every transcription unit in the chromosome and thus causing transcriptional chaos. This is achieved by use
of yet another control element called an insulator.^40 Insulators confine the effects of enhancers and
silencers to domains of effect (Figure 6.22). Insulators are believed to exert their effect by binding specific
proteins and by acting as a barrier to the migration of chromatin structure. When DNA is wrapped tightly
in nucleosomes (Section 6.4.2), then the genes contained within it are inaccessible to the transcription
machinery and cannot be expressed. Since a single chromosome contains many regions with differing
levels of chromatin condensation, there must be a mechanism for keeping these regions in the right places.
Insulators are believed to be an important component of this mechanism.
Genes and Genomes 229
Figure 6.20 Structure of a complex, upstream transcriptional control genethe even-skipped gene of Drosophila.
Coloured, shaded and hatched boxes indicate transcriptional control elements necessary for expression
in the cell types indicated. The scale is in kilobases upstream form the transcriptional start site (shown
by an arrow)
Figure 6.21 Transcriptional enhancers act at a distance by looping out intervening DNA
Figure 6.22 Transcriptional enhancers and insulators. Circled () indicates positive enhancer activity. X indicates
blocking of an enhancer activity by an intervening insulator