histone acetylation DNA methylation

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DNA methylation and histone deacetylation work together in gene silencing. DNA methylation involves the addition of a methyl group to the nucleotide base cytosine (pyrimidine ring). DNA methylation is a crucial part of normal organismal development and cellular differentiation in higher organisms. DNA methylation stably alters the gene expression pattern in cells such that cells can "remember where they have been"; in other words, cells programmed to be pancreatic islets during embryonic development remain pancreatic islets throughout the life of the organism without continuing signals telling them that they need to remain islets. DNA methylation also forms the basis of chromatin structure, which enables cells to form the myriad characteristics necessary for multicellular life from a single immutable sequence of DNA.

Histone Acetylation is when acetyl groups (-COCH2) are attached to the histone tails (particularly of lysine amino acids). Histone acetylation is associated with transcriptional activation because the affinity of acetylated histone protein for DNA is reduced and chromatin package is thus relaxed, so essentially it's promoting the genes to be "turned on".

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DNA methylation makes a chromosome more tightly packed because it makes the DNA more attracted to itself therefore limiting gene expression. Histone acetylation is when amino groups are added to histone proteins decreasing the attraction and making them unwind more. Genes in an area like this are more likely to be expressed. The mechanisms of cell specializing utilize these two types of increased or decreased expression.