The physics of chromatin

TL;DR

This paper reviews the physical properties of chromatin, focusing on nucleosome structure and the 30nm fiber, highlighting how chromatin achieves high DNA compaction while remaining accessible for gene regulation.

Contribution

It synthesizes recent physical insights into chromatin's structure and mechanics, emphasizing the balance between DNA compaction and accessibility.

Findings

Analysis of nucleosome overcharging and DNA unwrapping transitions

Mechanical properties of the 30nm fiber under stretching

Effects of ionic conditions on chromatin structure

Abstract

Recent progress has been made in the understanding of the physical properties of chromatin -- the dense complex of DNA and histone proteins that occupies the nuclei of plant and animal cells. Here I will focus on the two lowest levels of the hierarchy of DNA folding into the chromatin complex: (i) the nucleosome, the chromatin repeating unit consisting of a globular aggregate of eight histone proteins with the DNA wrapped around: its overcharging, the DNA unwrapping transition, the ''sliding'' of the octamer along the DNA. (ii) The 30nm chromatin fiber, the necklace-like structure of nucleosomes connected via linker DNA: its geometry, its mechanical properties under stretching and its response to changing ionic conditions. I will stress that chromatin combines two seemingly contradictory features: (1) high compaction of DNA within the nuclear envelope and at the same time (2) accessibility to genes, promoter regions and gene regulatory sequences.