Higher-order structure of mammalian chromatin deduced from viscoelastometry data

The results of viscoelastometry (VE) for mammalian DNA have been puzzling b ecause they have two orders of magnitude smaller measured viscoelastic rela xation times for mammalian chromosomes than that expected for DNA linear co ils of chromosomal size. In an attempt to resolve this discrepancy, we have applied a recent model of G1 chromosome structure (J.Y. Ostashevsky, Mol B iol. Cell 9, 3031-3040, 1998) in which the 30 nm chromatin fiber of each ch romosome forms a string of loop clusters (micelles). This model has two par ameters: the number of loops per micelle (f) and the average loop size (M-f ), which can be estimated independently from VE data. Using our VE data for plateau phase V79 Chinese hamster cells (unirradiated and X-irradiated wit h doses up to 40 Gy) we show that f similar to 13, which is close to other estimates made using the model (f ranges from 10 - 20), and M-f similar to 2 Mbp, which is similar to estimates made from our nucleoid data (1.3 Mbp) and to estimates made in the literature using a variety of techniques (1-3 Mbp).