An equiette cell , like the cells of the human organism, is characterized by having membranous compartments in its cytoplasm, the so-called organelles , highlighting the cell nucleus where DNA (deoxyribonucleic acid) is found that constitutes the genetic material of the cell.
DNA molecules are not found in the nucleus alone but are in the form of chromatin , a substance formed by molecular macrocomplexes of DNA, proteins and sometimes RNA .
The function of chromatin is to fundamentally pack the DNA with different structures depending on the phase of the cell cycle . Chromatin makes it possible for all DNA to fit in the nucleus, protects DNA from potential damage, modulates gene expression, and strengthens DNA molecules during cell division when subjected to traction forces.
The heterochromatin and euchromatin are the two forms or compaction levels having chromatin during interphase , between the end of a division and the start of the next. The set of the two is known as interphase chromatin. During this phase the cell grows, develops and exerts its physiological function; when it is ready to go into division it will begin to duplicate its DNA to enter the division phase again.
During the interface, gene expression and synthesis of cellular components is usually maximal. The chromatin adopts a lax conformation that allows the access of RNA polymerases for genetic transcription and repair factors if necessary. This rather compact conformation is known as euchromatin.
But the compaction of the interphase chromatin is not uniform . In areas where there are genes that the cell does not need, chromatin adopts a more compact conformation, heterochromatin, which does not allow genetic expression.
In addition, the conformation of the interphase chromatin is dynamic , which allows chromatin to play a central role in the modulation of gene expression depending on the conformation it adopts:
- Euchromatin : more lax conformation and frequently associated with RNA polymerases that allows the genetic expression . It is the most abundant form during the interphase, exceeding 90% of all chromatin.
- Heterochromatin : more compact conformation that does not allow the genetic expression. Two types of heterochromatin, constitutive and facultative, can be distinguished; the constitutive is never expressed, the facultative can be transferred to euchromatine and expressed.
Although the structure of chromatin is the subject of intense research, it is not yet understood in sufficient detail to understand how it performs most of its functions and the specific factors involved in the adoption of one or another conformation
The basic structural element of chromatin in all eukaryotic cells is the nucleosome . Each nuecleosoma consists of an octamer of histones , generally called nucleus, and a helix of DNA that surrounds it . The histone octamer is formed by two pairs of four histone types: H2A, H2B, H3 and H4. The DNA chain gives about 1.7 turns around it.
At the exit / entry of DNA into the nucleosome is located a H1-type histone called binding histone . The nucleosome and histone H1 complex is known as chromosome . Between each chromosome is a strand of DNA called linker DNA, spacer DNA or DNA linker. The linker DNA strands and intercalated chromosomes adopt a conformation often referred to as a ” bead necklace ” because of its characteristic shape.
There are differences between different texts as far as the exact definition of nucleosome is concerned. In some sources the nucleosome appears only as the nucleus of histones and the surrounding DNA without including the histone or the DNA of bond. In other sources the nucleosome includes the complete chromosome and the DNA of bond, all forming the basic unit of repetition of the chromatin.
The bead necklace that forms the euchromatin can be spirally wound up with histones H1 inwards to achieve a higher level of packaging and adopting the heterochromatin conformation. The heterochromatin fiber has a thickness of about 30 nm. In an interphase cell nucleus, heterochromatin usually appears more concentrated in the periphery and euchromatin in the interior. Heterochromatin is found only in eukaryotic organisms.
The conversion between euchromatin and heterchromatin is considered a mechanism of regulation of gene expression , specifically a chemical mechanism of epigenetic regulation . Although not all the mechanisms involved are well understood, it seems that the step between the two levels of packaging is due to chemical changes in the histones; for example, in heterochromatin there is a higher level of methylation and in euchromatin there is a higher level of acetylation .
Heterochromatin that can be converted to the form of echrochromatin is termed optional heterochromatin, but there is also heterochromatin that is never expressed and is not converted to euchromatin, the so-called constitutive heterochromatin.
Differences and key points
- Ecchromatin and heterochromatin are two levels of structural packing of the chromatin during the interphase.
- Euchromatin has a lower packing, higher heterochromatin.
- Euchromatin is associated with transcriptionally active regions, heterochromatin with inactive regions.
- The passage between euchromatin and heterochromatin is considered a mechanism of regulation of genetic expression.
- Euchromatin contains less DNA density than heterochromatin.
- Euchromatin can be found in prokaryotic and eukaryotic organisms, heterochromatin alone in eukaryotes .
- There is only one type of eucromtatin but two types of heterochromatin (facultative and constitutive).
- Euchromatin is the predominant form during the interphase.
- Euchromatin is present inside the nucleus, heterochromatin is more concentrated in the periphery of the nucleus.
During cell division the chromatin adopts a level of packing superior to the heterochromatin and forms the chromosomes typical of the cariograms. The formation of the chromosomes begins in the prophase and continues until the anaphase.
- MM Srinivas Bharath, Nagasuma R. Chandra and MRS Rao. (2003) Molecular modeling of the chromatosome particle. Nucleic Acids Research 31 (14): 4264-4274. PMCID: PMC167642 .
- C. David Allis and Thomas Jenuwein. (June 2016) The molecular hallmarks of epigenetic control. Nature Reviews Genetics 17: 487-500. doi: 101038 / nrg.2016.59 .
- Yael Katan-Khaykovich and Kevin Struhl. (June 2005) Heterochromatin formation involves changes in histone modifications over multiple cell generations. The Embo Journa 24 (12): 2138-2149. doi: 10.1038 / sj.emboj.7600692 .