Double helix
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.