Epigenetics is the 'study of heritable changes in gene function that occur without a change in the sequence of the DNA' (MedicineNet. Inc, 2009) This can occur by methylation, the addition of a methyl group to the molecule. As epigenetics has come to the forefront in recent years scientist have found that DNA is not only composed of the commonly known A, T, C and G but also of two additional nucleotides known as 5-methylcytosine (5-mC) and 5-hydroxymethylcytosine which can replace cytosine.
These additional nucleotides create another means for gene regulation. A methyl group is bound to cytosine using DNA methyltransferase which then creates 5-mc. This new stable nucleotide modifies the promoter region of genes and results in gene silencing.
Where DNA methylation occurs is thought to help account for the wide differentiation of stem cells we have which make up our organs, tissue and systems of the body. However it has also been shown in studies conducted on rats that environmental factors play a crucial part in DNA methylation.
5-hydroxymethylcytosine was discovered by accident when Kriaucionis was comparing 5-mc in two different neurons from a mouse. He came across substantial amounts of the unexpected nucleotide which he labelled 'x' which he then proceeded to conduct tests on and renamed it 5-hydroxymethylcytosine. It is not yet exactly known what this nucleotide does though it is thought to play a role in demethylating DNA and also significantly affecting gene expression.
The main reason why these nucleotides have remained hidden for so long is due to the method of studying such things. Typically research has been conducted using a procedure known as bisulfite which can't distinguish between 5-mc and 5-hydroxymethylcytosine. The new discovery may force investigators to revisit earlier works to ensure that 5-mc and 5-hydroxymethylcytosine are appropriately incorporated.
source:
http://www.sciencedaily.com/releases/2009/04/090416144639.htm
