DNA

DNA, short for deoxyribonucleic acid, is the molecule that stores hereditary information in almost all organisms and in many viruses. It is a polymer of nucleotides, each made of the sugar deoxyribose, a phosphate group and one of four bases: adenine (A), thymine (T), guanine (G) or cytosine (C). Two strands wind around each other in a double helix, like a twisted ladder. The sugar-phosphate backbones form the sides, and the bases pair across the middle as the rungs, A with T and G with C, held by hydrogen bonds. The strands run in opposite directions, so they are called antiparallel.

The order of bases spells out genes, the instructions for making RNA and proteins. Because the pairing is complementary, each strand can serve as a template for building a new partner, which is how DNA is copied before a cell divides. In eukaryotes, most DNA sits in the nucleus, wrapped around histone proteins in chromosomes, with a small amount in mitochondria. The human genome has about 3 billion base pairs, and the DNA of any two people is about 99.9 percent identical.

James Watson and Francis Crick proposed the double helix in 1953, relying on X-ray images made by Rosalind Franklin.