Left-handed Z-helices
One of the first DNA sequences to be crystallized was the
oligomer d(GCGCGC), as shown in Figure 1b. To many
people’s surprise, this structure was a left-handed helix,
opposite to that of the traditionalWatson–Crick helix. The
backbone is not a smooth helix, but is irregular and zigzag
in shape, hence its name. At the time, this structure was
quite controversial, but now it is generally accepted that
certain DNA sequences (in particular alternating purine–
pyrimidine tracts) can form left-handed Z-DNA, while most other sequences will readily form a right-handed
helix. The Z-helix is narrower than the A-and Bconformations,
and it has 12 bp per turn. The nucleotide
bases are flipped upside down, relative to the phosphate
backbone, in Z-DNA when compared with A-DNA and BDNA.