The left-handed Z-DNA double helix is held together by traditional Watson-Crick base pairs, but unlike righthanded B-DNA, which has major and minor grooves between the twists of its sugar-phosphate backbones, Z-DNA’s grooves show little difference in width. In addition, every other base in a stretch of Z-DNA takes on a different orientation relative to the sugar backbone than the arrangement in B-DNA, giving this alternative form of DNA the zig-zag shape for which it was named.
Z-DNA exists transiently in short stretches of up to 100 base pairs within some right-handed DNA molecules. The site where the DNA molecule switches chirality is called a B-Z junction. At this point in the polymer, one A-T base pair projects to the outside of the double helix.
Z-DNA in Action
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Rachel Brazil is a freelance science writer from London, UK. Reach her on Twitter @RachelBBrazil.