Epigenetics

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Epigenetics is the study of changes to the genome and the heritability of said alterations. Gene expression within an organism can be altered by changes to the genome, without the DNA sequence itself being changed in respond towards environment. This is mainly caused by the addition of specific chemical groups on to the genome, which can promote or inhibit the transcription of genes in to polypeptides. It is also due to positive feedback loops of gene regulatory proteins or to heritable modifications in chromatin [1]. The genome alterations are considered to be heritable, therefore the changes in gene expression of an organism may affect their offspring in subsequent generations[2]. In other words, parents' past experience could be passed on to next generation which will make them have better characteristics than their parents. 

The epigenome is a structure that consists of a histone octameron which tightly wrapped DNA is coiled. Change to the epigenome reults in conformational changes in the geometry of the genome. This “highlights” specific parts of the DNA sequence thus affecting transcription and, therefore, gene expression. An example of epigenetic change is DNA methylation, the most common epigenetic mark, which is the addition of a methyl group to a part in a DNA molecule. This prevents specific genomes from being expressed. Different cells have different active genes despite having similar DNA sequences. For example, brain cells and liver cells contain the same DNA sequence yet the expressed active genes are different.

Formerly, it was assumed that proper cellular development and distinction to occur in mammals, the epigenome was fully erased through 'reprogramming' process which happen twice, one during gamete formation and once during conception [3]. It is reconstructed between generations. The methylation marks are supposed to be converted to hydroxymethylation which will diluted as cell divide [4]. Now, however, incomplete removal of epigenomes in specific genes have been found, suggesting that certain gene profiles are inherited epigenetically[5]. Heritability of epigenetic modification has been observed in studies with mice. One such study revealed the transgenerational inheritance of the yellow coat colour phenotype in mice, resulting from the incomplete erasure of methylation marks when a silenced Avy allele is passed down through the female germline[6].

References

  1. Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts and Peter Walter. (2008) Molecular Biology of The Cell, Fifth Edition, New York: Garland Science.
  2. Carey N. The epigenetics revolution. United Kingdom: Icon Books Ltd; 2012.
  3. Alan Horsager(2014) Episona. Available at: https://www.episona.com/3-examples-transgenerational-epigenetic-inheritance/ (last accessed 18 Nov 2015)
  4. University of Cambridge (25 Jan 2013) Available at: http://www.cam.ac.uk/research/news/scientists-discover-how-epigenetic-information-could-be-inherited (last accessed 18 Nov 2015)
  5. Utahedu. 3. Utahedu. [Online]. Available from: http://learn.genetics.utah.edu/content/epigenetics/ [Accessed 20 November 2015].
  6. Morgan H, Sutherland H et al. Epigenetic inheritance at the agouti locus in the mouse. Nature Genetics 1999; 23(3)
  
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