In addition to chromatin, which provides the functional environment for the DNA, the DNA molecule itself is chemically modified by the addition of methyl residues at the 5' position of the cytosine rings in the CG sequence, resulting in methylated cytosine.93,94 Cytosine methylation, while chemically a rather simple modification, has remarkable importance for gene activity, or expression. Methylation of DNA is common in early development, is associated with gene silencing, and is assumed to be the mechanism for events such as parental imprinting, where the allele derived from one parent is silent. Moreover, DNA methylation is maintained by carbon-carbon bonds and therefore highly stable. Unlike the more transient histone modifications that redefine chromatin structure, DNA methylation is a reasonable candidate mechanism for environmental programming of gene expression.
