In higher eukaryotes, ‘replication factories’ coordinate DNA synthesis within local clusters of chromatin domains. Recent experiments(1,2) have confirmed the complexity of these clusters and established that the organization of sites labelled during S phase persists throughout the cell cycle. This implies that domain clusters are critical elements of an hierarchy that is fundamental to both nuclear and chromosome structure.

It may be that eukaryotic nuclei contain a collection of operationally independent units (genes), each controlled through its interactions with soluble protein factors which diffuse at random throughout the nucleoplasmic space. Alternatively, nuclei might be organized in such a sophisticated fashion that specific genes, occupy distinct sites and that spatially ordered RNA synthesis, processing and transport delivers mature RNAs to predestined sites in the cytoplasm.Different fields of research support each of these extreme views. Molecular biologists inspecting the precise details of (...) specific interactions, usually in vitro, inevitably favour the former, while cell biologists working with far more complicated systems generally assume that more elaborate arrangements exist. In considering the importance of nuclear architecture, I have attempted to relate a collection of experiments each of which intimates some close relationship between structural aspects of chromatin organization and the precise mechanisms underlying nuclear function. I will argue that higherorder structures are crucial for achieving the observed efficiency and coordination of many nuclear processes. (shrink)