Nucleic Acids

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Nucleic Acids, extremely complex molecules produced by living cells and viruses. Their name comes from their initial isolation from the nuclei of living cells. Certain nucleic acids, however, are found not in the cell nucleus but in cell cytoplasm. Nucleic acids have at least two functions: to pass on hereditary characteristics from one generation to the next, and to trigger the manufacture of specific proteins. How nucleic acids accomplish these functions is the object of some of the most intense and promising research currently under way. The nucleic acids are the fundamental substances of living things, believed by researchers to have first been formed about 3 billion years ago, when the most elementary forms of life began on earth. The origin of the so-called genetic code they carry has been accepted by researchers as being very close in time to the origin of life itself (see Evolution; Genetics). Biochemists have succeeded in deciphering the code, that is, determining how the sequence of nucleic acids dictates the structure of proteins.

The two classes of nucleic acids are the deoxyribonucleic acids (DNA) and the ribonucleic acids (RNA). The backbones of both DNA and RNA molecules are shaped like helical strands. Their molecular weights are in the millions. To the backbones are connected a great number of smaller molecules (side groups) of four different types (see Amino Acids). The sequence of these molecules on the strand determines the code of the particular nucleic acid. This code, in turn, signals the cell how to reproduce either a duplicate of itself or the proteins it requires for survival.

All living cells contain the genetic material DNA. The cells of bacteria may have but one strand of DNA, but such a strand contains all the information needed by the cell in order to reproduce an identical offspring. The cells of mammals contain scores of DNA strands grouped together in chromosomes. In short, the structure of a DNA molecule or combination of DNA molecules determines the shape, form, and function of the offspring. Some viruses, called retroviruses, contain only RNA rather than DNA, but viruses in themselves are generally not considered true living organisms (see Virus).

Certain kinds of RNA have a slightly different function from that of DNA. They take part in the actual synthesis of the proteins a cell produces. This is of particular interest to virologists because many viruses reproduce by “forcing” the host cells to manufacture more viruses. The virus injects its own RNA into the host cell, and the host cell obeys the code of the invading RNA rather than that of its own. Thus the cell produces proteins that are, in fact, viruses instead of the proteins required for cell function. The host cell is destroyed, and the newly formed viruses are free to inject their RNA into other host cells.

Silicon

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Silicon, symbol Si, semimetallic element that is the second most common element on earth, after oxygen. The atomic number of silicon is 14. Silicon is in group 14 (or IVa) of the periodic table. It was first isolated from its compounds in 1823 by the Swedish chemist Baron Jöns Jakob Berzelius.

PROPERTIES AND OCCURRENCE

Silicon is prepared as a brown amorphous powder or as gray-black crystals. It is obtained by heating silica, or silicon dioxide (SiO2), with a reducing agent, such as carbon or magnesium, in an electric furnace. Crystalline silicon has a hardness of 7, compared to 5 to 7 for glass. Silicon melts at about 1410° C (about 2570° F), boils at about 2355° C (about 4271° F), and has a specific gravity of 2.33. The atomic weight of silicon is 28.086.

Silicon is not attacked by nitric, hydrochloric, or sulfuric acids, but it dissolves in hydrofluoric acid, forming the gas, silicon tetrafluoride. It dissolves in sodium hydroxide, forming sodium silicate and hydrogen gas. At ordinary temperatures silicon is impervious to air, but at high temperatures it reacts with oxygen, forming a layer of silica that does not react further. At high temperatures it also reacts with nitrogen and chlorine to form silicon nitride and silicon chloride, respectively.

Silicon constitutes about 28 percent of the earth's crust. It does not occur in the free, elemental state, but is found in the form of silicon dioxide and in the form of complex silicates. Silicon-containing minerals constitute nearly 40 percent of all common minerals, including more than 90 percent of igneous-rock-forming minerals. The mineral quartz, varieties of quartz (such as carnelian, chrysoprase, onyx, flint, and jasper), and the minerals cristobalite and tridymite are the naturally occurring crystal forms of silica. Silicon dioxide is the principal constituent of sand. The silicates (such as the complex aluminum, calcium, and magnesium silicates) are the chief constituents of clays, soils, and rocks in the form of feldspars, amphiboles, pyroxenes, micas, and zeolites, and of semiprecious stones, such as olivine, garnet, zircon, topaz, and tourmaline.

See: Uses of Silicon

Uses of Silicon

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Silicon is used in the steel industry as a constituent of silicon-steel alloys. In steelmaking, molten steel is deoxidized by the addition of small amounts of silicon; ordinary steel contains less than 0.03 percent of silicon. Silicon steel, which contains from 2.5 to 4 percent silicon, is used in making the cores of electrical transformers because the alloy exhibits low hysteresis (see Magnetism). A steel alloy, known as duriron, containing about 15 percent silicon, is hard, brittle, and resistant to corrosion; duriron is used in industrial equipment that comes in contact with corrosive chemicals. Silicon is also used as an alloy in copper, brass, and bronze.

Silicon is a semiconductor, in which the resistivity to the flow of electricity at room temperature is in the range between that of metals and that of insulators. The conductivity of silicon can be controlled by adding small amounts of impurities, called dopants. The ability to control the electrical properties of silicon, and its abundance in nature, have made possible the development and widespread application of transistors and integrated circuits used in the electronics industry.

Silica and silicates are used in the manufacture of glass, glazes, enamels, cement, and porcelain, and have important individual applications. Fused silica, a glass made by melting quartz or hydrolyzing silicon tetrachloride, is characterized by a low coefficient of expansion and high resistance to most other chemicals. Silica gel is a colorless, porous, amorphous substance; it is prepared by removing part of the water from a gelatinous precipitate of silicic acid, which is formed by adding hydrochloric acid to a solution of sodium silicate. Silica gel absorbs water and other substances and is used as a drying and decolorizing agent.

Sodium silicate, an important synthetic silicate, is a colorless, water-soluble, amorphous solid that melts at 1088° C (1990° F). It is prepared by reacting silica (sand) and sodium carbonate at a high temperature or by heating sand with concentrated sodium hydroxide under pressure. The aqueous solution of sodium silicate, called water glass, is used for preserving eggs; as a substitute for glue in making boxes and other containers; as a binder in artificial gemstones; as a fireproofing agent; and as a binder and filler in soaps and cleansers. Another important silicon compound is the silicon-carbon compound Carborundum, which is used as an abrasive.

Silicon monoxide, is used as a coating to protect other materials, the outer surface oxidizing to the dioxide. Such layers are applied also as components of interference filters.