On the Melting of Copolymeric DNA

Abstract

A model for DNA molecules is introduced and treated by the methods of statistical mechanics. This model takes into account the difference in bonding free energy between adenine—thymine and guanine—cytosine base pairs. The model used is an extension of the Zimm—Bragg model for polypeptides. The partition function for a certain class (which probably corresponds to most DNA sequences) of molecules is evaluated numerically and the linear dependence of the temperature of half‐denaturation on base composition is obtained, in agreement with the experimental work of Marmur and Doty. A further, previously unnoted, result is that the breadth of the transition is a function of the base composition. Comparison of the predicted broadening with available experimental data is excellent for DNA from simple organisms which probably possess only one kind of DNA molecule each. However, for complex organisms, which are known to have compositionally heterogeneous DNA, the relationship is not obeyed. This suggests the possibility of using transition breadth as a simple and rapid test for compositional heterogeneity of DNA's.