What part of the DNA molecule did Watson and Crick determine to be on the inside?

The nitrogenous bases are on the inside of the DNA molecule. Watson and Crick’s 1953 double-helix model placed paired bases toward the center, while the sugar-phosphate backbones formed the molecule’s outer rails.

The four DNA bases are adenine, thymine, guanine, and cytosine. They pair specifically: adenine with thymine, and guanine with cytosine. Hydrogen bonds between these paired bases help hold the two strands together, but hydrogen bonds are not the structural component located inside; they are the forces linking the bases.

A common mix-up is confusing bases with nucleotides. A nucleotide is a larger unit made of a nitrogenous base, a deoxyribose sugar, and phosphate. The sugar and phosphate form the backbone, whereas the bases project inward and encode genetic information. Rosalind Franklin’s X-ray diffraction work helped reveal the helical arrangement and supported the placement of phosphates on the outside.