The Tetrahymena ribozymes, with secondary structures modified from [ ]. The L–21 (‘length minus 21’ nucleotides from the 5′ end of the in vivo intron) sequence is shown for the wild-type ribozyme, with the five mutations in the CaCl2-competent variant (PV)  indicated by the circles: A103G, A187U, A270G, U271C, and G312A. Numbering scheme follows the original ,,, with dots provided every ten nucleotides. The interaction between the internal guide sequence (IGS) and six nucleotides of the substrate is shown by the grey box; in the ‘pick-up-the-tail’ assay the hydroxyl of the 3′ nucleotide (G414, shown) attacks at the splice site in the substrate (lower-case letters) and transfers the 3′ portion of the substrate (the last 17 nucleotides of S-1t in this case = AAAUAAAUAAAUAAAUA) to the 3′ end of the ribozyme, thereby lengthening it making it detectable by gel electrophoretic analysis. In this drawing, 71 nucleotide near the 3′ end of the ribozyme were omitted for clarity. Bars between nucleotide pairs denote canonical Watson-Crick pairs, while dots denote non-canonical base-paring interactions. The active site of this ribozyme is the environment around the G414 and the stack of base-triples that is above and below it .