Chromatin is the substance used to create chromosomes. In a little more detail, chromatin is made up of DNA, RNA and various protein molecules. This is located in the nucleus of each cell that makes up the human being. This substance represents approximately two meters of DNA molecule, in hypercompact form. For its part, the nucleus of a cell has an approximate length of 5 to 7 micrometers.
What is chromatin
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In terms of chromatin biology definition, it refers to the way DNA is presented in the cell nucleus. It is the basic substance of eukaryotic chromosomes, and belongs to the union of DNA, RNA and proteins that are found in the interphase nucleus of eukaryotic cells and that constitutes the genome of these cells, whose function is to shape the chromosome so that it is integrate into the nucleus of the cell. Proteins are of two types: histones and non-histone proteins.
Chromatin history
This substance was discovered in 1880 thanks to Walther Flemming, the scientist who gave it that name, because of his fondness for dyes. However, Flemming's stories were discovered four years later, by researcher Albrecht Kossel. With regard to the advances that were made in the determination of the chromatin structure were very scarce, it was not until the 1970s, when the first observations of chromatin fibers could be made thanks to the already established electron microscopy, which that revealed the existence of the nucleosome, the latter being the base unit of chromatin, whose structure was more explicitly detailed by means of X-ray crystallography in 1997.
Chromatin types
It is classified into two types: euchromatin and heterochromatin. The basic units that make up chromatin are nucleosomes, which are made up of approximately 146 base pairs in length, which are in turn associated with a specific complex of eight nucleosomal histones. The types are detailed below:
Heterochromatin
- It is the most compact expression of this substance, it does not alter its level of compaction throughout the cell cycle.
- It consists of highly repetitive and inactive DNA sequences that do not replicate and form the centromere of the chromosome.
- Its function is to protect chromosomal integrity due to its dense and regular packing with genes.
It can be identified with a light microscope with a dark color due to its density. Heterochromatin is divided into two groups:
Constitutive
It appears highly condensed by repetitive sequences in all cell types and cannot be transcribed as it does not contain genetic information. They are the centromeres and telomeres of all chromosomes that do not express their DNA.
Optional
It is different in different cell types, it only condenses in certain cells or specific periods of cell development, such as the Barr corpuscle, which is formed because the optional heterochromatin has active regions that can be transcribed under certain circumstances and characteristics. It also includes satellite DNA.
Euchromatin
- Euchromatin is the part that remains in a less condensed state than heterochromatin and is distributed throughout the nucleus during the cell cycle.
- It represents the active form of chromatin in which genetic material is transcribed. Its less condensed state and its ability to change dynamically make transcription possible.
- Not all of it is transcribed, however the rest is generally converted to heterochromatin to compact and protect genetic information.
- Its structure is similar to a pearl necklace, where each pearl represents a nucleosome made up of eight proteins called histones, around them there are pairs of DNA.
- Unlike heterochromatin, the compaction in euchromatin is low enough to allow access to genetic material.
- In laboratory tests, this can be identified with an optical microscope, since its structure is more separated and it is impregnated with a light color.
- In prokaryotic cells, it is the only form of chromatin present, this may be due to the fact that the structure of heterochromatin evolved years later.
Chromatin role and importance
Its function is to provide the genetic information necessary for cell organelles to carry out protein synthesis and transcription. They also transmit and preserve the genetic information contained in DNA, duplicating DNA in cell reproduction.
In addition, this substance is also present in the animal world. For example, in the animal cell chromatin, sex chromatin forms as a condensed mass of chromatin in the interface nucleus, which represents an inactivated X chromosome that exceeds the number one in the nucleus of mammals. This is also known as Barr's corpuscle.
This plays a fundamental regulatory role in gene expression. The different states of compaction can be associated (although not unambiguously) with the degree of transcription exhibited by the genes found in these areas. Chromatin is strongly repressive for transcription, since the association of DNA with different proteins complicates the processing of different RNA polymerases. Therefore, there are a variety of chromatin remodeling and histone modification machines.
Currently there is what is known as a " histone code ". The different histones can undergo post-translational modifications, such as methylation, acetylation, phosphorylation, generally administered at lysine or arginine residues. Acetylation is associated with the activation of transcription, since when a lysine is acetylated, the overall positive charge of histone decreases, thus it has a lower affinity for DNA (which is negatively charged).
Consequently, the DNA is less bound, thus allowing access by the transcriptional machinery. In contrast, methylation is associated with transcriptional repression and stronger DNA-histone binding (although this is not always true). For example, in yeast S. pombe, methylation at lysine residue 9 of histone 3 is affiliated with repression of transcription in heterochromatin, whereas methylation at lysine residue 4 promotes gene expression.
The enzymes that carry out the functions of histone modifications are histone acetylases and deacetylases, and histone methylases and demethylases, which form different families whose members are responsible for modifying a particular residue in the long tail of histones.
In addition to histone modifications, there are also chromatin remodeling machines, such as SAGA, that are in charge of repositioning nucleosomes, either by displacing them, rotating them or even partially disarming them, removing some of the nucleosome constituent histones and then returning them. In general, chromatin remodeling machines are essential for the transcription process in eukaryotes, since they allow access and processivity of polymerases.
Another way of marking chromatin as "inactive" can occur at the level of DNA methylation, in the cytosines that belong to the CpG dinucleotides. In general, DNA and chromatin methylation are synergistic processes, since, for example, when DNA is methylated, there are histone methylating enzymes that can recognize methylated cytosines and methylated histones. Similarly, DNA methylating enzymes can recognize methylated histones and therefore continue methylation at the DNA level.
Chromatin FAQ
What are the characteristics of chromatin?
It is characterized by containing almost twice as many proteins as the genetic material. The most important proteins in this complex are histones, which are small positively charged proteins that bind to DNA through electrostatic interactions. Also, chromatin has over a thousand different histone proteins. The fundamental unit of chromatin is the nucleosome, which consists of the union of histones and DNA.How is chromatin made up?
It is made up of a combination of proteins called histones, which are basic proteins formed from arginine and lysine, with DNA and RNA, where the function is to shape the chromosome so that it is integrated into the cell nucleus.What is the structure of chromatin?
The ultrastructure of chromatin is based on: histones, forming nucleosomes (eight histone proteins + one 200 base pair DNA fiber). Each nucleosome associates with a different type of histone, H1, and condensed chromatin is formed.What is the difference between chromatin and chromosome?
As for chromatin, it is the fundamental substance of the cell nucleus, and its chemical constitution is simply DNA strands in different degrees of condensation.On the other hand, chromosomes are structures within the cell that contain genetic information and each chromosome is made up of a DNA molecule, associated with RNA and proteins.
