Definition
Epigenetics is a term in biology used today to refer to features such as chromatin and DNA modifications that are stable over rounds of cell division but do not involve changes in the underlying DNA sequence of the organism.
The word "epigenetic" can be used to describe any aspect other than DNA sequence that influences the development of an organism.
An epigenetic change alters the phenotype without changing the genotype.
Specific epigenetic processes include paramutation, bookmarking, imprinting, gene silencing, X chromosome inactivation, position effect, reprogramming, transvection, maternal effects, the progress of carcinogenesis, many effects of teratogens, regulation of histone modifications and heterochromatin, and technical limitations affecting parthenogenesis and cloning.
Mechanisms
Chemical processes used to regulate cromatin condensation :
- Methylation
- Phosphorylation
- Histone acetylation
Methylation and phosphorylation restrict access to genes by winding the DNA and histones thigter; acetylation does just the opposite.
Methylation is the epigenetic basis for imprinting:
- A DNA methyltransferase connects a methyl group (CH3-) to a cytosine in DNA
- Double helix shape modification
- Cromatin condensation
- No binding of accessory protein
- Gene activity regulation
Methylation can be maintained even if the DNA is copied.
External links:
Diet and the evolution of human amylase gene copy number variation.
Research uncovers considerable epigenetic variation between genetically-identical twins
16 August 2010
A longitudinal study of genetically identical (monozygotic) twins has shown that environmental influences play a role in mediating the function of genes. Chloe Wong, a PhD student at the Institute of Psychiatry (IoP) at King’s, found that so-called ‘epigenetic’ differences between monozygotic twins are already detectable in early childhood and can change dynamically over time. The science of epigenetics – the study of reversible changes in gene function that occur without a change in the DNA sequence – is transforming the nature-nurture debate. It has been speculated that dynamic epigenetic processes, operating at the interface between the genome (nature) and the environment (nurture), make an important contribution to physiological variability and the aetiology of complex diseases. At present, little empirical work has assessed the extent to which epigenetic changes occur over time, and the degree to which such changes are under the influence of heritable and environmental factors. The researchers addressed this by performing a systematic investigation of DNA methylation across three genes (SLC6A4/ SERT, DRD4 and MAOA) widely implicated in common psychiatric disorders using DNA collected at two ages (five and ten years), from a longitudinal study of monozygotic and non-identical (dizygotic) twins. DNA methylation is a naturally-occurring chemical process which modifies cytosine, one of the four bases of DNA. Once established, such modifications can be inherited through cell division, and play a crucial role in regulating cellular differentiation during normal development in higher organisms. Dr Jonathan Mill, who heads the Psychiatric Epigenetics team said: “Methylation is often associated with a more compact DNA structure, rendering the genome inaccessible, switching off gene expression or causing genes to be expressed at very low levels. This is important because even if you have two identical strands of DNA, DNA methylation will determine whether the genes are expressed or not.” Dr Mill added: “The findings from this study are important because they uncover considerable read more »»»
NIH Project: INTEGRATED GENETIC AND EPIGENETIC BIOMARKERS FOR MOLECULAR EPIDEMIOLOGY
Epigenetics provides a new generation of oncogenes and tumor-repressor genes, 2006
Genomic imprinting: parental influence on the genome, 2001
