DNA in cell nuclei is complexed into string of nucleosomes.
At physiologicial conditions this string folds - at least in vitro - into a 33nm thick chromatin
fiber. Recent experiments in the group of Daniela Rhodes show that depending on the
spacing of the nucleosomes along the DNA one
finds in fact either the classical 33nm fiber or a thicker 44nm fiber (Robinson et al., PNAS 103, 6506 (2006)).
We showed recently that these diameters correspond to a discrete set of magical structures where the
wedge-shaped nucleosomes are densely packed (see the figure for 3 example structures). 33nm wide
fibers would correspond to 5 ribbons
of stacked nucleosomes, 44nm correspond to a 7 ribbon structure. But the questions remains why 5 and 7
ribbon structures are preferred to other number of ribbons.
Currently we extend our model to include the elastic energy of the DNA that connects the nucleosomes.
Our results can predict the geometry and
energetics of chromatin fibers as a function of the linker length by combining geometrical
and mechanical arguments.
For details check out:
M. Depken and H. Schiessel: Nucleosome shape
dictates chromatin fiber structure, Biophys. J.
96, 777-784 (2009)
