There are four types of nitrogenous bases in DNA:

* Adenine (A)

* Guanine (G)

* Cytosine (C)

* Thymine (T)

These bases pair up in a specific way:

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

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

This pairing is called complementary base pairing. The two bases are held together by hydrogen bonds, forming the "rungs" of the DNA ladder.

The order of these bases along the DNA molecule forms the genetic code, which determines the instructions for building and maintaining an organism.