Pyrimidine

Pyrimidines are nitrogen-containing molecules built around a single six-membered ring with two nitrogen atoms. Cytosine, thymine, and uracil are pyrimidine bases found in nucleic acids.

Pyrimidine ring: A six-membered aromatic ring containing four carbon atoms and two nitrogen atoms. This ring is the defining structural framework shared by pyrimidine compounds.
Cytosine: A pyrimidine nucleobase found in DNA and RNA that pairs with guanine. Cytosine is one of the three canonical pyrimidine bases in nucleic acids.
Pyrimidine biosynthesis: The cellular pathway that constructs pyrimidine nucleotides from small precursor molecules. This pathway assembles the ring before attaching it to ribose phosphate.
Antimetabolite: A compound that resembles a natural metabolite and disrupts a biochemical pathway by interfering with its use or production. Several drugs mimic pyrimidines or block their synthesis to restrain cell growth.
Nucleobase: A nitrogen-containing molecule that forms part of a nucleic acid or nucleotide. Cytosine, thymine, and uracil are pyrimidine members of this broader group.
Thymine: A pyrimidine nucleobase used primarily in DNA, where it pairs with adenine. Thymine distinguishes DNA’s standard bases from those used in RNA.
De novo pyrimidine synthesis: The metabolic production of pyrimidine nucleotides from simpler molecules rather than recycled bases. It is the principal route for making pyrimidines when salvageable bases are unavailable.
5-Fluorouracil: A fluorinated uracil analogue used as a chemotherapy drug and converted into active metabolites in cells. Its resemblance to uracil disrupts thymidylate production and nucleic-acid metabolism.
Purine: A nitrogen-containing nucleobase framework made of fused five- and six-membered rings. Purines provide the two-ring counterpart to pyrimidines in nucleic acids.
Uracil: A pyrimidine nucleobase used in RNA, where it pairs with adenine. Uracil occupies the corresponding position to thymine in RNA.

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