It had two sugar-phosphate strands that twisted and spiraled to form a double-stranded helix. Protruding from these were the four bases in DNA: adenine, thymine, guanine, and cytosine, now commonly known by the letters A, T, G, and C. They came to agree with Franklin that the backbones were on the outside and the bases pointed inward, like a twisted ladder or spiral staircase.
