genetic code
the instructions in genes that “tell” the cell how to make specific proteins. The code resides in the sequence of bases occurring as constituents of the genetic material, DNA or RNA. These bases are represented by the letters A, T, G, and C (which stand for adenine, thymine, guanine, and cytosine, respectively). In messenger RNA, uracil (U) replaces thymine. Each unit, or codon, of the code consists of three consecutive bases. Hence, there are 64 possible triplet combinations of the four bases, which specify the amino acids that make up each protein molecule.