Tyrosinase is an oxidase, the rate-limiting enzyme for controlling the production of melanin. The enzyme is involved in two distinct reactions of melanin synthesis. O-Quinone undergoes several reactions to form melanin. Tyrosinase is a copper-containing enzyme present in plant and animal tissues that catalyzes the production of melanin and other pigments from tyrosine by oxidation, as in the blackening of a peeled or sliced potato exposed to air, it is found inside melanosomes. In humans, the tyrosinase enzyme is encoded by the TYR gene. A mutation in the tyrosinase gene resulting in impaired tyrosinase production leads to type I oculocutaneous albinism, a hereditary disorder that affects one in every 20,000 people. Tyrosinase activity is important. If uncontrolled during the synthesis of melanin, it results in increased melanin synthesis. Decreasing tyrosinase activity has been targeted for the betterment or prevention of conditions related to the hyperpigmentation of the skin, such as melasma and age spots.
Several polyphenols, including flavonoids or stilbenoid, substrate analogues, free radical scavengers, copper chelators, have been known to inhibit tyrosinase. Henceforth, the medical and cosmetic industries are focusing research on tyrosinase inhibitors to treat skin disorders. In food industry, tyrosinase inhibition is desired as tyrosinase catalyzes the oxidation of phenolic compounds found in fruits and vegetables into quinones, which gives an undesirable taste and color and decreases the availability of certain essential amino acids as well as the digestibility of the products; as such effective tyrosinase inhibitors are needed in agriculture and the food industry. Well known tyrosinase inhibitors include kojic acid, coumarins, vanillic acid and vanillic alcohol. Tyrosinase has a wide range of functions in insects, including wound healing, melanin synthesis and parasite encapsulation; as a result, it is an important enzyme. Some insecticides are aimed to inhibit tyrosinase. Tyrosinase dopamine using dioxygen.
In the presence of catechol, benzoquinone is formed. Hydrogens removed from catechol combine with oxygen to form water; the substrate specificity becomes restricted in mammalian tyrosinase which uses only L-form of tyrosine or DOPA as substrates, has restricted requirement for L-DOPA as cofactor. Tyrosinases have been isolated and studied from a wide variety of plant and fungal species. Tyrosinases from different species are diverse in terms of their structural properties, tissue distribution, cellular location. No common tyrosinase protein structure occurring across all species has been found; the enzymes found in plant and fungal tissue differ with respect to their primary structure, glycosylation pattern, activation characteristics. However, all tyrosinases have in type 3 copper centre within their active sites. Here, two copper atoms are each coordinated with three histidine residues. Human tyrosinase is a single membrane-spanning transmembrane protein. In humans, tyrosinase is sorted into melanosomes and the catalytically active domain of the protein resides within melanosomes.
Only a small, enzymatically inessential part of the protein extends into the cytoplasm of the melanocyte. As opposed to fungal tyrosinase, human tyrosinase is a membrane-bound glycoprotein and has 13% carbohydrate content; the derived TYR allele is associated with lighter skin pigmentation in human populations. It is most common in Europe, but is found at lower, moderate frequencies in Central Asia, the Middle East, North Africa, among the San and Mbuti Pygmies; the two copper atoms within the active site of tyrosinase enzymes interact with dioxygen to form a reactive chemical intermediate that oxidizes the substrate. The activity of tyrosinase is similar to a related class of copper oxidase. Tyrosinases and catechol oxidases are collectively termed polyphenol oxidases; the gene for tyrosinase is regulated by the microphthalmia-associated transcription factor. GeneReviews/NCBI/NIH/UW entry on Oculocutaneous Albinism Type 1 Tyrosinase at the US National Library of Medicine Medical Subject Headings
Anatomical terms of microanatomy
Anatomical terminology is used to describe microanatomical structures. This helps describe the structure and position of an object, minimises ambiguity. An internationally accepted lexicon is Terminologia Histologica. Epithelial cells line body surfaces, are described according to their shape, with three principal shapes: squamous and cuboidal. Squamous epithelium has cells. Cuboidal epithelium has cells whose height and width are the same. Columnar epithelium has cells taller. Endothelium refers to cells that line the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall, it is a thin layer of single-layered, squamous cells called endothelial cells. Endothelial cells in direct contact with blood are called vascular endothelial cells, whereas those in direct contact with lymph are known as lymphatic endothelial cells. Epithelium can be arranged in a single layer of cells described as "simple", or more than one layer, described as "stratified".
By layer, epithelium is classed as either simple epithelium, only one cell thick or stratified epithelium as stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium that are two or more cells thick, both types of layering can be made up of any of the cell shapes. However, when taller simple columnar epithelial cells are viewed in cross section showing several nuclei appearing at different heights, they can be confused with stratified epithelia; this kind of epithelium is therefore described as pseudostratified columnar epithelium. Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium. A mucous membrane or mucosa is a membrane that lines various cavities in the body and covers the surface of internal organs, it consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It is of endodermal origin and is continuous with the skin at various body openings such as the eyes, inside the nose, inside the mouth, the urethral opening and the anus.
Some mucous membranes a thick protective fluid. The function of the membrane is to stop pathogens and dirt from entering the body and to prevent bodily tissues from becoming dehydrated; the submucosa consists of a dense and irregular layer of connective tissue with blood vessels and nerves branching into the mucosa and muscular layer. It contains the submucous plexus, enteric nervous plexus, situated on the inner surface of the muscular layer; the muscular layer consists of two layers of the inner and outer layer. The muscle of the inner layer is arranged in circular rings around the tract, whereas the muscle of the outer layer is arranged longitudinally; the stomach has an inner oblique muscular layer. Between the two muscle layers are the myenteric or Auerbach's plexus; this controls peristalsis. Activity is initiated by the pacemaker cells; the gut has intrinsic peristaltic activity due to its self-contained enteric nervous system. The rate can of course be modulated by the rest of the autonomic nervous system.
The layers are not longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch. Serosa / Adventitia -- these last two tissue types differ in form and function according to the part of the gastrointestinal tract they belong to; the hollow inner part of a body organ or tube is called the lumen. The side of a cell facing the lumen is called the apical surface.
Melanin is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino acid tyrosine, is followed by polymerization; the melanin pigments are produced in a specialized group of cells known as melanocytes. There are three basic types of melanin: eumelanin and neuromelanin; the most common type is eumelanin, of which there are two types -- black eumelanin. Pheomelanin is a cysteine-derivative that contains polybenzothiazine portions that are responsible for the color of red hair, among other pigmentation. Neuromelanin is found in the brain. Research has been undertaken to investigate its efficacy in treating neurodegenerative disorders such as Parkinson's. In the human skin, melanogenesis is initiated by exposure to UV radiation, causing the skin to darken. Melanin is an effective absorbent of light; because of this property, melanin is thought to protect skin cells from UVB radiation damage, reducing the risk of folate depletion and dermal degradation, it is considered that exposure to UV radiation is associated with increased risk of malignant melanoma, a cancer of melanocytes.
Studies have shown a lower incidence for skin cancer in individuals with more concentrated melanin, i.e. darker skin tone. However. In humans, melanin is the primary determinant of skin color, it is found in hair, the pigmented tissue underlying the iris of the eye, the stria vascularis of the inner ear. In the brain, tissues with melanin include the medulla and pigment-bearing neurons within areas of the brainstem, such as the locus coeruleus and the substantia nigra, it occurs in the zona reticularis of the adrenal gland. The melanin in the skin is produced by melanocytes, which are found in the basal layer of the epidermis. Although, in general, human beings possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and ethnic groups produce variable amounts of melanin; some humans have little or no melanin synthesis in their bodies, a condition known as albinism. Because melanin is an aggregate of smaller component molecules, there are many different types of melanin with different proportions and bonding patterns of these component molecules.
