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5-nucleotides bulge.

A bulge is the structure formed where two stacks meet, with only unpaired bases on one side of the junction and no extra bases on the other side of the junction.  This differs from an internal loop, where unpaired bases are found on both sides. == Nomenclature == Bulges are typically described by the number of unpaired bases. In the illustration to the right, 5 bases (21 through 25) are unpaired.