a, Cryo-EM structures of a canonical nucleosome (PDB 6ESF24) and a partially unwrapped nucleosome (PDB 6ESH24). Rate constants of spontaneous unwrapping and rewrapping, determined by stopped-flow spectroscopy, are indicated. b, Locations of PTMs that have been studied for their effects on nucleosome unwrapping: at the DNA entry–exit site, at SHL ±3 (approximately 35 bp into the nucleosome) and at the dyad (ac, acetyl; ph, phosphoryl; me2a, asymmetrical dimethyl). The effects of these PTMs on nucleosome dynamics are indicated. From PDB 1KX5 (ref. 78).