File:Steps in nucleosome assembly.svg

English: Nucleosome assembly begins with the binding of acetylated histone H3–H4 complexes to the assembly factor CAF-1. This complex is then recruited to the replication fork by an interaction with the sliding clamp, and a tetramer of histones H3 and H4 is loaded onto the nascent DNA to form half of a new nucleosome. Dimers of histones H2A and H2B are then loaded onto the DNA, probably with the assistance of other assembly factors such as NAP-1, to form the complete histone octamer. Chromatin-remodeling complexes contribute to nucleosome assembly and spacing. The number of histone H3 and H4 subunits that interact with CAF-1 is not clear. These subunits form highly stable tetramers in solution (unlike histones H2A and H2B, which form dimers) and it is therefore thought that CAF-1 loads a complete H3–H4 tetramer as shown here. There is some evidence, however, that CAF-1 can interact with an H3–H4 dimer, in which case the loading of a tetramer would require two CAF-1–histone complexes.^[1]