DNA bases. Computer artwork of a human figure and the chemical structures of the four bases found in DNA (deoxyribonucleic acid). These are thymine, cytosine, adenine and guanine. These nucleotide bases comprise carbon (C), oxygen (O), nitrogen (N) and hydrogen (H). They form the links between the two sugar-phosphate strands (pink lines) in the DNA molecule. Thymine links with adenine and guanine links with cytosine. The attachment sites between the bases in these pairs is shown by the blue lines running down left and right. The sequence of these bases in DNA forms the genetic code.