The structural arrangement of DNA, which looks something like an immensely long ladder twisted into a helix, or coil. The sides of the "ladder" are formed by a backbone of sugar and phosphate molecules, and the "rungs" consist of nucleotide bases joined weakly in the middle by hydrogen bonds.

Dr. Francis Collins, director of the National Human Genome Research Institute, defines double helix.

Related terms: base pair, deoxyribonucleic acid (DNA), nucleotide.