Functions for DNA methylation in vertebrates.

DNA methylation is ancestrally a mechanism for neutralizing potentially damaging DNA elements in the genome. The genomes of most multicellular organisms contain a small fraction of methylated DNA that contains the methylated elements, whereas the organism's own genes remain free of methylation. Vertebrates are exceptional among animals in that their genomes, including genes, are predominantly methylated. They retain the ability to inactivate viral DNA but have recruited the DNA methylation system for new functions. Widespread low-density methylation can contribute to lowering of the level of transcriptional "noise" from cryptic or inappropriate promoters. This may be the major advantage of DNA methylation in these organisms and may be sufficiently beneficial to offset the disadvantage of m5C mutability. The other novel feature of DNA methylation in vertebrates is the capacity to de novo methylate certain CpG islands, causing long-term strong repression. These evolutionary innovations may explain the high complexity of vertebrate organs and cell types.