Epigenetics is the study of stable genetic modification that results in changes in gene expression and function without a corresponding alteration in DNA sequence.
例如我们熟悉的细胞分化（differentiation），就是通过选择性的switch on/off certain gene. 甚至是对与很多疾病的诊断与治疗中，例如癌症，也可以通过研究各种基因的epigenetics，来去诊断具体的病理，究竟是哪一个不该表达的基因表达了还是那一个该表达的基因没有正确表达出来，这样就可以更加具有针对性去进行治疗，这也是未来精准化医疗personalized medicine的发展基础。
一、Introns and exons
In most genes in humans and other multicellular organisms, the protein-coding sequence is split into segments (exons) that are separated by non-coding sequence (introns).
When a gene is transcribed, the RNA polymerase traverses the entire sequence, exons and introns, to make the primary transcript. This is then processed, within the nucleus, by being physically cut at exon-intron boundaries; the exons are spliced together to make the mature mRNA, and the introns are discarded. The machinery that does this, the spliceosome, is exceedingly complicated,Alternative splicing is often tissue-specific, and the different splice isoforms may have clearly different functions.
In order to transcribe a gene, the RNA polymerase must attach to the DNA just upstream of the transcription start site. This region is called the promoter. Binding is determined by the DNA sequence, but also by sequence-specific binding of a whole set of other proteins that together constitute the transcription initiation complex.
Enhancers are promoter-like sequences that are located some way away from the gene they regulate. They can be upstream or downstream of the gene, and in some cases up to a million base pairs away. Like promoters, they bind a variety of proteins, many of them tissue-specific, and the DNA loops round to bring them into contact with the promoter
五、Methylation and acetylation
Inactive genes can be wrapped tightly and become inaccessible – ensuring they are not read – (epigenetic silencing)
Active genes are ‘looser’ and are more easily read and transcribed
They add methyl (-CH3) groups to DNA, specifically to the 5-position of cytosines that lie immediately upstream of guanines (so-called CpG dinucleotides, the p representing the phosphate joining adjacent nucleotides). 5-methyl cytosine base-pairs with guanine exactly the same as normal cytosine, but the methyl groups act as a signal to methyl DNA binding proteins, which in turn recruit other regulatory proteins.
An acetyl group (-COCH3) is added to one of the lysines in the histone structure.
Open up the structure and activates the chromatins.
A methyl group (-CH3) is added to a lysine. Depending on the position of lysine
Activation or inactivation