A base pair is two bases that form a "rung of the DNA ladder". A DNA nucleotide is made of a molecule of a 5 carbon sugar, a molecule of phosphoric acid, and a molecule called a base. The bases are the "letters" that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine. In RNA A pairs with U (uracil).