Methylation and demethylation of the Arabidopsis genome.

The primary sequence of the genome is broadly constant and superimposed upon that constancy is the postreplicative modification of a small number of cytosine residues to 5-methylcytosine. The pattern of methylation is non-random; some sequence contexts are frequently methylated and some rarely methylated and some regions of the genome are highly methylated and some rarely methylated. Once established, methylation is not static: it can potentially change in response to developmental or environmental cues and this may result in correlated changes in gene expression. Changes can occur passively owing to a failure to maintain DNA methylation through rounds of DNA replication, or actively, through the action of enzymes with DNA glycosylase activity. Recent advances in genetic analyses and the generation of high resolution, genome-wide methylation maps are revealing in unprecedented detail the patterns and dynamic changes of DNA methylation in plants.