Chromatin condensation is confined to the loop and involves an all-or-nonestructural change

Using differential scanning calorimetry in combination with pulsed field ge l electrophoresis; we relate here the changes in the thermal profile of rat liver nuclei induced by very mild digestion of chromatin by endogenous nuc lease with the chain length distribution of the DNA fragments. The enthalpy of the endotherm at 106 degrees C, which reflects the denaturation of the heterochromatic domains, decreases dramatically after the induction of a ve ry small number of double-strand breaks per chromosome; the thermal transit ion disappears when the loops have undergone on average one DNA chain sciss ion event. Quantitative analysis of the experimental data shows that the lo op behaves like a topologically isolated domain. Also discussed is the proc ess of heterochromatin formation, which occurs according to an all-or-none mechanism. In the presence of spermine, a strong condensation agent, only t he loops that have undergone one break are able Id refold, in confirmation of the extremely cooperative nature of the transition. Furthermore, our res ults suggest a-relationship between the states that give rise to the endoth erms at 90 degrees C and 106 degrees C and the morphologies referred to as class II and dass III ina previous physicochemical study of the folding of chromatin fragments (Widom, 1986. J. Mel. Biol. 190:411-424) and support th e view that the overall process of condensation follows a sequential (two-s tep) pathway.