Figure 5.1. DNA has a double stranded helical structure with the two individual molecules oriented in opposite directions (i.e., 5' to 3' ends). Complementary nucleic acid base pairs are cross-linked by hydrogen bonds (three between cytosine and guanine and two between thymine and adenine). The sugar phosphate portion of each nucleotide forms the structural backbone of DNA via bonds through the phosphate of adjacent sugars (illustrated to the right). During RNA synthesis, the DNA strands separate, allowing one to be copied. As the RNA molecule is synthesized, the complementary base pair for each nucleic acid in the DNA molecule is added with the exception of thymine, which is replaced by uracil.