text: Naturally occurring bases can be divided into two classes according to their structure:
  * Pyrimidines are six-membered heterocyclic with nitrogen atoms in position 1 and 3.
  * Purines are bicyclic, consisting of a pyrimidine fused to an imidazole ring.
  Artificial nucleotides (Unnatural Base Pairs (UBPs) named d5SICS UBP and dNaM UBP) have been inserted into bacterial DNA but these genes did not template mRNA or induce protein synthesis. The artificial nucleotides featured two fused aromatic rings which formed a (d5SICS–dNaM) complex mimicking the natural (dG–dC) base pair.