The paper by J. T. Finch and A. Klug presents evidence for a solenoidal model of chromatin structure. Chromatin, prepared from rat liver nuclei using micrococcal nuclease and extracted in 0.2 mM EDTA, appears as filaments about 100 Å in diameter, which coil loosely. In the presence of 0.2 mM Mg++, these filaments condense into a supercoil or solenoidal structure with a pitch of about 110 Å, corresponding to the diameter of a nucleofilament. The authors propose that the x-ray reflections at 110 Å observed in chromatin originate from the spacing between turns of the solenoid rather than between nucleosomes along the nucleofilament. The solenoidal structure is independent of histone H1 for stabilization. Under certain conditions, isolated nucleosomes can also aggregate into a similar structure. The solenoidal structure is correlated with the "thread" of about 300 Å diameter observed by other workers in nuclei. The results support the idea that Kornberg's linear chain model represents only the first level of folding in chromatin, with nucleofilaments organized in a higher level of folding. The solenoidal structure is stable and can be observed in native chromatin, suggesting that it is a natural form of chromatin organization.The paper by J. T. Finch and A. Klug presents evidence for a solenoidal model of chromatin structure. Chromatin, prepared from rat liver nuclei using micrococcal nuclease and extracted in 0.2 mM EDTA, appears as filaments about 100 Å in diameter, which coil loosely. In the presence of 0.2 mM Mg++, these filaments condense into a supercoil or solenoidal structure with a pitch of about 110 Å, corresponding to the diameter of a nucleofilament. The authors propose that the x-ray reflections at 110 Å observed in chromatin originate from the spacing between turns of the solenoid rather than between nucleosomes along the nucleofilament. The solenoidal structure is independent of histone H1 for stabilization. Under certain conditions, isolated nucleosomes can also aggregate into a similar structure. The solenoidal structure is correlated with the "thread" of about 300 Å diameter observed by other workers in nuclei. The results support the idea that Kornberg's linear chain model represents only the first level of folding in chromatin, with nucleofilaments organized in a higher level of folding. The solenoidal structure is stable and can be observed in native chromatin, suggesting that it is a natural form of chromatin organization.