The "rungs" on DNA are actually nitrogenous bases, which are molecules that form the "steps" of the DNA ladder. There are four main types of nitrogenous bases:

* Adenine (A)

* Thymine (T)

* Guanine (G)

* Cytosine (C)

How they connect:

* Adenine (A) always pairs with Thymine (T) through two hydrogen bonds.

* Guanine (G) always pairs with Cytosine (C) through three hydrogen bonds.

These pairings are called complementary base pairing, and they ensure that the two strands of DNA are held together correctly.

So, to answer your question: The rungs of DNA are formed by the pairs of nitrogenous bases, with Adenine-Thymine and Guanine-Cytosine forming the "steps" of the DNA ladder.