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Chromatin contains DNA, basic proteins called histones, non-histone proteins and RNA. During cell division, the chromatin material becomes organised into distinct chromosomes.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cytoskeleton Cilia Flagella Centrosome Nucleus and Chromosomes
Electron microscopy, X-ray scattering and analytical ultracentrifugation support solenoid model where 30 nanometer fiber forms helix of nucleosomes approximated six per turn. Pitch about 11 nanometers corresponding to diameter of nucleosome disc itself. Six nucleosomes pack edge to edge per helical gyre producing packing ratio near forty-fold relative to naked DNA. Alternative two-start zigzag model proposes tetranucleosome repeating unit but still estimates similar density about six per 11 nanometer turn. Higher values eight to twelve would increase fiber diameter beyond 30 nanometer not observed. Therefore textbooks universally cite six nucleosomes per turn defining geometry.
Ref: Finch and Klug 1976 PNAS Model; Widom and Klug, 30 nm Fiber Six Nucleosomes per Turn
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