Replication and transcription: shaping the landscape of the genome.

As the relationship between nuclear structure and function begins to unfold, a picture is emerging of a dynamic landscape that is centred on the two main processes that execute the regulated use and propagation of the genome. Rather than being subservient enzymatic activities, the replication and transcriptional machineries provide potent forces that organize the genome in three-dimensional nuclear space. Their activities provide opportunities for epigenetic changes that are required for differentiation and development. In addition, they impose physical constraints on the genome that might help to shape its evolution.