Early studies of the developing embryo raised the question of how a fertilized egg could give rise to a complex multicellular organism containing many different kinds of cells. The term epigenetics originally referred to the study of these processes. With the advent of detailed knowledge of mechanisms of gene expression, this definition was superseded by another: epigenetics concerned the transmission of phenotype through mitosis or the germ line by mechanisms that did not involve changes in the DNA sequence. Much effort has been spent in attempting to identify and characterize these events. Work initially focused on DNA methylation as an epigenetic mark, but more recently there has been an emphasis on histone modifications as possible carriers of epigenetic information. However, there is confusion between situations in which the modifications may be propagated through cell division, thus helping to maintain a pattern of gene expression, and situations in which the modifications are simply part of the transcriptional apparatus. Arguments about the role of the histones have led to a reexamination of the definition of epigenetics and the primary events in development leading to cell type specific gene expression patterns.


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pp. 132-148
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