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Structure of eukaryotic chromosomes and chromatin part 4 molecular architecture of drosophila melanogaster chromatin



Structure of eukaryotic chromosomes and chromatin part 4 molecular architecture of drosophila melanogaster chromatin



Philosophical Transactions of the Royal Society of London B Biological Sciences 283(997): 407-410



The intact interphase genome of D. melanogaster has been isolated by sucrose gradient centrifugation after gentle lysis of tissue culture cells in 0.9 M NaCl-0.4% Nonidet P40. The non-viscous, folded DNA sediments as a single broad 5000 S peak in a complex with RNA (a fraction of the nuclear nascent RNA) and protein (all of the 4 intranucleosome histones: H2A, H2B, H3 and H4). The folded DNA is supercoiled and can be relaxed to more slowly sedimenting forms either by intercalating ethidium, or by nicking with DNase I. Incomplete DNase treatment gives partially relaxed complexes, indicating that each nick relaxes only a stretch of DNA (defined as a supercoiled DNA loop) without affecting the superhelical content of the rest of the genome. The concentration of superhelices in the Drosophila folded DNA is the same in Escherichia coli and SV40 closed circular DNA, namely about 1 negative turn every 200 base pairs in 0.15 M NaCl at 26.degree. C. The estimated average size of the supercoiled DNA loops, about 85,000 base pairs, equals the size of the larger Drosophila chromomeres. A model for the folded Drosophila genome is proposed which has the DNA constrained (by both protein-DNA and RNA-DNA interactions) into independent supercoiled loops containing an average 400 nucleosomes/loop. Each nucleosome is composed of a histone core with the DNA wound around it in a 360.degree. left handed toroidal supercoil; each nucleosome toroidal supercoil plus its relaxed internucleosome DNA contains an average 200 base pairs.

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