Quelle place pour l’épigénétique ?
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L’étude des modifications épigénétiques de lachromatine constitue aujourd’hui un domaine trèsactif de recherches. Mais l’intérêt pour l’épigénétiqueest plus large, ce dont ce cahier thématiques’efforce de rendre compte. Cette diversité ne permetpas une «définition » simple de l’épigénétique.L’étude historique montre comment l’épigénétiqueest toujours venue combler les insuffisances de lagénétique. La distinction entre génétique et épigénétiquetrouve aussi son sens par rapport à l’objectifde tout être vivant, qui est de se reproduire: à lagénétique, la reproduction de la structure primairedes composants macromoléculaires, à l’épigénétique,«le reste ».Today, epigenetics is a very fashionable field of research. Modification of DNA by methylation, and of chromatin by histone modification or substitution represents a major fraction of the studies; but this special issue shows that epigenetic studies are very diverse, and not limited to the study of chromatin. What is common behind these different uses of the word epigenetics ? A brief historical survey shows that epigenetics was invented twice, with different meanings: in the 1940s, by Conrad Waddington, as the study of the relations between the genotype and the phenotype; in the 1960s, as the global mechanisms of gene regulation involved in differentiation and development; what is common is that an approach distinct from genetics was in both cases considered as necessary because genetic models were incapable to address these problems. A good way to appreciate the relations between genetics and epigenetics is to realize that the main aim of organisms is to reproduce, and to consider the way organisms perform this task. Genetics is the precise means organisms have invented to reproduce the structure of their macromolecular components; the genome is also used to control the level and place of this reproduction. All the other means organisms have used to reproduce were more or less the result of tinkering, and constitute the field of epigenetics, with its diversity and richness.