The strand‐separation transition of T2 bacteriophage DNA

Strand separation of T2 DNA has been investigated in a helix‐destabilizing solvent. Temperature‐shift experiments in which the conformation of the DNA is monitored by its viscosity, sedimentation behavior, and kinetics of helix formation show that a well‐defined strand‐separation transition follows the helix–coil transition usually observed by changes in absorbance. For T2 DNA, this strand‐separation transition is 70% as broad as the helix–coil transition, and is characterized by extremely slow kinetics of conformational change in the population. Strand separation requires the expansion of the two‐stranded coil observed at the end of the helix–coil transition. This expansion is apparently coupled with the disurption of the last remaining base pairs in the molecule. The expansion process increases the viscosity, and can be readily followed as a function of time and/or temperature. Subsequent separation of the expanded form into complementary strands results in a viscosity decrease, the net result of a reduction in hydrodynamic volume and the halving of the molecular weight. Only under conditions where the driving force for strand separation is large are these events at all synchronous in the population. When the kinetics of conformational change are complete in the strand‐separation transition, a mixture of expanded forms and separate strands is observed; the breadth of the transition reflects differences in stability with respect to strand separation among the molecules in the population. The transition exhibits hysteresis and is not a reversible equilibrium between double‐stranded and single‐stranded forms. It appears that renucleation is kinetically forbidden within the strand‐separation region.