  Structural changes in giant DNA induced by the addition of the flexible
polymer 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 1000 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.
