Figure 1.

Characteristics of Z-DNA/RNA and Z-conformation adoption. (a) Both dsDNA (top) and dsRNA (bottom), normally in the B- and A-conformations, can adopt the higher-energy left-handed Z-form (right). Z-DNA and Z-RNA are structurally equivalent and will revert to the B/A-form in the absence of stabilizing factors. Pyrimidine (blue) – purine (green) repeats alternate between the C2′-endo and C3′-endo sugar pucker conformations along with the nucleobases between anti and the syn conformations. This leads to a zig-zagged backbone. B-DNA, A-RNA, Z-DNA, and Z-RNA models were made using PDBs 1N1K¹, 1PBM², 1QBJ³, and 2GXB⁴, respectively. (b) Zipper model for the conversion from B-DNA to Z-DNA⁵. First, a high-energy nucleation event allows for helical handedness conversion and a short Z-DNA stretch to be adopted. This can then propagate down the helix in a cooperative manner if the sequence allows it. At what points Zα plays a role in the zipper model is mostly unknown.