The Importance Of Epigenetics

INTRODUCTION

Epigenetics is the study of the ways in which changes alter cell and tissue-specific patterns of gene expression. C.H. Waddington coined the term epigenetics in the 1940s to describe how environmental influences on developmental events can affect the phenotype of the adult. He showed that environmental alterations during development induced alternative phenotypes in organisms with identical genotypes. Epigenetic regulation of gene expression uses reversible modifications of DNA and chromatin structure to mediate the interaction of the genome with a variety of environmental factors and to generate changes in the patterns of gene expression in response to these factors. The epigenome refers to the epigenetic state of a cell. During

DNA methylation also occurs during the differentiation of adult cells. In both instances, methylation takes place almost exclusively on cytosine bases adjacent to a guanine, a combination called a CpG dinucleotide. Many of these dinucleotides are clustered in regions, called CpG islands, located in and near promoter sequences adjacent to genes. Islands adjacent to essential genes (housekeeping genes) and cell-specific

Genes are unmethylated, making these genes available for transcription. Other genes with adjacent methylated CpG islands are transcriptionally silenced. The methyl groups in CpG islands occupy the major groove of DNA, and block the binding of transcription factors necessary to form transcription complexes. CpG islands are usually located upstream of promoter regions The bulk of methylated CpG dinucleotides are not adjacent to genes, and are found in repetitive DNA sequences located in heterochromatic regions of the genome, including the centromere. Methylation of these sequences contributes to silencing the transcription and replication of transposable elements such as LINE (long interspersed nuclear element) and SINE (short interspersed nuclear elements) sequences, which form a major part of the human genome. Heterochromatic methylation also maintains chromosome stability by preventing translocation and other chromosomal abnormalities. (Klug, 2012) (Simmons,