The D.N.A. molecule is a double helix. This means that it resembles a ladder that is twisted. The sides of the ladder consist of sugar and phosphate groups. In the center of the ladder are complementary base pairs. Adenine pairs with Thymine, Cytosine with Guanine. When the Hydrogen bonds holding these base pairs together are overcome, either strand of DNA can serve as a template for transcription. During transcription, messenger RNA also present in the nucleus will join to its complementary DNA base on the strand. For instance, if the DNA strands reads, A-T-C-G, the complementary RNA transcription will read U-A-G-C. Remember that RNA does not have the base Thymine(T), so in a pairing with A on the DNA strand, you must substitute the base Uracil(U). Because both sides of the DNA strand can serve as a template, it can easily be transcribed by RNA. Free nucleotides will be attracted to the complementary base and the message will be transcribed by the messenger RNA.