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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. Therefore optimal 11 nanometer fiber preservation requires low salt maintaining integrity while preventing higher order coiling.

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 illustration of first level packing and histone octamer as repeating bead.

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

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