|"The term epigenetics was coined by Conrad Waddington to describe âthe branch of biology which studies the casual interactions between genes and their products, which bring the phenotype into beingâ. Today the term broadly applies to changes in gene regulation and cellular phenotype without changes to the DNA sequence itself, as the phenotype of a cell is determined by its expression profile. Epigenetic marks drive much of this expression and provide diversity to this phenotype via chromatin alteration that affects gene transcription. An epigenome is the chromatin state found across the genome at a certain time point and cell type, and therefore thousands of epigenomes can exist for a single given genome. Even though there is no alteration in the DNA sequence itself, epigenetic marks, chromatin activity, and histone modifications are heritable during cell division, keeping these epigenetic marks intact and passed on to dividing cells. Some epigenetic modifications are stabilized and maintained throughout the life of an organism, while others change over time due to intrinsic or environmental factors.
An example of an epigenetic modification to DNA in mammals is the methylation of Cytosine to form 5-MethylCytosine at the C5 position in CpG dinucleotides. DNA methylation is thought occur primarily in CpG dinucleotides and is therefore correlated to the occurrence of this DNA motif, known as CpG islands. CpG islands are short stretches of DNA where the presence of the CpG sequence is higher than in other regions which are characterized by GC rich regions of the genome. These motifs are rarely larger than 5kbp and overlap with the promoter regions for 50% to 60% of human genes. Robert C. Barber, The Potential Role of Epigenetics in Alzheimerâs disease Etiology