X-RAY SMALL-ANGLE SCATTERING STUDY OF CHROMATIN AS A FUNCTION OF FIBER LENGTH

This work investigates the structure of native calf thymus chromatin a s a function of fiber length and isolation procedures by using X-ray s mall angle scattering technique. Two methods of chromatin isolation ha ve been compared in order to better understand the differences reporte d by various authors in terms of chromatin high order structure. In ad dition to these experimental results the effects of shearing have also been studied. In order to explain the differences among these chromat in preparations we built several models of chromatin fibers (represent ed as a chain of spherical subunits) assuming increasing level of cond ensation at increasing salt concentrations. For all these fiber models the corresponding theoretical X-ray scattering curves have been calcu lated and these results have been used to explain the influence of fib er length on the scattering profiles of chromatin. The comparison betw een experimental and theoretical curves confirms that the high molecul ar weight chromatin-DNA prepared by hypotonic swelling of nuclei (with out enzymatic digestion) displays a partially folded structure even at low ionic strength, whereas the low molecular weight chromatin-DNA pr epared by a brief nuclease digestion appears very weakly folded at the same ionic conditions.