Three dimensional association of double-stranded helices are produced in conditions for Z-DNA formation.

The Z form of alternating poly(dG-dC).poly(dG-dC) can be induced when the concentration of NaCl, MgCl2 or ethanol are increased. In order to obtain more information concerning this Z structure, the B----Z transition is analyzed on the same sample, both by UV spectrophotometry and electron microscopy. The procedures used in this work provide high resolution images with minimal alterations of the molecules. It is shown that at high values of cations or ethanol, the polymer makes complex associations of numerous molecules stuck together parallelly. By decreasing the salt or ethanol concentrations, a progressive decondensation of the molecules is obtained. At low concentrations of Mg++ (2.10(-2) M), alterations of the linear secondary structure of the molecules are observed, although the UV spectrum is of the B-type. In the presence of that low concentration of Mg++, natural DNAs (phi X174 and yeast mitochondrial DNA fragment inserted in pBR) exhibit structural modifications similar to those observed with the poly(dG-dC).poly(dG-dC). These structures mainly consist in four-stranded hairpins and loops built up by the sticking of two segments of DNA. The correlation between these intertwining of short DNA segments and the presence of potentially Z-forming sequences is discussed.