The physics of chromatin

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 it: its overcharging, the DNA unwrapping transition, the ‘sliding’ of the octamer along the DNA. (ii) The 30 nm chromatin fibre, the necklace-lik es tructure of nucleosomes connected via linker DNA: its geometry, its mechanical properties under stretching and its response to changing ioni cc onditions. 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. Contents