Nitrogenous bases are typically classified as the derivatives of two parent compounds, pyrimidine and purine. Also, folate is essential for purine and pyrimidine base biosynthesis, so synthesis will be inhibited. Uracil is a common and naturally occurring pyrimidine derivative. Pteridine is an aromatic chemical compound composed of fused pyrimidine and pyrazine rings. The conserved symmetrical Z-DNA was abbreviated ZZNZNZZ, where Z represents alternating purine/pyrimidine sequences. RNase A cleaves specifically after pyrimidine nucleotides. G mononucleotide repeat within one of the repetitive pyrimidine&colon;purine divergent domains. Transversions are replacement of a purine with a pyrimidine or vice versa. The synthesis of any pyrimidine nucleotide begins with the formation of uridine. Unrepaired pyrimidine dimers in humans may lead to melanoma. By contrast, an exchange between one purine and one pyrimidine is called a transversion. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. It is isomeric with two other (CH)N rings, pyrimidine and pyrazine. Synthesis of pyrimidine nucleotides is a much simpler process. R5P also serves as an important precursor to pyrimidine ribonucleotide synthesis.