Amphibian lampbrush chromosomes and meiotic prophase chromosomes of various insects and plants consist of a bundle of microfibrils about 500 A thick. These fibrils are double, being made of two closely associated fibrils 200 A thick. Fragments of interphase nuclei contain a mass of fibrils 200 A thick. Ultrathin sections through nuclei in prophase or interphase show sections of these double or single fibrils cut at various angles. A comparison of sections with the methacrylate left in and sections that were shadowed after removing the methacrylate suggests that the OsO4 reacts only with the outer part of the fibrils either because it does not penetrate, or as a result of a chemical difference of the inner core and the outside of the fibril. It is suggested that in analogy to the structure of the tobacco mosaic virus the chromosomal microfibril may have an inner core of DNA surrounded by a shell of protein.

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