Finch and Klug propose a solenoidal model for chromatin superstructure, based on electron microscopy and x-ray diffraction studies. Chromatin prepared by brief digestion with micrococcal nuclease and extracted in 0.2 mM EDTA appears as filaments of about 100 Å diameter that coil loosely. In the presence of 0.2 mM Mg²⁺, these "nucleofilaments" condense into a supercoil or solenoidal structure with a pitch of about 110 Å, corresponding to the diameter of a nucleofilament. The x-ray reflections at 110 Å are attributed to the spacing between turns of the solenoid rather than between nucleosomes along the nucleofilament. Histone H1 is required for the stabilization of the solenoidal structure. Isolated nucleosomes can also aggregate into a similar structure. The solenoidal structure correlates with the "thread" of about 300 Å diameter observed in nuclei. The model suggests that chromatin consists of a flexible chain of repeating structural units ("nucleosomes") of about 100 Å diameter, each containing a stretch of DNA 200 base pairs long condensed around a protein core made of eight histones. Nucleosomes are closely packed along the chain to form a "nucleofilament." Low-angle x-ray scattering of dilute chromatin solutions supports this model. Electron micrographs show that chromatin in 0.2 mM EDTA appears as uniform filaments of 100 Å diameter, while in 0.1 mM Mg²⁺, they coil tightly into supercoils. The 110 Å x-ray spacing is interpreted as arising from the spacing between turns of the solenoid rather than between nucleosomes. X-ray diffraction patterns show maxima at 110, 55, 37, 27, and 22 Å, consistent with the solenoidal structure. The solenoidal model is supported by x-ray studies showing that the 110 Å reflection arises from the spacing between turns of the solenoid. The solenoidal structure is thought to be stabilized by histone H1 and is observed in chromatin fibers of about 300 Å diameter. The model suggests that the solenoidal structure represents a higher level of folding than the nucleosome. The solenoidal structure is consistent with the "thread" observed in interphase chromosomes and is thought to have a diameter of about 300 Å. The model also suggests that the solenoidal structure may accommodate histone H1 and other proteins in its central hole. The packing ratio of DNA along the length of a densely packed nucleofilament is about 7:1, and when the nucleofilament is wound into a solenoid, the ratio increases to about 40:1. The model is supported byFinch and Klug propose a solenoidal model for chromatin superstructure, based on electron microscopy and x-ray diffraction studies. Chromatin prepared by brief digestion with micrococcal nuclease and extracted in 0.2 mM EDTA appears as filaments of about 100 Å diameter that coil loosely. In the presence of 0.2 mM Mg²⁺, these "nucleofilaments" condense into a supercoil or solenoidal structure with a pitch of about 110 Å, corresponding to the diameter of a nucleofilament. The x-ray reflections at 110 Å are attributed to the spacing between turns of the solenoid rather than between nucleosomes along the nucleofilament. Histone H1 is required for the stabilization of the solenoidal structure. Isolated nucleosomes can also aggregate into a similar structure. The solenoidal structure correlates with the "thread" of about 300 Å diameter observed in nuclei. The model suggests that chromatin consists of a flexible chain of repeating structural units ("nucleosomes") of about 100 Å diameter, each containing a stretch of DNA 200 base pairs long condensed around a protein core made of eight histones. Nucleosomes are closely packed along the chain to form a "nucleofilament." Low-angle x-ray scattering of dilute chromatin solutions supports this model. Electron micrographs show that chromatin in 0.2 mM EDTA appears as uniform filaments of 100 Å diameter, while in 0.1 mM Mg²⁺, they coil tightly into supercoils. The 110 Å x-ray spacing is interpreted as arising from the spacing between turns of the solenoid rather than between nucleosomes. X-ray diffraction patterns show maxima at 110, 55, 37, 27, and 22 Å, consistent with the solenoidal structure. The solenoidal model is supported by x-ray studies showing that the 110 Å reflection arises from the spacing between turns of the solenoid. The solenoidal structure is thought to be stabilized by histone H1 and is observed in chromatin fibers of about 300 Å diameter. The model suggests that the solenoidal structure represents a higher level of folding than the nucleosome. The solenoidal structure is consistent with the "thread" observed in interphase chromosomes and is thought to have a diameter of about 300 Å. The model also suggests that the solenoidal structure may accommodate histone H1 and other proteins in its central hole. The packing ratio of DNA along the length of a densely packed nucleofilament is about 7:1, and when the nucleofilament is wound into a solenoid, the ratio increases to about 40:1. The model is supported by