organic compound of the pyrimidine family that is a constituent of deoxyribonucleic acid ( DNA). DNA, along with RNA (ribonucleic acid), regulates hereditary characteristics in all living cells. Like the other nitrogenous components of nucleic acids, thymine is part of thymidine, a corresponding nucleoside (a structural unit composed of a nitrogen compound and a sugar), in which it is chemically linked with the sugar deoxyribose. It is also part of thymidylic acid, a nucleotide (a larger structural unit composed of a nucleoside and phosphoric acid), which is a phosphate ester of thymidine. The nucleotide, the nucleoside, or thymine itself may be prepared from DNA by selective techniques of hydrolysis.