English: A-DNA, B-DNA, and Z-DNA conformations of DNA. 12 base-pair steps composed by 13 base pairs are shown in a side view and top view. In the case of A-DNA and B-DNA, one strand (the one starting at the top left in the B-DNA) is pTpApCpGpTpApCpGpTpApCpGpT, while the other strand, going from the bottom to the top, is pApCpGpTpApCpGpTpApCpGpTpA. If a pA is added at the bottom (3' end) of the first strand and a pT at the bottom (5' end) of the second strand (giving pTpApCpGpTpApCpGpTpApCpGpTpA for both strands), then the DNA would have a two-fold rotational axis between the top seven base pairs and the bottom seven. When looking down the axis, this gives the appearance of a reflexion across a line. In the A-DNA image this line goes in a "westsouthwest-eastnortheast" direction, while in the B-DNA image it is 36° counterclockwise from the "east-west" direction. if the chain is lengthened with the same repeating pattern then there would be other two-fold rotational axes intersecting the axis of the double helix, and we can see imperfect reflexions in the axial view corresponding to those. In the Z-DNA shown, both strands are repeats of pCpG, which favors the formation of the Z configuration. The structure repeats every two base pairs, rather than every base pair. The cytosines are all near the axis, whereas the guanines are further away. Since the turn per two base pairs is 60° (at least in this image, though in theory the angle may be some irrational number of degrees), we see six apparent reflexions in the axial view.