Figure 2

An example mutation path for a TKF91 Structure Tree, illustrating the instantaneous transitions of the process. In each figure (N), gray arrows indicate the sites of past mutations in step (N - 1) → (N), while black arrows indicate the sites of upcoming mutations in step (N) → (N + 1). The types of mutation are as follows. (1) → (2): Point substitution of unpaired nucleotides in loop sequences. (2) → (3): Point and/or covariant substitution of paired nucleotides in stem sequences. (3) → (4): Insertions of single unpaired nucleotides in loop sequences. (4) → (5): Deletions of single unpaired nucleotides in loop sequences. (5) → (6): Insertion and deletion of paired nucleotides in stem sequences. (6) → (7): Insertion of stems into loop sequences. (7) → (8): Deletion of stems from loop sequences.