Three schematic illustrations of the structure of DNA, the substance of genetic inheritance. Left shows the molecule's basic double-helical structure; two strands of nucleotides (a sugar, phosphate group & base) joined by hydrogen bond bridges. Centre shows the sequence of the organic bases - each antiparallel strand is joined through hydrogen bonding between adenine (A), thymine (T), guanine (G) and cytosine (C). A always pairs with T, and C with G; the resulting base sequence along molecule determines genetic information. Diagram at right shows the phosphate components of the nucleotides.