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The folding of eukaryotic DNA into the 30 nm fibre comprises the first level of transcriptionally dormant chromatin. Understanding its structure and the processes of its folding and unfolding is a prerequisite for under-standing the epigenetic regulation in cell differentiation. | The sequentiallity of nucleosomes in the 30 nm chromatin fibre Dontcho Z. Staynov1 and Yana G. Proykova2 1 Imperial college London NationalHeart and Lung Institute UK 2 Schoolof Earth and EnvironmentalSciences University of Portsmouth UK Keywords 30 nm fibre chromatin structure nucleosome Correspondence D. Z. Staynov ImperialCollege London NationalHeart and Lung Institute Guy Scadding Building Dovehouse Street London SW3 6lY UK Tel 44 207 6223644 E-mail d.staynov@imperial.ac.uk Received 29 March 2008 revised 20 May 2008 accepted 23 May 2008 doi 10.1111 j.1742-4658.2008.06522.x The folding of eukaryotic DNA into the 30 nm fibre comprises the first level of transcriptionally dormant chromatin. Understanding its structure and the processes of its folding and unfolding is a prerequisite for understanding the epigenetic regulation in cell differentiation. Although the shape of the fibre and its dimensions and mass per unit length have been described the path of the internucleosomal linker DNA and the sequential-lity of the nucleosomes in the fibre are poorly understood. In the present study we have chemically crosslinked adjacent nucleosomes along the helix of chicken erythrocyte oligonucleosome fibres digested the inter-nucleosomal linker DNA and then examined the digestion products by sucrose gradient sedimentation. We found that the digestion products contain considerable amounts of mononucleosomes but less dinucleosomes which suggests that there are end-discontinuities in the fibres. This can be explained by a nonsequential arrangement of the nucleosomes along the fibre helix. The DNA is packed on several levels as chromatin in the eukaryotic nucleus. The first level of packing the highly conserved nucleosome allows transcription after remodelling and or histone modifications replacements. The nucleosome core particles have been reconstituted and crystallized and their structure solved in detail at 1.9 A resolution 1-3 . The second level of packing is the .
