Note that it is derivatives of pyrimidine and purine that are important
in the nulceic acids.
Pyrimidine
Purine
Uracil (U)
Thymine (T)
Cytosine (C)
Adenine (A)
Guanine (G)
The bases with the carbonyl groups (uracil, thymine, cytosine and guanine)
are not aromatic as drawn.
However, if we consider the important resonance contributor from the amide
systems, then we can see their aromatic character as shown for cytosine
below:
Study Tip: pyrimidine sounds most like pyridine, the structure it resembles
most closely... just an extra N.