(A) An RNA polymerase ribozyme (RPR; black circle) adds a mononucleotide triphosphate (PPPN) building block to the primer (brown line) in a primer/template duplex (left). This process can be repeated (right). (B) The triplet polymerase ribozyme developed by Attwater et al. (blue circle) adds trinucleotide triphosphate (PPPNNN) building blocks and has several advantages: for example, it does not always require a primer and can copy RNA sequences in both directions. (C) The chemical structure of a trinucleotide triphosphates building block, showing the three nucleotide bases.