DNA bases. Computer artwork of the chemical structures of the four bases found in DNA (deoxy- ribonucleic 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 lines running down left and right. For instance, the NH2 at the right of the cytosine molecule links with an O on the guanine. The sequence of these bases in DNA forms the genetic code.