Résumé
The regulation of gene expression occurs at multiple levels. The initial step consists of transcription, which converts DNA sequences into RNA products, which may subsequently be processed, transported, and/or translated into proteins. This chapter focuses on the role of enhancers as critical regulatory elements, together with transcription factors and structural regulatory factors as key organizers of the three-dimensional genomic regulatory landscape. Genome-wide studies have revealed that the number of transcriptional enhancers greatly exceeds the number of genes present in mammalian genomes. The enhancer landscape molecularly defines cell-type specificity and differences across tissues. During the course of embryonic development or cellular differentiation, the enhancer profiles show dramatic differences and dynamic re-organization. Increasing evidence indicates that chromatin loops are dynamic structures. Whereas an abundant literature details the dynamics of chromatin loops, several examples of stable structures have been reported.