Using fluorescence microscopy (FM), which permits the observation of single molecules, we found that a pearling structure is generated on a single long DNA molecule upon the addition of a gemini (dimeric) surfactant. This pearling structure was further investigated by performing atomic force microscopy measurements on the same DNA molecules as observed by FM. These observations revealed that the pearling structure is composed of many rings that are interconnected by elongated coil parts along a single DNA molecule, i.e., rings-on-a-string structure. The mechanism of the formation of such an intrachain segregated structure in terms of microphase separation on a single polyelectrolyte chain is discussed.

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