Figure 33.31 The complex structure of an RNA molecule. A single-stranded RNA molecule can fold back on itself to form a complex structure. (A) The nucleotide sequence showing Watson–Crick base pairs and other nonstandard base pairings in stem-loop structures. (B) The three-dimensional structure and one important long-range interaction between three bases. In the three-dimensional structure to the left, cytidine nucleotides are shown in blue, adenosine in red, guanosine in black, and uridine in green. In the blown-up view, hydrogen bonds within the Watson–Crick base pair are shown as dashed black lines; additional hydrogen bonds are shown as dashed green lines.