<P> Left - handed DNA was first discovered by Robert Wells and colleagues, during their studies of a repeating polymer of inosine--cytosine . They observed a "reverse" circular dichroism spectrum for such DNAs, and interpreted this (correctly) to mean that the strands wrapped around one another in a left - handed fashion . The relationship between Z - DNA and the more familiar B - DNA was indicated by the work of Pohl and Jovin, who showed that the ultraviolet circular dichroism of poly (dG - dC) was nearly inverted in 4 M sodium chloride solution . The suspicion that this was the result of a conversion from B - DNA to Z - DNA was confirmed by examining the Raman spectra of these solutions and the Z - DNA crystals . Subsequently, a crystal structure of "Z - DNA" was published which turned out to be the first single - crystal X-ray structure of a DNA fragment (a self - complementary DNA hexamer d (CG)). It was resolved as a left - handed double helix with two antiparallel chains that were held together by Watson--Crick base pairs (see X-ray crystallography). It was solved by Andrew Wang, Alexander Rich, and coworkers in 1979 at MIT . The crystallisation of a B - to Z - DNA junction in 2005 provided a better understanding of the potential role Z - DNA plays in cells . Whenever a segment of Z - DNA forms, there must be B--Z junctions at its two ends, interfacing it to the B - form of DNA found in the rest of the genome . </P> <P> In 2007, the RNA version of Z - DNA, Z - RNA, was described as a transformed version of an A-RNA double helix into a left - handed helix . The transition from A-RNA to Z - RNA, however, was already described in 1984 . </P> <P> Z - DNA is quite different from the right - handed forms . In fact, Z - DNA is often compared against B - DNA in order to illustrate the major differences . The Z - DNA helix is left - handed and has a structure that repeats every other base pair . The major and minor grooves, unlike A - and B - DNA, show little difference in width . Formation of this structure is generally unfavourable, although certain conditions can promote it; such as alternating purine--pyrimidine sequence (especially poly (dGC)), negative DNA supercoiling or high salt and some cations (all at physiological temperature, 37 ° C, and pH 7.3--7.4). Z - DNA can form a junction with B - DNA (called a "B - to - Z junction box") in a structure which involves the extrusion of a base pair . The Z - DNA conformation has been difficult to study because it does not exist as a stable feature of the double helix . Instead, it is a transient structure that is occasionally induced by biological activity and then quickly disappears . </P> <P> It is possible to predict the likelihood of a DNA sequence forming a Z - DNA structure . An algorithm for predicting the propensity of DNA to flip from the B - form to the Z - form, ZHunt, was written by P. Shing Ho in 1984 at MIT . This algorithm was later developed by Tracy Camp, P. Christoph Champ, Sandor Maurice, and Jeffrey M. Vargason for genome - wide mapping of Z - DNA (with Ho as the principal investigator). </P>

Differences between a b and z form of dna
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