The second lecture was more mechanistic, focusing on modern notions of epigenetics. At the turn of the 1980s-1990s, the term underwent a renaissance as its meaning changed. This semantic evolution followed the realization that certain changes in gene expression are transmitted across cell divisions, and even generations in some cases, without the DNA sequence itself being changed. The link between this transmission of an expression state and a DNA modification known as methylation led Australian geneticist Robin Holliday to redefine epigenetics in 1994 as the study of changes in gene expression that can be transmitted across cell divisions, or even generations, without changing the DNA sequence. Shortly afterwards, the idea of epigenetic memory emerged, based not only on DNA methylation, but also on chemical modifications of histones and other associated proteins. Current research is focusing on "writer" proteins, which establish these chemical modifications of histones, and "reader" proteins, which bind to the modified histones, as well as on the mechanisms that allow these modifications to propagate during cell divisions.
Finally, the exploration of chromatin in its intimate variations has led to a new use of the term epigenetics. In 2007, Adrian Bird formalized this second semantic change since Waddington: epigenetics can be considered as the study of "structural adaptations of chromosomal regions that allow altered states of gene activity to be recorded, marked or perpetuated". This definition thus goes beyond Holliday's stricter concept of heritable change in gene function. I have endeavored to provide an up-to-date review of the molecular basis of epigenetic processes, highlighting the major open questions that form the basis of our current research.
