Entry courtesy of
The National Human Genome Research Institute (NHGRI)

Double helix

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.

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