Other Conformations of DNA

A -DNA The Watson-Crick model of DNA is based on the x-ray diffraction patterns of B-DNA. Most DNA is B-DNA however, DNA may take on two other conformations, A-DNA and Z-DNA. These conformations are greatly favored by the base sequence or by bound proteins. When B-DNA is slightly dehydrated in the laboratory, it takes on the A conformation. A-DNA is very similar to B-DNA except that the base pairs are not stacked perpendicular to the helix axis rather, they are tilted because the deoxyribose moiety puckers differently. An A-DNA helix is wider and shorter than the B-DNA helix. 

In Z-DNA, guanine residues are syn, whereas cytosine and thymine residues are anti. Eukaryotic DNA contains several alternating purine-pyrimidine sequences consistent with die Z-DNA conformation however, the biologic significance of Z-DNA is still unclear. 

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