A (adenine): In genetics, A stands for adenine, one member of the A-T (adenine-thymine) base pair in DNA. The other base pair in DNA is G-C (guanine-cytosine).
Each base pair forms a "rung of the DNA ladder." A DNA nucleotide is made of a molecule of 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 DNA base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.
Adenine is also one of the bases in RNA. There it always pairs with uracil (U). The base pairs in RNA are therefore A-U and G-C.