Large Discrete Transition in a Single DNA Molecule Appears Continuous in the Ensemble

We observed giant double-stranded DNA chains by fluorescence microscopy in an aqueous environment. We found that the coil-globule transition of T4DNA, 166kbp, induced by spermidine is markedly discrete for individual chains, and continuous for their ensemble average. Simple theoretical consideration is given taking account of the hierarchy of the system. It is suggested that the unique characteristics of the transition for long DNA has a general significance for the coil-globule transition in other biological and synthetic ``stiff polymers.''