Nucleic acids (DNA and RNA) are formed by the union of nucleotides, each of which in turn consists of phosphate, pentose and nitrogenous base.
Nitrogenous bases are classified as purine and pyrimidine as follows:
Purine bases: Adenine (A) and Guanine (G)
Pyrimidine bases: Cytosine (C), Thymine (T) and Uracilla (U)
In both DNA and RNA, the Guanine nitrogen base (G) binds exclusively to cytosine (C). The difference occurs in relation to the other bases: whereas in DNA adenine (A) binds only to thymine (T), in RNA adenine (A) will always be linked to the nitrogenous base Uracila (U).
