There are two types of nitrogenous bases that form the DNA/RNA. These include the purines and pyrimidines.
Guanine is a derivative of purine and is made up of a fused pyrimidine-imidazole ring system with double bonds in conjugation. The molecular formula is C5H5N5O. Here, O being more electronegative than N will carry a partial negative charge while N will carry a partial positive charge.