The rungs of the DNA ladder would be made up of nitrogenous bases. There are four nitrogenous bases in DNA - Thymine binds to Adenine and Guanine binds to Cysteine. These bases join together to meet in the centre of the DNA helix - making the 'rungs'.
The rungs of the ladder are pairs of 4 types of nitrogen bases (thymines, adenines, guanines, cytosines). --- The pairs are guanine and cytosine (G-C), or adenine and thymine (A-T). The rungs of the dna ladder are made of alternating sugars and phosophate groups.