Transition of Chromatin from the “10 nm” Lower Order Structure, to the “30 nm” Higher Order Structure, as Followed by Small Angle X-Ray Scattering
Double helical Watson-Crick B DNA is a stiff macro-molecule which, in accordance with its basic structure (1), should proceed in free space in strictly linear fashion. Though abnormally migrating DNA has recently been discovered (2), and naturally curved DNA can presently be considered a highly fashionable topic of investigation (3), it must be considered an important exception, rather than the general rule. Still, even unrestricted in dilute solution, DNA is less rigid than could be gathered from its multiply fixed hydrogen bonded structure, and bends considerably as a result of disordering due to thermal Brownian motion. To describe the coiling behavior of DNA quantitatively the persistent chain or wormlike coil of Kratky and Porod (4) has been introduced. In this description the persistence length a equals about 40 nm in 0.2M NaCl (5), corresponding to a curvature radius of this order. Short DNA chains are almost rodlike in solution.
KeywordsSucrose EDTA Sedimentation MgCl2 Fractionation
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