DNA
deoxyribonucleic acid: one of the two types of nucleic acid found in living organisms; it is the principal carrier of genetic information in chromosomes and, to a much lesser extent, in mitochondria. Certain segments of the DNA molecules constitute the organism’s genes. Structurally, DNA consists of two intertwined, helically coiled strands of nucleotides—the double helix. The nucleotides each contain one of four bases: adenine, guanine, cytosine, or thymine. Each base forms hydrogen bonds with the adjacent base on the other, sister strand, producing consecutive base pairs arranged rather like the “rungs” on a helical ladder. Adenine (A) is always paired with thymine (T), and guanine (G) with cytosine (C). DNA can undergo self-replication in such a way that each strand serves as the template for the assembly of a complementary matching strand, resulting in two molecules exactly like the
original helix in terms of base pairing. The sequence of bases in the DNA of genes contains information according to the genetic code. Each gene specifies the manufacture of a particular protein or ribosome. Because of DNA’s ability to conserve its base sequence when replicating, the genetic instructions it carries are also conserved, both during cell division within a single organism and for that organism’s offspring following reproduction. Compare RNA. See also recombinant DNA.