Fig. 1.

Cryo-EM images of chromatin in chicken erythrocytes. (A) A 2D overview image of an approximately 50-nm-thick cryosection of a chicken erythrocyte nucleus. Cross-sections of fibers (top views) have higher contrast compared to the longitudinally sectioned fibers and are thus much more obvious, but all possible orientations are visible. Some longitudinally sectioned fibers are highlighted by red boxes; some cross-sectioned fibers are indicated by red circles. The box length is 200 nm, and the width is 50 nm. Inset: A 1D rotationally averaged power spectrum of the nucleus shows a strong peak in the 30–40-nm range. (B) A 10-nm-thick slice through a 3D reconstruction of a chicken erythrocyte nucleus. The nucleus contains compact yet distinct and well-separated 30-nm fibers in which individual nucleosomes are visible. A longitudinal and a cross-sectioned fiber are indicated by a red box and a red circle, respectively. (C and D) Cross-sections of individual fibers; the slices are 1.2 nm thick. (E and F) Longitudinal sections of individual fibers; the slices are 1.2 nm thick. The size and shape of the high-density positions correspond well to individual nucleosomes.