Organization of mammalian chromosomal DNA: supercoiled and folded circular DNA subunits from interphase cell nuclei

Chinese hamster cells (line V79/4) in the G2 phase of the mitotic cycle were lysed onto neutral sucrose gradients and the released chromosomal DNA was characterized according to its size and shape by sedimentation velocity studies. Using the intercalating agent, ethidium bromide, in the gradients and the induction of DNA single- and double-strand breaks by irradiation, the DNA was proved to be released into the gradient in supercoiled circular subunits whose homogenous size corresponds to 2.8 . 10(9) Dalton. Supercoiling of these DNA subunits was found to be confined to smaller regions sized on the average about 9 . 10(7) Dalton and maintained by folding the DNA into loops. The average superhelix density was determined to be--0.09 turns per 10 base pairs of DNA. The functional aspects of the experimental findings are discussed in terms of replicative and transcriptional units.