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 2 supercoil turns of DNA are wound about the nucleosome core. The structure of nucleosomes from putative transcriptionally active genes, fractionated by preferential sensitivity to DNase II and solubility in 2 mM MgCl2, was 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 had a less recognizable organization, lacking a 2-turn supercoil/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.