What is Epigenetics?
Conrad Waddington, originally defined the term epigenetics as 'the interactions of genes with their environment that bring the phenotype into being'. Today, the term is used to describe the study of heritable changes in genome function that occur without a change in DNA sequence. This includes; the study of how patterns of gene expression are passed from one cell to its descendants, how gene expression changes during the differentiation of one cell type into another, and how environmental factors can change the way genes are expressed. There are far-reaching implications of epigenetic research for agriculture and for human biology and disease, including our understanding of stem cells, cancer and ageing.
Genomes Vs. Epigenomes
In the nucleus of eukaryotic cells, genomic DNA is highly folded and compacted with histone and non-histone proteins into a dynamic polymer called chromatin. Gene expression, chromosome segregation, DNA replication, repair, and recombination all act, not on DNA alone, but on this chromatin template. The discovery that enzymes can (re)organise chromatin into accessible and inaccessible configurations revealed epigenetic mechanisms that considerably extend the information potential of the genetic code. Thus, one genome can generate many 'epigenomes', as the fertilised egg progresses through development and translates its information into a multitude of cell fates.
Since the early discoveries of Gregor Mendel, Walther Flemming, Theodore Boveri and others, Europe has been a world leader in unravelling the principles of heredity through genetic and chromosome research. Studies of chromosomes and heredity now have added significance, following the complete sequencing of many eukaryotic genomes, including that of humans. These 'genome projects' have shown that although humans may have a much bigger genome than less complex organisms, we do not have that many more protein coding genes. For example, the 140 million base pair (Mb) long genome of the fruit fly contains ~17,000 genes. The human genome is 21x larger (3000Mb), but still contains only ~ 40,000 genes. This suggests that "We are more than just the sum of our genes", and that biological complexity depends less on gene number, and more on how those genes are used (expressed), which is largely due to epigenetic mechanisms.