**1.2. Alternative coding by swinger polymerization**

Further little known mechanisms increase numbers of proteins potentially coded by single sequences. Polymerization occasionally exchanges systematically between nucleotides during DNA replication [44–46] or RNA transcription [47–53] for long sequence stretches (23 exchange rules are possible, nine symmetric, e.g., A<>C, and fourteen asymmetric, e.g., A > C > G > A), producing swinger sequences. Swinger replication, in particular the double symmetric exchange A<>T + C<>G, seems most frequent for mitochondrial ribosomal RNAs [46]. This increases the coding potential of rRNAs, strengthening the hypothesis that rRNAs are modern remnants of protogenomes that templated for translational molecules (tRNA-like and rRNA-like) and protein coding genes [54–58] by dense overlap coding. This is compatible with the occurrence of protein coding regions within modern rRNAs [8, 59–61].

I stress here that the exchange A<>T + C<>G is not trivial: this creates the complement of the template sequence, which is not the regular inverse (or reverse) complement. "Complement" is frequently used as shortcut for inverse complement, but the A<>T + C<>G transformed sequence is a different sequence because it lacks the 3′-to-5′ inversion combined with nucleotide complementarity. The shortcut has been used because 3′-to-5′ inverted sequences had not been described previous to the description of A<>T + C<>G transformed sequences.

It seems regular transcription occasionally switches abruptly to swinger transcription (and vice versa), as indicated by chimeric RNAs. These RNAs correspond in part to regular DNA, and an adjacent part corresponds to DNA only if accounting for swinger transformation [62]. Corresponding chimeric peptides have also been detected [63]. Chimeric DNA also exists: the mitogenome of the stonefly *Kamimuria wangi* (NC\_024033) is regular, beside its 16S rRNA, which is entirely swinger transformed along transformation A<>T + C<>G [46].
