Structural changes in giant DNA induced by the addition of the flexible polymer Polyethylene Glycol (PEG) were examined by the method of single-DNA observation. In dilute DNA conditions, individual DNA assumes a compact state via a discrete coil-globule transition, whereas in concentrated solution, DNA molecules exhibit an extended conformation via macroscopic phase segregation. The long-axis length of the stretched state in DNA is about 103 times larger than that of the compact state. Phase segregation at high DNA concentrations occurs at lower PEG concentrations than the compaction at low DNA concentrations. These opposite changes in the conformation of DNA molecule are interpreted in terms of the free energy, including depletion interaction.

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