We have obtained tilt series of thin sections of chromatin fibers embedded in araldite-epon and stained with uranyl acetate. The tilt series contain between 12 and 18 micrographs, spanning tilt ranges between 84 and 117 degrees. Reconstructions in three dimensions have been obtained by filtered back-projection from each tilt series. The reconstructions have been low-pass filtered in order to reduce the amount of noise. In the reconstructions it is possible to approximately localize the nucleosomes. In several regions they show a clear zigzag arrangement, but in other regions it is difficult to determine the sequence of the nucleosomes. In any case there is no clear indication of a solenoidal arrangement. We discuss the rules which may give rise to a 3-D arrangement of the nucleosomal zigzag.
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