Thymine molecule. Computer artwork showing the structure of a molecule of the nucleobase thymine. Atoms are colour-coded spheres: carbon (green), nitrogen (blue), oxygen (red), and hydrogen (white), with the bonds between them as rods. Thymine is one of the four nitrogenous bases found in DNA (deoxyribonucleic acid), which form complementary pairs. Thymine's complementary base in DNA is adenine. In RNA, thymine is replaced by uracil.