NUCLEIC ACID

any of the substances that comprise the genetic material of living cells. Nucleic acids direct the course of protein synthesis, thereby regulating all cell activities. Furthermore, by their transmission from one generation to the next, nucleic acids are the vehicle for inherited characteristics. The molecules of nucleic acids are long chains consisting of repeating structural units called nucleotides. Each nucleotide is made up of a central sugar group, to which are attached a phosphate group and a nitrogenous organic base. Although oversimplified, it is helpful to visualize the phosphate group and the nitrogenous base as being oriented at right angles to each other. The backbone of the nucleic acid molecule is formed by the bonding of the sugar group of one nucleotide to the phosphate group of the next. The nitrogenous bases protrude from the backbone. The two classes of nucleic acidsribonucleic acids (RNAs) and deoxyribonucleic acids (DNAs)differ in that the sugar in RNA is ribose, while in DNA the sugar is deoxyribose. They also differ in their nitrogenous bases: while the bases adenine, guanine, and cytosine are present in both DNA and RNA, the base uracil is found only in RNA and the base thymine occurs only in DNA. (Also, RNA may contain some additional, unusual bases.) DNA is the major constituent of chromosomes in all cells, including those of bacteria and blue-green algae. DNA is also present in certain cell organelles, namely mitochondria and chloroplasts. RNA is commonly present in the nucleus and cytoplasm of cells. Most cytoplasmic RNA is associated with ribosomes, small particles that are the site of protein synthesis. Viruses, which are noncellular, contain a single nucleic acid molecule, either RNA or DNA. The biological function of DNA is the preservation and transmission of a genetic message, which is encoded in the sequence of its bases. In 1953 J.D. Watson and F.H.C. Crick announced that the DNA molecule consists of two chains, or strands, of polynucleotides coiled around each other to form a double helix in which the bases of one strand are weakly boundby hydrogen bondsto complementary bases of the other. Adenine is paired with thymine, and cytosine with guanine. The entire configuration resembles a twisted ladder, with sides composed of sugar and phosphate groups and rungs made up of paired bases. The WatsonCrick model helps to show how DNA passes along its encoded message from one generation of cells to the next: prior to cell division (mitosis), the double strands disengage, and each separate strand serves as a template upon which a new, complementary strand forms. Thus, one molecule of DNA forms two identical molecules of DNA, each containing one strand from the original DNA molecule and a newly synthesized strand. When the cell divides in two, each daughter cell receives one of the identical copies. DNA replication also precedes meiosis, the process by which sex cells (sperm and eggs) are formed. Meiosis, however, involves two divisions, so that each sperm or egg winds up with half the DNA of the original cell. The union of a sperm and an egg marks the beginning of a new individual. This new organism has a full complement of DNAhalf contributed by its male parent and half contributed by its female parent. Most DNA molecules share a common genetic function and structural pattern, but RNA molecules perform several functions in the cell and their properties vary accordingly. RNA exists in three forms in cells, and all are involved in protein synthesis. Messenger RNA (mRNA) is a single-stranded molecule that carries genetically coded information from DNA to the ribosomes. A molecule of mRNA forms by transcription. In this process, a portion of the double-stranded DNA molecule unwinds, exposing its nitrogenous bases. A complementary strand of mRNA forms along one of the unwound DNA strands. This mRNA molecule carries the DNA instructions for the synthesis of a particular protein (or part of a protein). The mRNA molecule detaches from its DNA template and travels to the ribosomes. Ribosomes consist of various proteins and the second form of RNAribosomal RNA (rRNA). The precise function of rRNA is uncertain, but protein synthesis cannot occur in its absence. The third form of RNAtransfer RNA (tRNA)transports amino acids to the proper site along the mRNA. Amino acids are the structural components of proteins. Sets of three bases in a molecule of mRNA code for a particular amino acid; these three-base sequences are called codons. The tRNA has complementary sequences of three bases, known as anticodons, which signal the identity of the amino acid it carries; these anticodons link up