Sinister depictions of DNA

Abstract

At the centre of molecular genetics stands the DNA molecule. Students learn about the structure of double-stranded DNA from textbooks and lectures. They are taught that the helical structure of DNA inside a living cell is in the right-handed or B-form. They also learn about left-handed DNA, termed Z-DNA, which was first proposed by Pohl and Jovin in 1972 (Ref. 1xSalt-induced co-operative conformational change of a synthetic DNA: Equilibrium and kinetic studies with poly (dG-dC). Pohl, F.M. and Jovin, T.M. J. Mol. Biol. 1972; 67: 357–369Crossref | Scopus (945)See all ReferencesRef. 1). Under certain circumstances, a transition from B- to Z-DNA can occur in stretches of alternating purine and pyrimidine residues. Whether Z-DNA has a biological function, however, is still not known.B- and Z-DNA can easily be distinguished from each other in diagrams if the two strands are marked in different shades (Fig. 1Fig. 1). But wait – unexpected figures depicting left-handed DNA seem to be cropping up in unusual places. In Richard R. Sinden’s textbook DNA Structure and Function2xSee all References, for example, one figure (5.7) intends to demonstrate that the formation of left-handed DNA depends on the topological state of the DNA molecule. However, the starting DNA molecule is already in the left-handed confirmation. Another prominent example is found on the cover of a recent issue of Science (23 October 1998). Even the next generation of molecular biologists is being prepared for an onslaught of left-handed DNA. In a textbook distributed by CSHL Press3xSee all References, and designed to teach children ‘the secret of DNA’, most of the figures, including that on the cover, depict Z-DNA.Figure 1The B to Z of DNA. (a) Right-handed (B-form) and (b) left-handed (Z-form) DNA.View Large Image | Download PowerPoint SlideThe most spectacular example of this sinister trend is found in the sixth edition of Benjamin Lewin’s text book, Genes VI (Ref. 4xSee all ReferencesRef. 4). Most of the figures have been redone, but unfortunately so has the handedness of the DNA. In 176 of 179 figures, left-handed DNA is shown, instead of right-handed DNA. More curiously still, in some figures, DNA is neither right- nor left-handed; the two strands are not intertwined at all (e.g. Fig. 4.21). Most of the time the figures are just wrong without mechanistic consequences, but sometimes there is a consequence. For example, in Fig. 17.14 rotation of the DNA strands does not lead to strand separation, as indicated in the text (p. 551), but to DNA supercoiling. Beware students – these figures are sold as transparencies and are used in teaching! While preparing this brief note, it was brought to our attention that others too have been struck by the growing appearance of left-handed DNA. In a beautifully illustrated homepage entitled The left-handed hall of fame (http://www-lecb.ncifcrf.gov/∼toms/LeftHanded.DNA.html), Tom Schneider provides an impressive collection from the past 34 years of illustrations depicting left-handed DNA. Based on this sudden prevalence of left-handed DNA, it appears that more than 25 years after its discovery, Z-DNA has finally found its functional role, if only in text books and scientific journals.