Epigenetics

Epigenetics is the study of changes in gene activity that are not caused by changes in the DNA sequence. By "DNA sequence," we mean the sequence of nucleotide base pairs in an exon, which is the part of a gene that determines the sequence of amino acids in the coded protein. Epigenetics focuses on gene expression, the way genes bring about their phenotypic effects.

These changes in gene activity may persist for the remainder of the cell's life and may also last for many generations of cells, through cell divisions. However, there is no change in the underlying DNA sequence of the organism. Instead, non-hereditary factors cause the organism's genes to behave (express themselves) differently. Epigenetics operates through specific mechanisms, such as DNA methylation.

Epigenetic factors may last into adulthood.

Definitions

A general definition of epigenetics is "the study of... gene activity during the development of complex organisms." Thus, epigenetic can be used to describe anything other than DNA sequence that influences the development of an organism.

A stricter or narrower definition is "the study of mitotically and/or meiotically heritable changes in gene function that cannot be explained by changes in DNA sequence."

The term "epigenetics" has been used to describe processes that are not heritable. An example is histone modification. So some definitions do not require heritability. Adrian Bird defined epigenetics as "the structural adaptation of chromosomal regions so as to register, signal, or perpetuate altered activity states." This definition includes DNA repair or cell division phases and stable changes across cell generations. It excludes others such as prions unless they affect chromosome function.

The NIH Roadmap Epigenomics Project uses the definition: "...For purposes of this program, epigenetics refers to both heritable changes in gene activity and expression (in the progeny of cells or individuals) and also stable, long-term alterations in the transcriptional potential of a cell."

In 2008, a consensus definition of the epigenetic trait, "stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence," was made at a Cold Spring Harbor meeting.

A reliable and readable account is from The Guardian newspaper.

Examples

The best example of epigenetic changes in eukaryotes is the process of cell differentiation. During morphogenesis, generalised stem cells become the cell lines of the embryo which in turn become fully differentiated cells. In other words, a single fertilized egg cell – the zygote – divides and changes into all the many cell types: neurons, muscle cells, epithelium, blood vessels, etc.

As the embryo develops, some genes get switched on, while others are switched off or moderated. This process is called gene regulation. There are many molecules inside the cell nucleus which do the job of adjusting the genes' output.

DNA and histones make up what is called chromatin. Epigenetic modifications to the chromatin are copied during cell division. This produces a line of cells, all of which are alike. This is called a tissue.

Meiosis cancels epigenetic changes, and resets the genome to its baseline state, allowing the process to unfold in each new generation. There are some exceptions to this rule, but none of these exceptions involve changes to DNA base pair sequences.

This process is different from mutations of the DNA. Genetic mutations change the primary DNA sequence, and mutations can happen in any cell. However, only mutations in cells involved in reproduction can affect the offspring.

Epigenetics has important implications for understanding complex processes such as aging, development, and diseases like cancer. It also raises questions about the impact of environmental factors on gene expression and the potential for interventions to modify or reverse epigenetic changes.

Epigenetics Resources
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