Certain structures in Eterna can only be solved using one or a few sequences.
Main article: Zigzag
Zigzags have a large number of solutions, but they are not always immediately apparent and are unlikely to be found by random chance.
Tricks Involving External Loops
|The 1-nt bulge attached to an external loop requires one GC pair in the orientation shown. Whe the unpaired base is on the 3' side of the bulge, as shown at right, the GU pair and unpaired U are required. When the unpaired base is on the 5' side of the bulge, an AU or GU pair will suffice.|
The 2-1 loop attached to an external loop or multiloop is best solved by the UU/U mismatch, but it can be done with a GG/A mismatch. It requires a GC solitary pair.
<img title="aa" src="/wiki/images/1516183662.png" border="0" alt="aa" width="998" height="440" />