Epigenetics of physical exercise is the study of epigenetic modifications to the cell genome resulting from physical exercise. Environmental factors, including physical exercise, have been shown to have a beneficial influence on epigenetic modifications. Generally, it has been shown that acute and long-term exercise has a significant effect on DNA methylation, an important aspect of epigenetic modifications. The broader field of epigenetics studies heritable alterations to genes that do not involve changing the DNA sequence itself. The next section briefly discusses two important mechanisms involved in epigenetic modifications. Epigenetic Modifications Epigenetic modifications such as DNA methylation and histone modifications alter DNA accessibility and change chromatin structure, thereby regulating patterns of gene expression. These modifications can be heritable, thus passing from parent to offspring. As mentioned previously, environmental factors can modulate epigenetic alterations. Factors such as diet, exposure to environmental toxins, and stress have all been shown to play a role in affecting epigenetic modifications, especially by influencing methylation patterns in the DNA. Physical exercise is one such factor that also has been shown to affect methylation and chromatin modifications. DNA Methylation DNA methylation occurs when a methyl group is covalently attached to the C5 position of a cytosine nucleotide by a DNA methyltransferase (DNMT) enzyme. These dinucleotide repeats where cytosine is followed by an adjunct guanine nucleotide are referred to as CpG sites, with the 'p' indicating a phosphodiester linkage. Parts of the genome that contain a high amount of CpG sites are named CpG islands, and these islands often overlap with many of the core promoter regions found in the human genome. There are also CpG sites that are not a part of CpG islands, and these are constitutively methylated to indicate to transcription machinery to not allow transcription to be initiated there. In contrast, CpG islands are the areas in the genome where CpG sites are usually unmethylated until DNMTs methylate them, resulting in affects on gene expression. The methylation of CpG islands typically results in the transcriptional silencing of a promoter, which can be done in two ways. Methylation can recruit histone deacetylases (HDAC), which work to form tightly condensed heterochromatin, which is transcriptionally inactive. Methylating a CpG island will also cause the methyl group to wedge itself into the DNA major groove, which effectively blocks binding sites for proteins that can activate transcription. Post-translational Histone Modifications Histones are proteins that help organize DNA. Eight histone proteins