The strand‐separation transition of T7 DNA

The strand‐separation transition of T7 DNA has been investigated by temperature shift and viscosity measurements in two formamide–water solvents. The strand‐separation region is quite narrow, and follows directly at the end of the denaturation transition observed by absorbance. The kinetics of strand separation of T7 DNA are slow and complex in the strand‐separation transition. Similarities and differences in the behavior of T2 and T7 DNA in strand separation are indicated and discussed. Briefly, the time course of strand separation and the conformational changes observed in the population undergoing strand separation are similar for the two molecules. However, the transition breadths and the interval between the helix–coil transition and the strand‐separation transition differ markedly. Both DNA molecules exhibit hysteresis in the strand‐separation region. For both molecules, it appears that strand separation involves the coupled denaturation and disentanglement of the two‐stranded form found at the end of the helix–coil transition.