DNA local helical periodicity on the histone surface

Introduction the linking number paradox and DNA local helical periodicity on the histone surface 

In spite of these progresses, and perhaps because of them, some early basic problems have not received all the attention they deserved, even if their solutions are likely to fill crucial gaps in our understanding of chromatin function. Among them, the so-called linking number paradox [6,7], previously reviewed by one of the authors [8], has raised heated debate in the past [9-11]. As will be shown below, this paradox is at the heart of nucleosome conformational dynamics within the context of chromatin superstructure. 

This problem first emerged from the necessity to reconcile topological and structural data of nucleosomes and chromatin. As soon as a minichromosome could be reconstituted from pure DNA and histones, the total reduction of the DNA linking number (Lk) was found to be equal to the number of nucleosomes, which was also true for the native Hl-bearing SV40 minichromosome [12]. On the other hand, the first low-resolution crystal of the core particle showed that DNA was wrapped with 1 3/4 turns of a left-handed superhelix. Assuming linker DNAs 

The proposed solution to that paradox was contained in the classical equation [14-16]  

In conclusion, nucleosome cleavage periodicity data, together with direct measurements of a defined nucleosome crystal structure, suggest that wrapped DNA may be overtwisted to different extents, but in no case do these overtwistings appear sufficient to explain the paradox ( 0.5 bp/turn overtwisting required, against 0.2 bp/turn observed at the most). 

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