key concept 16.4 Epigenetic Changes Regulate Gene Expression

In the mid-twentieth century, the developmental biologist Conrad Hal Waddington coined the term “epigenetics” and defined it as “that branch of biology which studies the causal interactions between genes and their products which bring the phenotype into being.” Today epigenetics is defined more specifically, referring to the study of changes in gene expression that are prompted without changes in the DNA sequence.

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  • Methylation of cytosine bases in DNA can enhance the binding of repressor proteins to promoter regions, resulting in silencing of gene expression.

  • Acetylation and deacetylation of histone proteins alter the affinity of the histones for DNA, changing the accessibility of regions of the DNA to RNA polymerase.

  • Environmental factors can cause epigenetic changes.

  • Some heterochromatin such as the inactive X chromosome in female mammals results from extensive DNA methylation.

Epigenetic changes are reversible but sometimes are stable and heritable. You saw an example of this phenomenon in the opening story of this chapter, which described how genes inherited from mothers who had particularly stressful pregnancies were expressed at different levels than they were in children whose mothers had not suffered during pregnancy. In that example, stress—an environmental change—prompted a higher degree of DNA methylation in the promoter, reducing expression of a gene associated with behavior in teenagers. Here we examine more closely how epigenetic effects are caused either by DNA methylation or by alterations in chromosomal proteins.