Both pheomelanin and eumelanin are found in human skin and hair, but eumelanin is the most abundant melanin in humans, as well as the form most to be deficient in albinism. Eumelanin polymers have long been thought to comprise numerous cross-linked 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid polymers. Pheomelanins impart a pink depending upon the concentration. Pheomelanins are concentrated in the lips, glans of the penis, vagina; when a small amount of brown eumelanin in hair, which would otherwise cause blond hair, is mixed with red pheomelanin, the result is strawberry blonde. Pheomelanin is present in the skin, redheads often have a more pinkish hue to their skin as well. In chemical terms, pheomelanins differ from eumelanins in that the oligomer structure incorporates benzothiazine and benzothiazole units that are produced, instead of DHI and DHICA, when the amino acid L-cysteine is present. Trichochromes are pigments produced from the same metabolic pathway as the eumelanins and pheomelanins, but unlike those molecules they have low molecular weight.
They occur in some red human hair. Neuromelanin is a dark insoluble polymer pigment produced in specific populations of catecholaminergic neurons in the brain. Humans have the largest amount of NM, present in lesser amounts in other primates, absent in many other species. However, the biological function remains unknown, although human NM has been shown to efficiently bind transition metals such as iron, as well as other toxic molecules. Therefore, it may play crucial roles in the related Parkinson's disease. Melanins have diverse roles and functions in various organisms. A form of melanin makes up the ink used by many cephalopods as a defense mechanism against predators. Melanins protect microorganisms, such as bacteria and fungi, against stresses that involve cell damage such as UV radiation from the sun and reactive oxygen species. Melanin protects against damage from high temperatures, chemical stresses, biochemical threats. Therefore, in many pathogenic microbes melanins appear to play important roles in virulence and pathogenicity by protecting the microbe against immune responses of its host.
In invertebrates, a major aspect of the innate immune defense system against invading pathogens involves melanin. Within minutes after infection, the microbe is encapsulated within melanin, the generation of free radical byproducts during the formation of this capsule is thought to aid in killing them; some types of fungi, called radiotrophic fungi, appear to be able to use melanin as a photosynthetic pigment that enables them to capture gamma rays and harness this energy for growth. The darker feathers of birds owe their color to melanin and are less degraded by bacteria than unpigmented ones or those containing carotenoid pigments. Feathers that contain melanin are 39% more resistant to abrasion than those that do not because melanin granules help fill the space between th
Embryology is the branch of biology that studies the prenatal development of gametes and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology. Embryology has a long history. Aristotle proposed the accepted theory of epigenesis, that organisms develop from seed or egg in a sequence of steps; the alternative theory, that organisms develop from pre-existing miniature versions of themselves, held sway until the 18th century. Modern embryology developed from the work of von Baer, though accurate observations had been made in Italy by anatomists such as Aldrovandi and Leonardo da Vinci in the Renaissance. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end. In bilateral animals, the blastula develops in one of two ways that divide the whole animal kingdom into two halves. If in the blastula the first pore becomes the mouth of the animal, it is a protostome.
The protostomes include most invertebrate animals, such as insects and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula; the gastrula with its blastopore soon develops three distinct layers of cells from which all the bodily organs and tissues develop: The innermost layer, or endoderm, give rise to the digestive organs, the gills, lungs or swim bladder if present, kidneys or nephrites. The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, blood system; the outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, skin or carapace and hair, bristles, or scales. Embryos in many species appear similar to one another in early developmental stages; the reason for this similarity is. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.
Drosophila melanogaster, a fruit fly, is a model organism in biology on which much research into embryology has been done. Before fertilization, the female gamete produces an abundance of mRNA - transcribed from the genes that encode bicoid protein and nanos protein; these mRNA molecules are stored to be used in what will become the developing embryo. The male and female Drosophila gametes exhibit anisogamy; the female gamete is larger than the male gamete because it harbors more cytoplasm and, within the cytoplasm, the female gamete contains an abundance of the mRNA mentioned. At fertilization, the male and female gametes fuse and the nucleus of the male gamete fuses with the nucleus of the female gamete. Note that before the gametes' nuclei fuse, they are known as pronuclei. A series of nuclear divisions will occur without cytokinesis in the zygote to form a multi-nucleated cell known as a syncytium. All the nuclei in the syncytium are identical, just as all the nuclei in every somatic cell of any multicellular organism are identical in terms of the DNA sequence of the genome.
