Higher-order structure of mammalian chromatin deduced from viscoelastometry data

The results of viscoelastometry (VE) for mammalian DNA have been puzzling because they have two orders of magnitude smaller measured viscoelastic relaxation times for mammalian chromosomes than that expected for DNA linear coils of chromosomal size. In an attempt to resolve this discrepancy, we have applied a recent model of G1 chromosome structure (J.Y. Ostashevsky, Mol Biol. Cell 9, 3031-3040, 1998) in which the 30 nm chromatin fiber of each chromosome forms a string of loop clusters (micelles). This model has two parameters: the number of loops per micelle (f) and the average loop size (Mf), which can be estimated independently from VE data. Using our VE data for plateau phase V79 Chinese hamster cells (unirradiated and X-irradiated with doses up to 40 Gy) we show that f approximately 13 , which is close to other estimates made using the model (f ranges from 10-20), and Mf approximately 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).