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#11 nm fiber

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Nucleosome Packing

11 nm fiber is best observed when chromatin is isolated in:

Isolation ionic conditions profoundly affect visualization of chromatin hierarchical levels. In low salt buffer like 1 millimolar EDTA or 10 millimolar Tris-HCl, electrostatic repulsion between linker DNA segments prevents folding and H1-mediated compaction weak, leaving extended 11 nanometer beads-on-string fibers easily observed by electron microscopy with clear spacing. Adding NaCl to 100 millimolar or Mg2+ to 2 millimolar plus stoichiometric H1 induces coiling into 30 nanometer dense fiber. High salt beyond 0.6 molar strips histones, detergent solubilizes membranes, heat denatures proteins

Ref: Olins and Olins 1974 J Cell Biol EM; Alberts et al., Chapter 4: 11 nm Fiber Observed in Low Salt Buffer

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Nucleosome Packing

The 11 nm fiber is also called:

Electron microscopy of chromatin isolated in low ionic strength buffer reveals necklace-like organization: 11 nanometer particles spaced by thin linker DNA resembling beads on string. This configuration corresponds to extended primary chromatin fiber without H1-mediated coiling, exposing nucleosomes individually. Solenoid model describes 30 nanometer helix with six nucleosomes per turn, zig-zag describes two-start helix of 30 nanometer fiber, loop fiber denotes scaffold-anchored 300 nanometer domain. Beads-on-string visualization requires low salt to prevent folding, providing classic illustra

Ref: Olins and Olins 1974 J Cell Biol; Alberts et al., Chapter 4: 11 nm Fiber Called Beads-on-String

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