An exact statistical-mechanical treatment is given for the nearest-neighbor coupled model of DNA, allowing for the difference in bonding stability of the two base pairs, adenine–thymine and gaunine–cytosine. Treatments are given for random as well as pair-correlated sequences, both for finite and infinite degrees of polymerization. In addition, sequence-ensemble average thermal properties are compared with the corresponding properties of single molecules. The breadth in temperature of the helix–coil transition is shown to be a marked function of base composition. It is also shown that nonrandomness in nearest-neighbor base sequence of the magnitude observed in typical bacterial and viral DNA's has little effect on the thermodynamic properties.