Nucleosomal anatomy – where are the histones?

The recent surge of discoveries concerning the structural organization of nucleosomes, together with genetic evidence of highly specialized roles for the histones in gene regulation, have brought a renewed need for a detailed understanding of nucleosomal anatomy. Here we review recent structural advances leading to a new level of understanding of the nucleosome and chromatin fibre structure. We discuss the problems and challenges for existing models of chromatin structure and, in particular, consider how linker histones may bind within the nucleosome, together with the implications of their association for the structure of the chromatin fibre.