Comparison of DNA loop size and super-coiled domain size in human cells.

DNA loop size and super-coiled domain size in HeLa cells were investigated by the nucleoid sedimentation technique as well as by analysis of the DNA-nuclear-matrix complex. After X-irradiation, the relaxation of super-coils was not correlated with the release of DNA from the nuclear matrix. The super-coiled domain size was much larger than the average DNA loop size. To explain the results, we propose that unwinding of super-coils in a DNA loop also causes unwinding in adjacent DNA loops in spite of existing attachment sites. Alternatively, the results may be explained by assuming that only very large DNA loops determine the sedimentation behaviour of nucleoids.