| Abstract: | Fragmented and mitomycin C cross-linked E. coli DNA was fractionated according to base composition by means of hydroxylapatite chromatography and density-gradient centrifugation in order to determine the effect of compositional heterogeneity on the breadth of the helix–coil transition. The transitions of some of the fractions are broader than that of the unfractionated DNA, due, presumably, to nonrandom sequences in molecules of 5 × 105 daltons. Analysis of the transition breadths in terms of the known heterogeneity leads to reconsideration of current DNA helix–coil transition theory. We propose that partially denatured states include those for which the chains do not remain in strict register. Denaturation profiles are comprehensible only if this multitude of entropically favorable, degenerate states is included in the theory. |
