FIGURE 6.

Schematic of the proposed mechanisms of DNA methylation and demethylation involved in acute and chronic stress responses of birds. During initial or acute stress, DNA methylations are caused by the de novo DNA methyltransferases DNMT3A and DNMT3B. When DNA replication occurs in the acute stress state, these methyl marks are maintained by the DNMT1, which has preferences for hemi-methylated DNA. If DNMT1 is inhibited by chronic stress, the newly synthesized strand of DNA will not be methylated and results in passive demethylation for maintaining of methylation marks (methyl groups in DNA). In contrast to the passive demethylation, active demethylation in acute stress can erase and remove the DNA methylation marks (methyl groups in DNA) to maintain homeostasis of methylation status. Figure adapted from Wu and Zhang (2010). DNMT1, DNA methyltransferase 1; DMAP1, DNA methyltransferase 1 associated protein 1; DNMT3A, DNA methyltransferase 3 alpha; DNMT3B, DNA methyltransferase 3 beta. TET, Ten–eleven translocation.