Before the nuclei can differentiate in transcriptional activity, the embryo must be divided into segments. In each segment, a unique set of regulatory proteins will cause specific genes in the nuclei to be transcribed; the resulting combination of proteins will transform clusters of cells into early embryo tissues that will each develop into multiple fetal and adult tissues in development. Outlined below is the process that leads to tissue differentiation. Maternal-effect genes - subject to Maternal inheritance Egg-polarity genes establish the Anteroposterior axis. Zygotic-effect genes - subject to Mendelian inheritance Segmentation genes establish 14 segments of the embryo using the anteroposterior axis as a guide. Gap genes establish 3 broad segments of the embryo. Pair-rule genes define 7 segments of the embryo within the confines of the second broad segment, defined by the gap genes. Segment-polarity genes define another 7 segments by dividing each of the pre-existing 7 segments into anterior and posterior halves.
Homeotic genes use the 14 segments as pinpoints for specific types of cell differentiation and the histological developments that correspond to each cell type. Humans are deuterostomes. In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception, the developing human is called a fetus; as as the 18th century, the prevailing notion in western human embryology was preformation: the idea that semen contains an embryo – a preformed, miniature infant, or homunculus – that becomes larger during development. Until the birth of modern embryology through observation of the mammalian ovum by von Baer in 1827, there was no clear scientific understanding of embryology. Only in the late 1950s when ultrasound was first used for uterine scanning, was the true developmental chronology of human fetus available; the competing explanation of embryonic development was epigenesis proposed 2,000 years earlier by
MHC class II
MHC class II molecules are a class of major histocompatibility complex molecules found only on professional antigen-presenting cells such as dendritic cells, mononuclear phagocytes, some endothelial cells, thymic epithelial cells, B cells. These cells are important in initiating immune responses; the antigens presented by class. Loading of a MHC class II molecule occurs by phagocytosis. In humans, the MHC class II protein complex is encoded by the human leukocyte antigen gene complex. HLAs corresponding to MHC class II are HLA-DP, HLA-DM, HLA-DOA, HLA-DOB, HLA-DQ, HLA-DR. Mutations in the HLA gene complex can lead to bare lymphocyte syndrome, a type of MHC class II deficiency. Like MHC class I molecules, class II molecules are heterodimers, but in this case consist of two homogenous peptides, an α and β chain, both of which are encoded in the MHC; the subdesignation α2, etc. refers to separate domains within the HLA gene. These molecules have both extracellular regions as well as a transmembrane sequence and a cytoplasmic tail.
The α1 and β1 regions of the chains come together to make a membrane-distal peptide-binding domain, while the α2 and β2 regions, the remaining extracellular parts of the chains, form a membrane-proximal immunoglobulin-like domain. The antigen binding groove, where the antigen or peptide binds, is made up of two α-helixes walls and β-sheet; because the antigen-binding groove of MHC class II molecules is open at both ends while the corresponding groove on class I molecules is closed at each end, the antigens presented by MHC class II molecules are longer between 15 and 24 amino acid residues long. These molecules are constitutively expressed in professional, immune antigen presenting cells, but may be induced on other cells by interferon γ, they are expressed on APCs in the periphery. MHC class II expression is regulated in APCs by CIITA, the MHC class II transactivator. CIITA is expressed on professional APCs however, non-professional APCs can regulate CIITA activity and MHC II expression; as mentioned interferon-y triggers the expression of CIITA and is responsible for converting monocytes which are MHC class II negative cells into functional APCs that express MHC class II on their surfaces.
MHC class II is expressed on group 3 innate lymphoid cells. Having MHC class II molecules present proper peptides that are bound stably is essential for overall immune function; because class II MHC is loaded with extracellular proteins, it is concerned with presentation of extracellular pathogens. Class II molecules interact with immune cells, like the T helper cell; the peptide presented regulates. Stable peptide binding is essential to prevent detachment and degradation of a peptide, which could occur without secure attachment to the MHC molecule; this would prevent T cell recognition of the antigen, T cell recruitment, a proper immune response. The triggered appropriate immune response may include localized inflammation and swelling due to recruitment of phagocytes or may lead to a full-force antibody immune response due to activation of B cells. During synthesis of class II MHC in the endoplasmic reticulum, the α and β chains are produced and complexed with a special polypeptide known as the invariant chain.
