Superhelicity of nucleosomal DNA changes its double-helical repeat.
Winding DNA in a superhelix can be considered a process consisting of two smooth deformations: bending and twisting. The extra twist angle introduced by winding DNA into the nucleosomal superhelix is calculated by means of the Crick formula to be -0.5 degrees per base pair (bp). This is equivalent to a change of -0.15 +/- 0.015 bp in the DNA double-helical repeat. Free DNA in solution is known to have a helical repeat of 10.55 +/- 0.1 bp. On the other hand, a weighted average of various estimates of the DNA repeat in the nucleosome is 10.38 +/- 0.02. The difference happens to be perfectly accounted for by the superhelicity of the nucleosomal DNA. This implies that the latter is essentially nonconstrained .
D001482:Base Composition; D004278:DNA, Superhelical / Q000235:genetics*; D008433:Mathematics; D008957:Models, Genetic; D009690:Nucleic Acid Conformation; D009707:Nucleosomes / Q000648:ultrastructure*; D012091:Repetitive Sequences, Nucleic Acid
D004278:DNA, Superhelical; D009707:Nucleosomes