underneath the carapace is the mouth and six appendages referred to in Roman numerals I-VI; each appendage in turn is composed of nine segments labeled in Arabic numerals 1-9. The first segments which connect the appendages to the body are known as the coxa; the first pair are the chelicerae, small pincer-like arms used for tearing food apart during feeding. After the chelicerae are three pairs of short legs, they are spiniferous, with predominantly two spines on each podomere and with the tipmost segment having a