The rungs of the DNA “ladder” are made of pairs of nitrogenous bases that bond together in the center of the molecule.

More specifically:

  • Each rung is a base pair : two nitrogenous bases joined by hydrogen bonds.
  • The four bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G).
  • A always pairs with T, and C always pairs with G, forming the steps of the twisted ladder (the double helix).

The sides (or “rails”) of the ladder, in contrast, are made of a sugar–phosphate backbone, but the rungs themselves are the paired bases in the middle.

Information gathered from public forums or data available on the internet and portrayed here.