Definition of Helix, double

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Helix, double: The structure of DNA with the two strands of DNA spiraling about one other.

The double helix 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.

"The Double Helix" is the title of a excellent book by James Watson telling the story of his and Francis Crick's discovery of the correct model for DNA.