Euchromatin (also called " open chromatin") is a lightly packed form of chromatin (DNA, RNA, and protein) that is enriched in , 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 , 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 Cell nucleus, possibly as a mechanism to handle increasing genome size.
Structure
Euchromatin is composed of repeating subunits known as
, reminiscent of an unfolded set of beads on a string, that are approximately 11 nm in diameter.
At the core of these nucleosomes are a set of four
histone protein pairs: H3, H4, H2A, and H2B.
Each core histone protein possesses a 'tail' structure, which can vary in several ways; it is thought that these variations act as "master control switches" through different
methylation and
acetylation states, which determine the overall arrangement of the chromatin.
Approximately 147 base pairs of
DNA are wound around the histone octamers, or a little less than 2 turns of the helix.
Nucleosomes along the strand are linked together via the histone, H1,
and a short space of open
linker DNA, ranging from around 0–80 base pairs. The key distinction between the structure of euchromatin and
heterochromatin is that the nucleosomes in euchromatin are much more widely spaced, which allows for easier access of different protein complexes to the DNA strand and thus increased gene transcription.
Appearance
Euchromatin resembles a set of beads on a string at large magnifications.
From farther away, it can resemble a ball of tangled thread, such as in some
Micrograph visualizations.
In both optical and electron microscopic visualizations, euchromatin appears lighter in color than
heterochromatin - which is also present in the
Cell nucleus and appears darkly
- due to its less compact structure.
When visualizing
, such as in a
Karyotype,
Cytogenetics is used to stain the chromosomes. Cytogenetic banding allows us to see which parts of the chromosome are made up of euchromatin or heterochromatin in order to differentiate chromosomal subsections, irregularities or rearrangements.
One such example is
G banding, otherwise known as
where euchromatin appears lighter than heterochromatin.
+Appearance of Heterochromatin and Euchromatin Under Various Visualization Techniques |
| Euchromatin | Lighter | Darker | Lighter | Dull | Light |
| Heterochromatin | Darker | Lighter | Darker | Bright (Fluorescent) | Darker (Faint) |
Function
Transcription
Euchromatin participates in the active transcription of
DNA to
mRNA products. The unfolded structure allows gene regulatory proteins and
RNA polymerase complexes to bind to the DNA sequence, which can then initiate the transcription process.
While not all euchromatin is necessarily transcribed, as the euchromatin is divided into transcriptionally active and inactive domains,
euchromatin is still generally associated with active gene transcription. There is therefore a direct link to how actively productive a cell is and the amount of euchromatin that can be found in its nucleus.
It is thought that the cell uses transformation from euchromatin into heterochromatin as a method of controlling gene expression and DNA replication, since such processes behave differently on densely compacted chromatin. This is known as the 'accessibility hypothesis'.
Epigenetics
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
Ageing, especially in diseases known to resemble premature aging.
Research has shown epigenetic markers on histones for a number of additional diseases.
Regulation
Euchromatin is primarily regulated by post-translational modifications to its nucleosomes'
, conducted by many histone-modifying enzymes. These modifications occur on the histones'
N-terminus 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-terminus 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.
Another method of regulation that incorporates a negative charge, thereby favoring the "open" form, is ADP-ribosylation.
See also
-
Histone Modifying Enzymes
-
Constitutive Heterochromatin
Further reading
-
Heterochromatin formation involves changes in histone modifications over multiple cell generations –
-
Chromatin Velocity reveals epigenetic dynamics by single-cell profiling of heterochromatin and euchromatin –
-
Epigenetic inheritance and the missing heritability –
-
Histone epigenetic marks in heterochromatin and euchromatin of the Chagas' disease vector, Triatoma infestans –
