Figure 2. Nested and non-nested rRNA helices.

A 2° structure with four helical regions is shown in the top panel. A topology diagram, illustrating the nesting concept, is shown in the bottom panel. The green and yellow helices are nested within the red helix, with base pairs (i,q) (red) and (j,p) (yellow or green) where i<j<p<q. The blue helix is non-nested within the other helices, with base pairs (i′,q′) (red) and (j′,p′) (blue) where i′<j′<q′<p′. The red, green and yellow helices are commonly considered to be 2° structural helices. The blue helix is non-nested and is considered to be a tertiary helix.