EPIGENETIC CONTROL OF DEVELOPMENT

Epigenetics refers to hereditary modifications of gene expression whic h are not due to a change of the nucleotid sequence in DNA. In eukaryo tes, epigenetic processes are often accompanied by changes in chromati n configuration that prevent access to the genes of transcription fact ors present in the cell. Such changes are frequently associated with t he methylation of cytosines DNA. In mammals, chromosome X inactivation and parental genomic imprinting are two examples of epigenetic inacti vation involving only one of the two alleles in the diploid cell. Chan ges in chromatin configuration during development are accompanied by c hanges 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 th eir expression state during embryonic development. Two groups of prote ins (Polycomb and Trithorax) play an antagonistic role in maintaining chromatin states associated with the expression and the silencing of t hese genes. The study of HML genes in yeast constitutes a model system for the determination of DNA signals and proteins involved in gene si lencing, and the relationship between the establishement of silenced s tates and DNA replication. Different models aimed at explaining how si lenced chromatin states are maintained through DNA replication are bri efly presented.