Additional DNA long-range folding is an early process during stimulation of human lymphocytes

There is disagreement about the existence of permanent single-strand breaks in the nuclear DNA of quiescent lymphocytes, possibly due to different experimental approaches. Using the nucleoid sedimentation technique, no evidence is found for spontaneous DNA single-strand breakage in unstimulated human lymphocytes and for quick break-sealing after mitogenic stimulation. An increased sedimentation rate of nucleoids, which can be monitored as soon as 3 h after stimulation, is shown to be due rather to an additional long-range folding of nuclear DNA. Such temporal folding appears to be related to RNA transcription and may interfere with detection of DNA single-strand breakage.