One of four base molecules that form part of the base pairs of the nucleic acids of DNA, RNA and some nucleotides, as well as their derivatives. In DNA, it is always paired with the purine thymine and in RNA with the pyrimidine uracil.
See DNA (deoxyribonucleic acid), Methylation, Nucleic acid, Nucleotide, Purines, Pyrimidines, RNA (ribonucleic acid), Thymine, Uracil