The nascent MHC class II protein in the rough ER has its peptide-binding cleft blocked by the invariant chain to prevent it from binding cellular peptides or peptides from the endogenous pathway. The invariant chain facilitates the export of class II MHC from the ER to the golgi, followed by fusion with a late endosome containing endocytosed, degraded proteins; the invariant chain is broken down in stages by proteases called cathepsins, leaving only a small fragment known as CLIP which maintains blockage of the peptide binding cleft on the MHC molecule. An MHC class II-like structure, HLA-DM, facilitates CLIP removal and allows the binding of peptides with higher affinities; the stable class II MHC is presented on the cell surface. After MHC class II complexes are synthesized and presented on APCs they are unable to be expressed on the cell surface indefinitely, due to the internalization of the plasma membrane by the APCs. In some cells, antigens bind to recycled MHC class II molecules while they are in the early endosomes.
While other cells such as dendritic cells internalize antigens via receptor-mediated endocytosis and create MHC class II molecules plus peptide in the endosomal-lysosomal antigen processing compartment, independent of the synthesis of new MHC class II complexes. These suggest that after the antigen is internalized existent MHC class II complexes on mature dendritic cells can be recycled and developed into new MHC class II molecules plus peptide. Several molecules are involved in this pathway.- PIK3R2 and PIP5K1A are two kinases that create substrates for PSD4. - PSD4 is a GEF that loads ARL14/ARF7 with GTP. - ARL14/ARF7 is a Small GTPase protein, selectively expressed in immune cells. This protein is localized within MHC-II compartments in immat
Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen and nitrogen, although other elements are found in the side chains of certain amino acids. About 500 occurring amino acids are known and can be classified in many ways, they can be classified according to the core structural functional groups' locations as alpha-, beta-, gamma- or delta- amino acids. In the form of proteins, amino acid residues form the second-largest component of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. In biochemistry, amino acids having both the amine and the carboxylic acid groups attached to the first carbon atom have particular importance, they are known as α-amino acids. They include the 22 proteinogenic amino acids, which combine into peptide chains to form the building-blocks of a vast array of proteins.
These are all L-stereoisomers, although a few D-amino acids occur in bacterial envelopes, as a neuromodulator, in some antibiotics. Twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as "standard" amino acids; the other two are selenocysteine, pyrrolysine. Pyrrolysine and selenocysteine are encoded via variant codons. N-formylmethionine is considered as a form of methionine rather than as a separate proteinogenic amino acid. Codon–tRNA combinations not found in nature can be used to "expand" the genetic code and form novel proteins known as alloproteins incorporating non-proteinogenic amino acids. Many important proteinogenic and non-proteinogenic amino acids have biological functions. For example, in the human brain and gamma-amino-butyric acid are the main excitatory and inhibitory neurotransmitters. Hydroxyproline, a major component of the connective tissue collagen, is synthesised from proline. Glycine is a biosynthetic precursor to porphyrins used in red blood cells.
Carnitine is used in lipid transport. Nine proteinogenic amino acids are called "essential" for humans because they cannot be produced from other compounds by the human body and so must be taken in as food. Others may be conditionally essential for medical conditions. Essential amino acids may differ between species; because of their biological significance, amino acids are important in nutrition and are used in nutritional supplements, fertilizers and food technology. Industrial uses include the production of drugs, biodegradable plastics, chiral catalysts; the first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus, subsequently named asparagine, the first amino acid to be discovered. Cystine was discovered in 1810, although its monomer, remained undiscovered until 1884. Glycine and leucine were discovered in 1820; the last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth.
The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. Usage of the term "amino acid" in the English language is from 1898, while the German term, Aminosäure, was used earlier. Proteins were found to yield amino acids after enzymatic acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "peptide". In the structure shown at the top of the page, R represents a side chain specific to each amino acid; the carbon atom next to the carboxyl group is called the α–carbon. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as alpha amino acids; these include amino acids such as proline which contain secondary amines, which used to be referred to as "imino acids". The alpha amino acids are the most common form found in nature, but only when occurring in the L-isomer.
