What is the difference between heterochromatin and euchromatin?

A cell ecuariotelike the cells of the human organism, is characterized by having membranous compartments in its cytoplasm, the so-called organelles, highlighting the cell nucleus where is the DNA (deoxyribonucleic acid) that constitutes the genetic material of the cell.

DNA molecules are not found in the nucleus by themselves, but rather are in the form of chromatina substance made up of molecular macrocomplexes of DNA, proteins, and sometimes RNA.

The role of chromatin is primarily package DNA with different structures in function of the cell cycle phase. Chromatin makes it possible for all DNA to fit into the nucleus, protects DNA from damage, modulates gene expression, and strengthens DNA molecules during cell division when subjected to tensile forces.

The heterochromatin and the euchromatin are both forms or compaction levels that presents the chromatin during the interface, between the end of one division and the beginning of the next. The set of the two is known as interphase chromatin. During this phase the cell grows, develops and performs its physiological function; when it is ready to go into division it will begin to duplicate its DNA to enter the division phase again.

interphase chromatin

During the interface, the gene expression and synthesis of cellular components is usually maximal. The chromatin adopts a loose conformation that allows access of RNA polymerases for gene transcription and repair factors if necessary. This loose conformation is known as euchromatin.

But compaction of interphase chromatin it is not uniform. In areas where there are genes that the cell does not need, the chromatin adopts a more compact conformation, heterochromatin, which does not allow gene expression.

In addition, the conformation of interphase chromatin it is dynamicwhich allows chromatin to play a central role in modulating gene expression depending on the conformation it adopts:

  • euchromatin: plus conformation lax and frequently associated with RNA polymerases that allows gene expression. It is the most abundant form during interphase, exceeding 90% of all chromatin.
  • heterochromatin: plus conformation compact that does not allow gene expression. Two types of heterochromatin can be distinguished, constitutive and facultative; the constitutive is never expressed, the facultative can pass to euchromatin and express itself.

Structure

Although the structure of chromatin is the subject of intense research, it is still not 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 nucleosome. Each nucleosome is made up of a histone octamerusually called the core, and a DNA helix that wraps around it. The histone octamer is made up of two pairs of four types of histones: H2A, H2B, H3 and H4. The DNA chain gives approximately 1.7 laps around it.

An H1-type histone called histone link. The complex of the nucleosome and histone H1 is known as chromatosome. Between each chromatosome there is a strand of DNA called linking DNA, spacer DNA or linker DNA. The DNA linker strands and intercalated chromatosomes adopt a conformation often referred to as "Pearl Necklace» for its characteristic shape.

There are differences between different texts when it comes to the exact definition of a nucleosome. In some sources, the nucleosome appears only as the histone core and surrounding DNA, not including the histone and linking DNA. In other sources, the nucleosome includes the entire chromatosome and the linking DNA, all of which form the basic repeating unit of chromatin.

The pearl necklace that forms euchromatin can coil into a spiral with histones H1 inward achieving a higher level of packing and adopting the heterochromatin conformation. The heterochromatin fiber is about 30 nm thick. In an interphase cell nucleus, heterochromatin tends to appear more concentrated in the periphery and euchromatin in the interior. Heterochromatin is only found in eukaryotic organisms.

The conversion between euchromatin and heterochromatin It is considered a mechanism of regulation of gene expressionspecifically a chemical regulation mechanism epigenetic. Although all the mechanisms involved are not well understood, it seems that the passage between both packaging levels 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 convert to eurchromatin is called facultative heterochromatin, but there is also heterochromatin that is never expressed and does not convert to euchromatin, the so-called constitutive heterochromatin.

Differences and key points

  • Ecuchromatin and heterochromatin are two levels of structural packaging of chromatin during interphase.
  • Euchromatin has less packing, heterochromatin more.
  • Euchromatin is associated with transcriptionally active regions, heterochromatin with inactive regions.
  • The passage between euchromatin and heterochromatin is considered a mechanism for regulating gene expression.
  • Euchromatin contains lower density of DNA than heterochromatin.
  • Euchromatin can be found in both prokaryotic and eukaryotic organisms. heterochromatin only in eukaryotes.
  • There is only one type of euchromatin but two types of heterochromatin (facultative and constitutive).
  • Euchromatin is the predominant form during interphase.
  • Euchromatin occurs within the nucleus, heterochromatin is more concentrated at the periphery of the nucleus.

metaphase chromatin

During cell division, chromatin adopts a higher level of packaging than heterochromatin and forms cells. chromosomes typical of karyograms. Chromosome formation begins in prophase and continues until anaphase.

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