Figure 2.

A schematic of the hierarchy of forces that determine RNA folding. Electrostatic repulsion between helices (depicted as cylinders) dictates the energetic cost of forming a compact structure. Electrostatic screening from an associated ion atmosphere (blue dots) mitigates this cost (A). Junctions that join helical regions bias the relative positions of the helices that they join, bringing different regions of secondary structure in close proximity (B). Recurring tertiary motifs bind secondary structure elements together and stabilize the final folded state. Some motifs, such as the tetraloop/tetraloop receptor (whose secondary structure is pictured) are highly conserved and ubiquitous throughout RNA structure (C).