D.N.A. consists of a double helix. One can think of a ladder, which has two sides, with steps in between. Since the molecule has two sides, each side is the complement of the other and can function as a template for transcription(copying)by messenger R.N.A. D.N.A. remains in the nucleus, however, mRNA can transcribe it and take the message to the ribosomes for translation. Since D.N.A. only has four bases, adenine, guanine, cytosine and thymine, and they pair according to the following rules, A pairs with T and C pairs with G, then, only the complementary R.N.A. base will copy the D.N.A. instruction. For example, if the D.N.A. code reads A-T-G-C, the complementary R.N.A. that is transcribed will read U-A-C-G. *Remember that R.N.A. doesn't have the base thymine, so substitute uracil instead when transcription occurs.