The alpha carbon is a chiral carbon atom, with the exception of glycine which has two indistinguishable hydrogen atoms on the alpha carbon. Therefore, all alpha amino acids but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other. While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amin
Ultraviolet designates a band of the electromagnetic spectrum with wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight, contributes about 10% of the total light output of the Sun, it is produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, black lights. Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce; the chemical and biological effects of UV are greater than simple heating effects, many practical applications of UV radiation derive from its interactions with organic molecules. Suntan and sunburn are familiar effects of over-exposure of the skin to UV, along with higher risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earth's atmosphere.
More energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so that it is absorbed before it reaches the ground. Ultraviolet is responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans; the UV spectrum thus has effects both harmful to human health. The lower wavelength limit of human vision is conventionally taken as 400 nm, so ultraviolet rays are invisible to humans, although some people can perceive light at shorter wavelengths than this. Insects and some mammals can see near-UV. Ultraviolet rays are invisible to most humans; the lens of the human eye blocks most radiation in the wavelength range of 300–400 nm. Humans lack color receptor adaptations for ultraviolet rays; the photoreceptors of the retina are sensitive to near-UV, people lacking a lens perceive near-UV as whitish-blue or whitish-violet. Under some conditions and young adults can see ultraviolet down to wavelengths of about 310 nm. Near-UV radiation is visible to insects, some mammals, birds.
Small birds have a fourth color receptor for ultraviolet rays. "Ultraviolet" means "beyond violet", violet being the color of the highest frequencies of visible light. Ultraviolet has a higher frequency than violet light. UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more than violet light itself, he called them "oxidizing rays" to emphasize chemical reactivity and to distinguish them from "heat rays", discovered the previous year at the other end of the visible spectrum. The simpler term "chemical rays" was adopted soon afterwards, remained popular throughout the 19th century, although some said that this radiation was different from light; the terms "chemical rays" and "heat rays" were dropped in favor of ultraviolet and infrared radiation, respectively. In 1878 the sterilizing effect of short-wavelength light by killing bacteria was discovered.
By 1903 it was known. In 1960, the effect of ultraviolet radiation on DNA was established; the discovery of the ultraviolet radiation with wavelengths below 200 nm, named "vacuum ultraviolet" because it is absorbed by the oxygen in air, was made in 1893 by the German physicist Victor Schumann. The electromagnetic spectrum of ultraviolet radiation, defined most broadly as 10–400 nanometers, can be subdivided into a number of ranges recommended by the ISO standard ISO-21348: A variety of solid-state and vacuum devices have been explored for use in different parts of the UV spectrum. Many approaches seek to adapt visible light-sensing devices, but these can suffer from unwanted response to visible light and various instabilities. Ultraviolet can be detected by suitable photodiodes and photocathodes, which can be tailored to be sensitive to different parts of the UV spectrum. Sensitive ultraviolet photomultipliers are available. Spectrometers and radiometers are made for measurement of UV radiation.
Silicon detectors are used across the spectrum. Vacuum UV, or VUV, wavelengths are absorbed by molecular oxygen in the air, though the longer wavelengths of about 150–200 nm can propagate through nitrogen. Scientific instruments can therefore utilize this spectral range by operating in an oxygen-free atmosphere, without the need for costly vacuum chambers. Significant examples include 193 nm photolithography equipment and circular dichroism spectrometers. Technology for VUV instrumentation was driven by solar astronomy for many decades. While optics can be used to remove unwanted visible light that contaminates the VUV, in general, detectors can be limited by their response to non-VUV radiation, the development of "solar-blind" devices has been an important area of research. Wide-gap solid-state devices or vacuum devices with high-cutoff photocathodes can be attractive compared to silicon diodes. Extreme UV is characterized by a transition in the physics of interaction with matter. Wavelengths longer than about 30 nm interact with the outer valence electrons of atoms, while wavelengths shorter than that interact with inner-shell electrons and nuclei.
The long end of the EUV spectrum is set by a prominent He+ spectr