title: Structure of chromatin and the linking number of DNA.
abstract: Recent observations suggest that the basic supranucleosomal structure of chromatin is a zigzag helical ribbon with a repeat unit made of two nucleosomes connected by a relaxed spacer DNA. A remarkable feature of one particular ribbon is that it solves the apparent paradox between the number of DNA turns per nucleosome and the total linking number of a nucleosome-containing closed circular DNA molecule. We show here that the repeat unit of the proposed structure, which contains two nucleosomes with -1 3/4 DNA turns per nucleosome and one spacer crossover per repeat, contributes -2 to the linking number of closed circular DNA. Space-filling models show that the cylindrical 250-A chromatin fiber can be generated by twisting the ribbon.
mesh_terms: D000818:Animals; D002642:Chick Embryo; D002843:Chromatin / Q000648:ultrastructure*; D004247:DNA / Q000032:analysis*; D004270:DNA, Circular; D004912:Erythrocytes / Q000648:ultrastructure; D006657:Histones; D008433:Mathematics; D008854:Microscopy, Electron; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D009707:Nucleosomes / Q000648:ultrastructure
chemical_list: D002843:Chromatin; D004270:DNA, Circular; D006657:Histones; D009707:Nucleosomes; D004247:DNA