Guanine: One of the four bases in DNA that make up the letters ATGC. Guanine is the "G". The others are adenine, cytosine, and thymine. Guanine always pairs with cytosine to form the base pair G-C (guanine-cytosine) in the DNA. The other base pair in the DNA is A-T (adenine-thymine). Each base pair forms a rung in the DNA ladder. Guanine is also one of the four bases in RNA.