Euchromatin

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Distinction between Euchromatin and Heterochromatin

Euchromatin (also called "open chromatin") is a lightly packed form of

transcription. Euchromatin stands in contrast to heterochromatin, which is tightly packed and less accessible for transcription. 92% of the human genome is euchromatic.[1]

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.

Structure

Euchromatin is composed of repeating subunits known as nucleosomes, reminiscent of an unfolded set of beads on a string, that are approximately 11 nm in diameter.[2] At the core of these nucleosomes are a set of four histone protein pairs: H3, H4, H2A, and H2B.[2] 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.[2] Approximately 147 base pairs of DNA are wound around the histone octamers, or a little less than 2 turns of the helix.[3] Nucleosomes along the strand are linked together via the histone, H1,[4] 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.[2]

Appearance

Microscopy of heterochromatic versus euchromatic nuclei (H&E stain).

Euchromatin resembles a set of beads on a string at large magnifications.[2] From farther away, it can resemble a ball of tangled thread, such as in some electron microscope visualizations.[5] In both optical and electron microscopic visualizations, euchromatin appears lighter in color than heterochromatin - which is also present in the nucleus and appears darkly[6] - due to its less compact structure.[5] When visualizing chromosomes, such as in a karyogram, cytogenetic banding 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.[7] One such example is G banding, otherwise known as Giemsa staining where euchromatin appears lighter than heterochromatin.[8]

Appearance of Heterochromatin and Euchromatin Under Various Visualization Techniques[8][9][10][11][12][2]
Giemsa (G-) Banding Reverse (R-) Banding Constitutive Heterochromatin (C-) banding Quinacrine (Q-) banding Telomeric R (T-) banding
Euchromatin Lighter Darker Lighter Dull Light
Heterochromatin Darker Lighter Darker Bright (Fluorescent) Darker (Faint)

Function

Schematic karyogram of a human, showing an overview of the human genome using G banding, which is a method that includes Giemsa staining, wherein the lighter staining regions are generally more euchromatic, whereas darker regions generally are more heterochromatic.
Further information: Karyotype

Transcription

Euchromatin participates in the active

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.[2] While not all euchromatin is necessarily transcribed, as the euchromatin is divided into transcriptionally active and inactive domains,[13]
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

housekeeping genes, which code for the proteins needed for basic functions of cell survival.[15]

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.[16] Regarding euchromatin, post-translational modifications of the histones can alter the structure of chromatin, resulting in altered gene expression without changing the DNA.[17] 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.[18] Research has shown epigenetic markers on histones for a number of additional diseases.[19][20]

Regulation

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.[21] Histone acetylation, for instance, is typically associated with euchromatin structure, whereas histone methylation promotes heterochromatin remodeling.[22] 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.[21] 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.[21]

kinases and phosphatases, which add and remove the phosphate groups respectively. This can occur at serine, threonine, or tyrosine residues present in euchromatin.[21][22] Since the phosphate groups added to the structure will incorporate a negative charge, it will promote the more relaxed "open" form, similar to acetylation.[22] In regards to functionality, histone phosphorylation is involved with gene expression, DNA damage repair, and chromatin remodeling.[22]

Another method of regulation that incorporates a negative charge, thereby favoring the "open" form, is

DNA damage response pathway.[22]

See also

References

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  3. ^ "Definition: nucleosome/nucleosomes". Scitable Nature Education. Retrieved 2021-10-06.
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  5. ^ a b "The cell. 4. Nucleus. Chromatin. Atlas of plant and animal histology". mmegias.webs.uvigo.es. Retrieved 2021-12-02.
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  8. ^ a b "Giemsa banding". Biology Articles, Tutorials & Dictionary Online. 2019-10-07. Retrieved 2021-12-02.
  9. ^ "Reverse banding - Definition and Examples - Biology Online Dictionary". Biology Articles, Tutorials & Dictionary Online. 2020-09-18. Retrieved 2021-12-02.
  10. ^ "Constitutive heterochromatin banding". Biology Articles, Tutorials & Dictionary Online. 2019-10-07. Retrieved 2021-12-02.
  11. ^ "Quinacrine banding". Biology Articles, Tutorials & Dictionary Online. 2019-10-07. Retrieved 2021-12-02.
  12. ^ "T-banding". Biology Articles, Tutorials & Dictionary Online. 2019-10-07. Retrieved 2021-12-02.
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  19. ^ Simmons D (2008). "Epigenetic Influences and Disease". Nature Education. 1 (1): 6. Retrieved 2021-12-02.
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Further reading

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