The rungs of the DNA ladder are made up of nitrogenous bases. These bases are paired together in a specific way:

* Adenine (A) always pairs with Thymine (T)

* Guanine (G) always pairs with Cytosine (C)

These pairs are held together by hydrogen bonds.

So, each rung of the DNA ladder consists of one purine base (A or G) and one pyrimidine base (T or C) connected by hydrogen bonds.