Epigenetic refers to heritable changes in gene expression not attributable to nucleotide sequence variation. Epigenetic mechanisms include DNA methylation, genome imprinting and dosage compensation.
DNA methylation is essential for development of mammals. DNA cytosine methyltransferase catalyses the addition of methyl groups to the 5-carbon position of cytosine ring in DNA, which alters the chromatin structures. These epigenetic "markers" on DNA can be copied after DNA synthesis, resulting in heritable changes in chromatin structure. Methylation of CpG-rich promoters is used by mammals to prevent transcriptional initiation and to ensure the silencing of genes on the inactive X chromosome, imprinted genes, and parasitic DNAs. Methylation has significant role in tissue-specific gene expression. There is also tantalizing evidence that normal chromosome structure may be affected by methylation and that human diseases, including cancer, are caused and impacted by abnormal methylation.
Chromatin structure plays an important role in the expression of genes. Less condensed euchromatic regions are the most accessible for transcription, where as highly condensed heterochromatic regions are refractory to transcription. Thus, the same gene can be either well expressed or transcriptionally silent depending on whether it lies in euchromatic or heterochromatic region resulting in epigenetic variations.
Allopolyploid speciation is wide spread in plants, the molecular requirements for successful orchestration of coordinated gene expression of two divergent and reunited genomes in single nucleus have significant role in evolution of novel allopolyploids. It is likely that the evolutionary success of allopolyploidy is in part attributable to epigenetic phenomena.
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