Five nitrogen-containing heterocyclic bases form the nucleotide components. The three pyrimidine derivatives found in nucleic acids are uracil (U) thymine (T), cytosine (C).  The remaining two are derivatives of purine viz. Adenine (A), and guanine (G). Cytosine, guanine, adenine are found in both DNA and RNA. Thymine is found only in DNA and uracil only in RNA.

 Pyrimidine derivatives