What actually forms code found in DNA?

The code in DNA is stored in the four chemical bases (Adenine, guanine, cytosine and thymine) which are found in the genes. These include the information the cell needs to make specific proteins. Each base pair is made from 2 complementary nucleotides (either A+T or G+C) and bound together with hydrogen bonds.  

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands