Euchromatin structure and functions

The euchromatin is the portion of eukaryotic chromosomes that is composed of loosely packed chromatin and that contains most of the coding gene sequences of the genome of many organisms.

This region of eukaryotic chromosomes is associated with transcriptionally active areas, which is why it is of great importance for the cells of an organism. It is clearly visible in cells that are not dividing, as it becomes heterochromatin when condensing or compacting, a previous step to mitotic and / or meiotic cell division.

So, euchromatin is one of the two types of structural organization of chromatin, the second being heterochromatin, which can be facultative or constitutive.

Article index

• 1 Structure
  • 1.1 Chromatin
  • 1.2 The octamer of histones
  • 1.3 Euchromatin and heterochromatin
• 2 Functions of euchromatin
  • 2.1 why?
• 3 References

Structure

The structure of euchromatin can be described exactly like the structure of chromatin found in many textbooks, since one of the few differences between the latter and heterochromatin is the level of compaction or condensation of the DNA + protein strand..

Chromatin

The DNA of eukaryotic organisms is found in the nucleus, in close association with a large number of proteins. Among these proteins there are some of considerable importance, histones, which are responsible for "organizing" and condensing the chromosomal DNA strands, allowing these large molecules to "enter" in such a small space and controlling the expression of genes.

Each eukaryotic chromosome is made up of a single strand of DNA and a large number of histone proteins. These structures are significantly dynamic, since their degree of compaction is modified not only depending on the cellular transcriptional needs, but also depending on the moment of the cell cycle and some environmental signals..

Alterations in chromatin compaction affect, in one way or another, the level of genetic expression (in some regions more than in others), therefore it corresponds to a level of epigenetic regulation of information..

Histones make it possible to shorten the length of the DNA strands of each chromosome by almost 50 times, which is particularly important during cell division, since chromatin compaction ensures the correct segregation of chromosomes between daughter cells.

The histone octamer

The DNA molecules of eukaryotic chromosomes wrap around a "cylindrical" structure made up of eight histone proteins: H2A, H2B, H3, and H4. The octameric nucleus is composed of two dimers of H2A and H2B and a tetramer of the H3 and H4 proteins..

Histones are basic proteins, as they have a large number of positively charged amino acid residues, such as lysine and arginine, for example. These positive charges interact electrostatically with the negative charges of DNA molecules, favoring its union with the protein nucleus..

Each histone octamer coils about 146 base pairs, forming what is known as a nucleosome. Chromatin is made up of consecutive nucleosomes, linked together by a short piece of DNA and a histone bridging or binding protein called H1. This configuration decreases the length of the DNA about 7 times compared to the initial length..

Histone proteins also have amino acid "tails" that protrude from the nucleosomes and that can undergo covalent modifications that can modify the level of compaction of chromatin (compaction is also affected by covalent modifications of DNA, such as , cytokine methylation, which favors compaction).

Depending on the time of life of each cell, the strand made up of nucleosomes can further compact, forming a fibrous structure known as the "30 nm fiber", which shortens the length of the DNA molecule another 7 times..

This 30 nm fiber can be organized inside the core in the form of radial loops; these loops are characterized by harboring transcriptionally active genes and correspond to euchromatin.

Euchromatin and heterochromatin

Euchromatin and heterochromatin are the two types of chromatin organization. Heterochromatin is the most compact or "closed" part of a chromosome; characterized by biochemical marks of hypoacetylation and hypermethylation (in higher eukaryotes the methylation of residue 9 of histone H3).

Transcriptionally silent genomic regions, regions of repetitive sequences, and “vestigial” regions of invading transposable elements and retrotransposons are associated with heterochromatin, to name a few..

Heterochromatin composes the telomeric and centromeric regions of chromosomes, which are functionally important for the protection of the ends of these structures and for their correct segregation during cell division events..

Additionally, depending on the transcriptional needs of a cell, a portion of the chromatin can be heterochromatinized at one time and release this compaction at another..

Euchromatin, on the other hand, is characterized by hyperacetylation and hypomethylation, more specifically by the acetyl group “tags” on the lysine 4 residue of histones H3 and H4..

It corresponds to the "looser" regions of chromatin and usually represents the most transcriptionally active portions, that is, where the largest number of coding genes are grouped..

Functions of euchromatin

Euchromatin is very abundant within the cell nucleus when the cells are not dividing, that is, when the chromosomes are neither condensed nor show their characteristic shape..

Given that this portion of chromatin is the one that contains the largest number of transcriptionally active genes, euchromatin has important functions in development as well as in metabolism, physiology and the regulation of vital biological processes inherent in cells..

Why?

Because “active” genes code for all the proteins and enzymes necessary to carry out all the metabolic and physiological processes of a cell.

Those genes that do not encode proteins, but are also active from the transcriptional point of view, usually have regulatory functions, that is, they encode for small RNA molecules, for transcription factors, ribosomal RNAs, etc..

Therefore, the regulation of transcriptional processes also depends on the information contained in euchromatin, as well as the regulation of processes related to cell division and growth..

References

1. Brooker, R., Widmaier, E., Graham, L., Stiling, P., Hasenkampf, C., Hunter, F.,… & Riggs, D. (2010). Biology.
2. Eissenberg, J., Elgin, S. (2005) Heterochromatin and Euchromatin. Encyclopaedia of Life Sciences. John Wiley & Sons, Ltd.
3. Griffiths, A. J., Wessler, S. R., Lewontin, R. C., Gelbart, W. M., Suzuki, D. T., & Miller, J. H. (2005). An introduction to genetic analysis. Macmillan.
4. Grunstein, M., Hecht, A., Fisher-Adams, G., Wan, J., Mann, R. K., Strahl-Bolsinger, S., ... & Gasser, S. (1995). The regulation of euchromatin and heterochromatin by histones in yeast. J Cell Sci, 1995 (Supplement 19), 29-36.
5. Tamaru, H. (2010). Confining euchromatin / heterochromatin territory: jumonji crosses the line. Genes & development, 24 (14), 1465-1478.