Fig. 1. Schematic representation of the three levels in genome organization. (A) The sequence level. The black line is the DNA (fat black lines represent coding regions, thin line intergenic DNA), green bars are promoters and enhancers and blue boxes are boundary elements. (B) The chromatin level. Red, yellow and white boxes depict nucleosomes, blue boxes are boundary elements, green bars are recruitment sites for proteins that change the histone code, or otherwise induce a change in the structural and functional state of chromatin. From here the change in chromatin state may spread along the chromatin fiber, as indicated by the arrows. The red and the yellow chromatin domains are in a different state. (C) The nuclear level. The image shows part of a HeLa cell nucleus in which chromatin was labelled by integration of GFP-histone H2B (Verschure et al., 1999). Light areas (local high GFP-concentration) represent condensed chromatin, whereas black areas correspond to the interchromatin compartment. The red and the yellow chromatin domains depicted in B have been drawn in the image. The yellow domain is epigenetically silenced and is part of condensed chromatin. The red domain is permissive for transcription and loops out into the interchromatin space.

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