DNA organization in nucleosomes

Abstract

A new technique known as electron spectroscopic imaging has allowed the direct visualization of DNA within the nucleosomes of chromatin. The results presented here confirm the model which suggests that approximately two supercoil turns of DNA are wound about the nucleosome core. The structure of nucleosomes from putative transcriptionally active genes, fractionated by preferential sensitivity to DNAase II and solubility in 2 mM MgCl2, has been examined using both dark field electron microscopy and electron spectroscopic imaging. Oligomeric strands of nucleosomes in this fraction have a less distinct beaded appearance than those of bulk chromatin. The phosphorus distribution in this chromatin suggests that the DNA has a less recognizable organization, lacking a two-turn supercoil per subunit. The unique appearance of this fraction in 30 mM NaCl is reversibly changed to the classical beaded appearance when dialyzed into 0.4 M NaCl.