1. Histone modifications can affect transcription by altering chromatin structure or interaction with other regulatory proteins. The addition of positive or negative charges via modifications disrupts the electrostatic interaction between histones and DNA, which modulates chromatin structure and thus the accessibility of DNA to regulatory proteins. DNA methylation is an epigenetic modification that can influence the interaction between transcription factors and CpG dinucleotides, chromatin structure, or the binding between methylated DNA and methylation recognition factors. The different types of histone modifications are: Acetylation is mediated by histone acetyltransferases (HATs) which act as coactivators for transcription by interacting with DNA binding activators. Acetylation catalyzes the transfer of an acetyl group to the ε-amino group of the lysine residue with the use of acetyl CoA as a cofactor. Example: H3K56 acetylation by p300/CBP. HDACs have opposite effects to HATs and remove the acetyl group from lysine residues. Phosphorylation is mediated by kinases through the transfer of the phosphate group to the hydroxyl group of the histone amino acid side chain of ATP and may play an activating or repressive role. This causes negative charges to be added to histones, thereby affecting chromatin structure. Example: Phosphorylation of H3 on Ser 10 is associated with transcriptional activation. Phosphorylation of H2A by MSK1 represses transcriptional activation. Phosphatases reverse the effect of kinases by removing the phosphate group. Histone methylation by methyltransferases occurs at lysine or arginine residues, which can undergo mono-, di...... middle of paper ......level of transcription, protein synthesis or the genome. The effects of RNAi may be contrary to chromatin and DNA modifications in mediating mRNA degradation, translation inhibition, DNA elimination and the like in the formation of heterochromatin. RNAi is post-transcriptional, histone modification can be transcriptional and post-translational modification and DNA modification are at the transcriptional level. RNAi can be reversible or irreversible, while histone and DNA modifications are reversible processes. RNAi involves complementary base pairing with the target RNA to cause repression, while DNA and chromatin modification requires bromodomains, chromodomains, specific amino acid residues, and chemical groups for protein interaction. -DNA and-histone. RNAi, DNA and chromatin modification are involved in heterochromatin formation, gene regulation and genome stability..