or deoxyribonucleic acid
One of two types of nucleic acid (the other is RNA ); a complex organic compound found in all living cell s and many virus es.
It is the chemical substance of gene s. 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 nucleotide s: 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 chromosome s. In eukaryote s these are in the nucleus , and DNA also occurs in mitochondria and chloroplast s (if any). Prokaryote s have a single circular chromosome in the cytoplasm . Some prokaryotes and a few eukaryotes have DNA outside the chromosomes in plasmid s. See also Rosalind Franklin ; genetic engineering ; mutation ; Maurice Wilkins .