Interplay between folding/unfolding and helix/coil transitions in giant DNA

It has been well established that double-stranded DNA undergoes a melting, or helix/coil, transition into a single-stranded coil state with an increas e in temperature. On the other hand, it has recently been found that, at a fixed temperature, long DNA, larger than several kilobase pairs, exhibits a discrete transition, or switching, between elongated and folded states, pr eserving its double-stranded structure, with the addition of various conden sation agents, such as alcohol, hydrophilic polymer, multivalent cation, an d cationic surfactant. In the present study, we examined the interplay betw een the folding/unfolding transition and the helix/coil transition in indiv idual giant DNA molecules, by observing the conformation of single molecula r chains with fluorescence microscopy. The results indicate that the helix- to-coil transition tightly cooperates with the unfolding transition in DNA.