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Lightly packed form of chromatin that is enriched in genes

Euchromatin (also called "open chromatin") is a lightly packed form of chromatin (DNA, RNA, and protein) that is enriched in genes, and is often (but not always) under active transcription. Euchromatin stands in contrast to heterochromatin, which is tightly packed and less accessible for transcription. 92% of the human genome is euchromatic. In eukaryotes, euchromatin comprises the most active portion of the genome within the cell nucleus. In prokaryotes, euchromatin is the only form of chromatin present; this indicates that the heterochromatin structure evolved later along with the nucleus, possibly as a mechanism to handle increasing genome size.

Epigenetics involves changes in the phenotype that can be inherited without changing the DNA sequence. This can occur through many types of environmental interactions. Regarding euchromatin, post translational modifications of the histones can alter the structure of chromatin, resulting in altered gene expression without changing the DNA. Additionally, a loss of heterochromatin and increase in euchromatin has been shown to correlate with an accelerated aging process, especially in diseases known to resemble premature aging. Research has shown epigenetic markers on histones for a number of additional diseases.

Euchromatin is primarily regulated by post translational modifications to its nucleosomes' histones, conducted by many histone modifying enzymes. These modifications occur on the histones' N terminal tails that protrude from the nucleosome structure, and are thought of to recruit enzymes to either keep the chromatin in its open form, as euchromatin, or in its closed form, as heterochromatin. Histone acetylation, for instance, is typically associated with euchromatin structure, whereas histone methylation promotes heterochromatin remodeling. Acetylation makes the histone group more negatively charged, which in turn disrupts its interactions with the DNA strand, essentially "opening" the strand for easier access. Acetylation can occur on multiple lysine residues of a histone's N terminal tail and in different histones of the same nucleosome, which is thought to further increase DNA accessibility for transcription factors. Phosphorylation of histones is another method by which euchromatin is regulated. This tends to occur on the N terminal tails of the histones, however some sites are present in the core. Phosphorylation is controlled by kinases and phosphatases, which add and remove the phosphate groups respectively. This can occur at serine, threonine, or tyrosine residues present in euchromatin. Since the phosphate groups added to the structure will incorporate a negative charge, it will promote the more relaxed "open" form, similar to acetylation. In regards to functionality, histone phosphorylation is involved with gene expression, DNA damage repair, and chromatin remodeling. Another method of regulation that incorporates a negative charge, thereby favoring the "open" form, is ADP ribosylation. This process adds one or more ADP ribose units to the histone, and is involved in the DNA damage response pathway.