Sage Congress: Stephen Friend, Sage Bionetworks

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DNA

DNA double helix. A. Molecular model of DNA. The molecules include (1) hydrogen, (2) oxygen (3) …© Merriam-Webster Inc.One of two types of nucleic acid (the other is RNA); a complex organic compound found in all living cells and many viruses. It is the chemical substance of genes. Its structure, with two strands wound around each other in a double helix to resemble a twisted ladder, was first described (1953) by Francis Crick and James D. Watson. Each strand is a long chain (polymer) of repeating nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). The two strands contain complementary information: A forms hydrogen bonds (see hydrogen bonding) only with T, C only with G. When DNA is copied in the cell, the strands separate and each serves as a template for assembling a new complementary strand; this is the key to stable heredity. DNA in cells is organized into dense protein-DNA complexes (see nucleoprotein) called chromosomes. In eukaryotes these are in the nucleus, and DNA also occurs in mitochondria and chloroplasts (if any). Prokaryotes have a single circular chromosome in the cytoplasm. Some prokaryotes and a few eukaryotes have DNA outside the chromosomes in plasmids. See also Rosalind Franklin; genetic engineering; mutation; Maurice Wilkins.

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genetics

Study of heredity in general and of genes in particular. Modern genetics began in the 19th century with the work of Gregor Mendel, who formulated the basic concepts of heredity. In 1909 the word gene was coined by Wilhelm Johannsen, thus giving genetics its name. In the same year, Thomas Hunt Morgan provided evidence that genes occur on chromosomes and that adjacent genes on the same chromosome form linkage groups. This led to the important discovery that genes affect molecular action at the cell level, as evidenced by human hereditary diseases such as inborn errors of metabolism. Molecular genetics began in earnest in the 1940s when Oswald Avery showed that DNA is the chromosome component that carries genetic information. The molecular structure of DNA was deduced by James D. Watson, Francis Crick, and Maurice Wilkins. These and other developments led to the deciphering of the genetic code of the DNA molecule, which in turn made possible the recombination techniques of genetic engineering, discovered in the 1970s. An understanding of genetics is necessary for the diagnosis, prevention, and treatment of hereditary diseases, the selective breeding of plants and animals, and the development of industrial processes that use microorganisms. See also behaviour genetics; biotechnology.

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