Figure 1.

Epigenetic and genetic alterations of DNA. Epigenetic changes, including methylation and hydroxymethylation of cytosines and other nucleotides, chromatin condensation and opening, and the shortening and lengthening of telomeres, are reversible and thus provide a capacity to rapidly adapt to changes in the environment (lightning bolt). Genetic changes, including DNA substitutions, insertion/deletions (not shown), recombination, and viral integration/transposition, are primarily irreversible. For example, it is rare that a second point mutation exactly reverses a mutation or that a second recombination event occurs at precisely the same location as a previous recombination event. The magnitude of reversibility is shown by the length of the blue arrow.