Figure 1.

Schematic diagram of duplex formation by hybridization of two entirely complementary circular ssDNA under topological constraints. The two possible cases of Z-conformation (I) and non-Z-formation (II) are shown. For structure I, the left-handed part takes the Z-conformation. For II, base pairs are not formed in the left-handed part. If mismatched sequences are put at the left-handed part, structure of form II prefers to form (another possibility is to form a mismatched duplex with Z-conformation).