DNA contains two strands of building blocks called nucleotides, a range like a spiral staircase. Each nucleotide includes three parts, a phosphate group, a sugar molecule, and one of four bases, adenine, guanine, cytosine, or thymine. The sugar phosphate bonds form the double backbone of the molecule, the handrails of the staircase. But we find the genetic key to DNA in the steps of this stairway, the nitrogen containing bases. These bases link up using hydrogen bonds in a very specific way. Adenine will bond only with thymine, a2t, cytosine only bonds with guanine, c2g. While these basic pairings never change, the order of the pairs along each strand varies greatly from one species to the next. In this elegant design, we see how nature stores the instructions to build all living things.