with the appropriate codons on the mRNA. Once in place, the amino acids are linked together. The synthesis of proteins according to the instructions coded in the mRNA is known as translation. Mistakes can occur during the replication, transcription, and translation of genetic information. Mutations are changes in the structure of the DNA and are passed to the cell's descendants. One base may replace another (base substitution), or one or more bases may be added (insertion) or lost (deletion). naturally occurring complex phosphorus compound, acidic in character, and capable of being broken down chemically to yield phosphoric acid, sugars, and a mixture of organic bases (purines and pyrimidines). Nucleic acids are of interest because they provide the genetic material of the cell and, by directing the process of protein synthesis, determine its inherited characteristics. About 1868, nuclei isolated from pus cells were found to contain an unusual phosphorus compound, which was named nuclein. In later years, complex phosphorus-containing acid materials were also isolated from a wide variety of cells; these appeared to be chemically similar to nuclein and came to be called nucleic acids. Additional reading Comprehensive works are Henry R. Mahler and Eugene H. Cordes, Basic Biological Chemistry (1968); Abraham White et al., Principles of Biochemistry, 6th ed. (1978); Albert L. Lehninger, David L. Nelson, and Michael L. Cox, Principles of Biochemistry, 2nd ed. (1993); Thomas Briggs and Albert M. Chandler (eds.), Biochemistry, 2nd ed. (1992); John W. Hill, Dorothy M. Feigl, and Stuart J. Baum, Chemistry and Life, 4th ed. (1993); Lubert Stryer, Biochemistry, 3rd ed. (1988); Donald Voet and Judith G. Voet, Biochemistry (1990); Geoffrey Zubay, Biochemistry, 3rd ed. (1993); and Laurence A. Moran et al., Biochemistry, 2nd ed. (1994). David J. Holme and Hazel Peck, Analytical Biochemistry, 2nd ed. (1993), covers newer methods of analysis. The Editors of the Encyclopdia BritannicaStudies focusing specifically on nucleic acids include Erwin Chargaff and J.N. Davidson (eds.), The Nucleic Acids: Chemistry and Biology, 3 vol. (195560), a classical modern text, its basic chemistry valid but biological sections out of date; Erwin Chargaff, Essays on Nucleic Acids (1963); D. Cohen, The Biological Role of the Nucleic Acids (1965), a short general monograph; Progress in Nucleic Acid Research and Molecular Biology (irregular); D.W. Hutchinson, Nucleotides and Coenzymes (1964), an elementary monograph; James D. Watson, The Double Helix: A Personal Account of the Discovery of the Structure of DNA (1968, reissued 1980); John C. Kendrew, The Thread of Life: An Introduction to Molecular Biology (1966), a popular book based on a series of TV programs; Arthur Kornberg and Tania A. Baker, DNA Replication, 2nd ed. (1992); Raymond F. Gesteland and John F. Atkins (eds.), The RNA World (1993); Gary Parker, W. Ann Reynolds, and Rex Reynolds, DNA: The Key to Life, 2nd ed. (1975), a programmed series of questions and answers; C.R. Calladine and Horace R. Drew, Understanding DNA, 2nd ed. (1992); Karl Drlica, Understanding DNA and Gene Cloning: A Guide for the Curious (1992); R.M.S. Smellie, A Matter of Life: DNA (1969), a popular work; and T.L.V. Ulbricht, Introduction to Nucleic Acids and Related Natural Products (1965), an elementary monograph. College-level textbooks on this subject include Roger L.P. Adams, John T. Knowler, and David P. Leader, The Biochemistry of the Nucleic Acids, 11th ed. (1992); Eberhard Harbers, Gtz F. Domagk, and Werner Mller, Introduction to Nucleic Acids: Chemistry, Biochemistry and Functions (1968; originally published in German, 1964); Vernon M. Ingram, The Biosynthesis of Macromolecules, 2nd ed. (1972); A.M. Michelson, The Chemistry of Nucleosides and Nucleotides (1963); A.R. Peacocke and R.B. Drysdale, The Molecular Basis of Heredity (1965); and Stephen Neidle (ed.), Topics in Nucleic Acid Structure (1981).Topics of recent interest are covered in Necia Grant Cooper (ed.), The Human Genome Project: Deciphering the Blueprint of Heredity (1994); Thomas F. Lee, The Human Genome Project: Cracking the Genetic Code of Life (1991); James D. Watson et al., Recombinant DNA, 2nd ed. (1992); Lorne T. Kirby, DNA Fingerprinting: An Introduction (1990); M.J. McPherson, P. Quirke, and G.R. Taylor (eds.), PCR: A Practical Approach (1991), on the polymerase chain reaction; and Bernd Herrmann and Susanne Hummel (eds.), Ancient DNA: Recovery and Analysis of Genetic Material from Paleontological, Archaeological, Museum, Medical, and Forensic Specimens (1994). James Norman Davidson Robert Young Thomson The Editors of the Encyclopdia Britannica