There are four different nitrogenous bases in DNA:

* Adenine (A)

* Guanine (G)

* Cytosine (C)

* Thymine (T)

These bases pair up in a specific way: Adenine always pairs with Thymine (A-T), and Guanine always pairs with Cytosine (G-C). These pairs are held together by hydrogen bonds, forming the "rungs" of the DNA ladder.

The sides of the ladder are made up of alternating sugar (deoxyribose) and phosphate molecules.