The nitrogenous base you're referring to that pairs with cytosine is Guanine. Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (in DNA) or uracil (in RNA). In the DNA double helix, guanine forms three hydrogen bonds with cytosine, making the G-C base pair one of the fundamental building blocks of the DNA structure.
