**3.4.** *Anti*↔*syn* **conformational transitions of the long purine-purine DNA mismatches**

All long purine·purine DNA base mispairs can acquire enzymatically competent conformations—А\*·Аsyn(TF), G·Аsyn, A\*·G\*syn and G·G\*syn—through the A\*·A(WC)↔A\*·Asyn(TF), G·A(WC)↔G·Asyn, A\*·G\*(WC)↔A\*·G\*syn and G·G\*(WC)↔G·G\*syn conformational transitions [77], eventually guaranteeing their chemical incorporation into the newly synthesized structure of the DNA double helix (TF-Topal-Fresco nucleobase pair [10]; *syn*-*syn*-orientation of the base according the sugar-phosphate moiety) (**Figure 4**). Characteristic time of these nondissociative conformational transitions (~10−7 s) is much less than the period of time the high-fidelity DNA polymerase spends on incorporating one nucleotide into the DNA double helix (~8.3 × 10−4 s [67]). So-called long A\*·A(WC), G·A(WC), A\*·G\*(WC) and G·G\*(WC) DNA base mispairs have been outlined as "node stations" on the way of the formation of the enzymatically competent conformations arising in the recognition pocket of the high-fidelity DNA polymerase at its transition from the open to closed state.
