Adenine is a nitrogen base present in nucleic acids. This belongs to the purine group. Purine groups are nitrogen bases with two rings. A four-membered ring is attached to a six-membered ring. Purines are guanine and adenine and these are present in both DNA's and RNA's. The main difference between adenine and guanine is that guanine shows an extra = O group. Both the nitrogen bases have $–{\mathrm{N}\mathrm{H}}_2$ groups but present on different carbon atoms. Thus the 'x' in the figure represents the amino group. Adenine is called as 6-amino purine.