Dissertation Information for Brian David Athey
MPACT Status: Incomplete - Not_Inspected
Title: An EM study of chromatin fibers: Evidence in support of the double-helical crossed-linker model
Abstract: "Electron microscopy experiments were performed to test the validity of the proposed models for chromatin fiber structure. In one such experiment, chromatin fiber specimens from Thyone (sea cucumber) and Necturus (mudpuppy) were embedded into vitrified aqueous solutions of physiological ionic strength (124 mM) and imaged using cryo-electron microscopy procedures. Diameter measurements were performed using interactive digital image-processing techniques. Thyone and Necturus fibers had significantly different estimated mean outer diameters of 43.5 nm (CD = 4.2 nm; SEM = 0.61 nm) and 32.0 nm (SD = 3.0 nm; SEM = 0.36 nm), respectively. The same specimens were negatively-stained, and had estimated outer diameters of 38.7 nm (SD = 4.0 nm, SEM = 0.8 nm) and 29.0 nm (SD = 2.2 nm; SEM = 0.8 nm), respectively. These fibers suffered $\sim$10% shrinkage, and were found to be significantly flattened. The DNA linker length of Thyone briareus sperm ($N$ = 87 bp) is significantly longer ($\sim$13 nm) than that of Necturus maculosus erythrocytes ($N$ = 48 bp). The diameter difference of $\sim$12 nm suggested that a direct relationship between fiber diameter and DNA linker length exists. These diameter measurements and the published EM and x-ray data regarding chromatin fiber diameter support this hypothesis. The constant 30 nm diameter solenoid and twisted-ribbon models are inconsistent with these diameter data.
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