AFM visualization of salt-induced chromatin fiber compaction

AFM visualization of salt-induced chromatin fiber compaction 

The structure of the condensed chromatin fiber is still under discussion [1,23,54], with two competing models the original solenoid model of Finch and Klug [16], and the straight-linker model [12,14,55]. Assessing the structure in vivo or in situ has proven impossible thus far, due to technical limitations. Chromatin fibers released from nuclei into solution by nuclease treatment have been widely used as models for fiber structure such fibers are extended at low ionic strength and condensed at ionic strengths believed to be close to those found in vivo ( 150 mM Na or 0.35 mM Mg ). The salt-induced fiber compaction has been extensively studied in the past but is still poorly understood in terms not only of the details of the structure but also in terms of the molecular mechanisms of the compaction process. 

Chromatin fiber assembly under applied force 

Finally, the data gave a clear indication of the reversibility of the assembly process. As illustrated in the magnified portion of the assembly curve in Fig. 7a (see inset), the path of the curve was not smooth but exhibited occasional upward stretches, indicating that in an overall assembly process there were occasional dissociation steps involving a few nucleosomes at a time. Thus, we believe that we see the first real-time demonstration of the dynamic equilibrium between an assembled and a disassembled state of individual nucleosomes in the fiber context such equilibrium has been previously suggested on the basis of 

Distance between external magnet and cuvette tmmt anct rorresnnnriinn fnrne rnNt 

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