Two Types Of Epigenetics
Genetics alone cannot explain all the biological processes as predicted and assumed earlier. Then epigenetics, a relatively young science comes to help genetics explain the biology. Epigenetics is any covalent modification of DNA, RNA and protein, resulting changes in their functions without modifying their sequences (Bird, 2002). Some time epigenetic modifications passed to future generations, but in other instances they change with environmental stimuli. Epigenetics can explain some of the exciting biological phenomena like formation of different types of tissue and organ from cell having same DNA sequence, phenotypic differences in identical twins, X chromosome inactivation, genomic imprinting, why mutation in same gene result in two types
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Methylation of cytosines within DNA is found in most eukaryotes (plants, animals and fungi) (Klose and Bird, 2006), but not in Caenorhabditis elegans and Saccharomyces cerevisae (Feng et al., 2010). CpG islands are DNA sequences of 200 – 3000 bp characterized by higher CG content (at least 50 %) (Illingworth and Bird, 2009). Almost half of CpG islands are associated with transcription start sites, TSS (Deaton and Bird, 2011) and many are linked to developmental regulators and housekeeping genes (Meissner, 2011). CpG island methylation is associated with X-chromosome inactivation in females (Reik and Lewis, 2005) and genomic imprinting (Kacem and Feil, 2009). Some studies reported that methylation positively correlates with active transcription of genes (Hellman and Chess, 2007). DNA methylation does not occur only in CpG islands, but also in CpG island shores, that are frequently associated with transcriptional repression (Irizarry et al., 2009). All these studies suggest that regulation of DNA methylation is very important for normal development. For example, knockout mutations of any of the three DNA methyl transferase genes Dnmt1, DNMT3a and DNMT3b in mice are deadly (Okano et al., 1999), demonstrating that irregular DNA methylation negatively affect normal development. Mutations in DNMT1 and CMT3 effect the normal development of Arabidopsis embryo (Xiao et al., 2006). Although DNA methylation occurs mainly in CpG sites, non-CG methylation, which is widely studied in plants, has recently been found in humans at CHH and CHG sites (H = A, C or T) (Lister et al., 2009).
Methylation of cytosines within DNA is found in most eukaryotes (plants, animals and fungi) (Klose and Bird, 2006), but not in Caenorhabditis elegans and Saccharomyces cerevisae (Feng et al., 2010). CpG islands are DNA sequences of 200 – 3000 bp characterized by higher CG content (at least 50 %) (Illingworth and Bird, 2009). Almost half of CpG islands are associated with transcription start sites, TSS (Deaton and Bird, 2011) and many are linked to developmental regulators and housekeeping genes (Meissner, 2011). CpG island methylation is associated with X-chromosome inactivation in females (Reik and Lewis, 2005) and genomic imprinting (Kacem and Feil, 2009). Some studies reported that methylation positively correlates with active transcription of genes (Hellman and Chess, 2007). DNA methylation does not occur only in CpG islands, but also in CpG island shores, that are frequently associated with transcriptional repression (Irizarry et al., 2009). All these studies suggest that regulation of DNA methylation is very important for normal development. For example, knockout mutations of any of the three DNA methyl transferase genes Dnmt1, DNMT3a and DNMT3b in mice are deadly (Okano et al., 1999), demonstrating that irregular DNA methylation negatively affect normal development. Mutations in DNMT1 and CMT3 effect the normal development of Arabidopsis embryo (Xiao et al., 2006). Although DNA methylation occurs mainly in CpG sites, non-CG methylation, which is widely studied in plants, has recently been found in humans at CHH and CHG sites (H = A, C or T) (Lister et al., 2009).