Contrôle épigénétique du développement.

Abstract

Epigenetics refers to hereditary modifications of gene expression which are not due to a change of the nucleotid sequence in DNA. In eukaryotes, epigenetic processes are often accompanied by changes in chromatin configuration that prevent access to the genes of transcription factors present in the cell. Such changes are frequently associated with the methylation of cytosines in DNA. In mammals, chromosome X inactivation and parental genomic imprinting are two examples of epigenetic inactivation involving only one of the two alleles in the diploid cell. Changes in chromatin configuration during development are accompanied by changes in the nature and the acetylation state of histones, and other chromatin components. In Drosophila, the expression of homeotic genes is controlled by chromatin changes ensuring an epigenetic memory of their expression state during embryonic development. Two groups play an antagonistic role in maintaining chromatin states associated with the expression and the silencing of these genes. The study of HML genes in yeast constitutes a model system for the determination of DNA signals and proteins involved in gene silencing, and the relationship between the establishment of silenced states and DNA replication. Different models aimed at explaining how silenced chromatin states are maintained through DNA replication are briefly presented.