In cells genes are turned off or on according to the requirements and type of cell. Clearly an intestinal cell will behave differently to a nerve cell yet arising from the same germinal cell line. Yet the genes are the same.
The silencing of genes can be controlled by the Polycomb group of proteins. These alter the structure of chromatin the DNA – histone complex. Such silencing may be thorough the addition of specific small molecular weight molecules to the histone. This can be reversible.
The manner in which genes are arrayed within the cell determines transcription. The suppressing tags include acetyl, methyl and phosphate groups added to specific amino acids in histones. Methylation is the most stable addition and be retained through many life cycles. There are histone demethylase enzymes which remove the methyl additions to the histone.
A newly described demethylase enzyme acts with a retinoic acid receptor to prevent expression of retinoic acid responsive genes. Retinoic acid can signal to the Polycomb group proteins to remove histone tags in nerve cell releasing more specialised stem cells. Retinoic acid also has a role in bone marrow and macrophage gene unlocking .
Jones (2007) Reversing the irreversible Nature vol 450 pp 357-9

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