Fig. 4. Model for the organization of the chromatin fibre in a HeLa nucleus. DNA is wound into a nucleosome, and then a zig-zagging string of nucleosomes is tied to a factory through a cluster of transcription factors (diamond) or an active polymerase (oval). Components of the factory exchange with the soluble pool, and attachments to the factory are made and broken as factors dissociate and transcription terminates. 10-20 loops (only three are shown) of 5-200 kbp form a cloud around the transcription factory; long, static, loops are likely to become heterochromatic and attached to the lamina. 50-100 clouds then form a chromosome territory. Modified, with permission, from Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc. (Cook, 2001).