The thermal denaturation of chromatin core particles

The thermal denaturation of chicken erythrocyte core particles has been followed using changes in absorption and circular dichroism. The absorption-denaturation curve is biphasic, with transitions centred at 59 degrees C and 74 degrees C. The first of these transitions is reversible, whereas heating into the second transition produces irreversible changes in DNA and histone secondary structure. After heating to temperatures within the second transition and then cooling, two components are observed in the analytical centrifuge: a slow-sedimenting species which is present in proportions corresponding to the extent of irreversible denaturation and a fast sedimenting species which is present in inverse proportion to the extent of denaturation. The slow-sedimenting component was primarily denatured DNA. The results suggest that thermal denaturation of core particles is a cooperative, 'all-or-none' process.