The letters in DNA stand for the four nitrogenous bases that make up the building blocks of DNA:

* A - Adenine

* T - Thymine

* C - Cytosine

* G - Guanine

These bases pair up in a specific way: Adenine (A) always pairs with Thymine (T), and Cytosine (C) always pairs with Guanine (G). This pairing is crucial for the structure and function of DNA.