The DNA helix
DNA stands for deoxyribose nucleic acid, or deoxyribonucleic acid.
Its structure was discovered by James D. Watson and Francis H.C. Crick in 1953 with the assistance of Rosalind Franklin, but the knowledge of DNA was first discovered in 1871.
The DNA molecule has a polymer backbone of deoxyribose molecules (Ribose in RNA), a five carbon sugar, connected together by a phosphate group (see phosphorylation).
The sugar also connects to a nucleobase (or simply called "base", for short). There are 5 different bases used for coding, 4 of which are used in DNA (the other, uracil, is exclusive to RNA).
The four bases are adenine, guanine, cytosine and thymine. These are represented by the letters A, G, C, & T and carry all information found in the DNA (see nucleic acid nomenclature).
The structure of DNA is a double helix. This means that there are two strands coiled around each other. The molecule is bonded together by the bases with hydrogen bonds. Guanine pairs with Cytosine by three hydrogen bonds while Adenine bonds with Thymine by two hydrogen bonds (see base pair).
Watson and Crick's insight into the double helical structure of the DNA molecule was based upon Erwin Chargaff noting that these pairs of bases were always in the same concentration.