Cytosine is a pyrimidine

Pyrimidine & Purine Bases

Pyrimidine and Purine form the basic bodies of the organic bases that occur in nucleic acids (DNA and RNA).

Hydroxyl and / or amino groups are bonded to the heteroaromatic rings. Tautomers can occur here.


 
Pyrimidine

Purine
The bases are derived from the pyrimidine Cytosine, Thymine and Uracil where thymine only occurs in DNA, uracil only in RNA. All three bases can exist in two different tautomeric forms. The hydroxyl group donates a proton to a nitrogen atom in the ring. The double bond is rearranged, resulting in a carbonyl group. The system thus corresponds to a cyclic acid amide.

Cytosine
 
Thymine
 
Uracil
Adenine and Guanine have a purine system. Adenine only has one amino group attached, there are no tautomers. Guanine has one amino and one hydroxyl group attached to the six-membered ring of the purine. A tautomer occurs here, which also corresponds to a cyclic acid amide.

Adenine
 
Guanine
The keto forms are each the more stable tautomer; the bases are therefore predominantly in the keto form. The formation of hydrogen bonds in the nucleic acids is also only possible with this form.