CHROMATIN: TYPES, CHARACTERISTICS, STRUCTURE, FUNCTIONS - BIOLOGY - 2025

The chromatin is the complex formed by DNA and proteins only in eukaryotic organisms. In terms of proportion, it contains almost twice as many proteins as it does genetic material. The most important proteins in this complex are histones - small, positively charged proteins that bind to DNA through electrostatic interactions. Additionally, chromatin has more than a thousand proteins other than histones.

The fundamental unit of chromatin is the nucleosome, which consists of the union of histones and DNA. This arrangement is reminiscent of the beads on a necklace. After going through all the higher levels of DNA organization, we will arrive at the chromosomes.

Source: Chromatin_nucleofilaments.png: Chris Woodcockderivative work: Gouttegd

Chromatin structure is closely related to the control of gene expression. There are two main types: euchromatin and heterochromatin.

Euchromatin is characterized by having a low degree of compaction, which translates into high levels of transcription. In contrast, heterochromatin is transcriptionally inactive, due to its high degree of compaction.

Structurally there are certain epigenetic marks of the histones typical of both types of chromatins. While euchromatin is associated with acetylation, heterochromatin is associated with a decrease in chemical groups.

There are some regions of heterochromatin with structural functions, such as telomeres and centromeres.

Historical perspective

The study of genetic material and its structural organization began in 1831 when the researcher Robert Brown described the nucleus. One of the immediate questions to this discovery was to inquire about the biological and chemical nature of this structure.

These questions began to be elucidated between 1870 and 1900, with the experiments of Friedrich Miescher, who introduced the word nuclein. However, Walther Flemming modifies the term and uses chromatin to refer to the nuclear substance.

With the passage of time, one begins to have a deeper knowledge of the genetic material and its properties. It was not until 1908 when the Italian researcher Pasquale Baccarini noticed that the chromatin was not homogeneous and managed to visualize small bodies inside the nucleus.

The types of chromatins - euchromatin and heterochromatin - were initially proposed by Emil Heitz in 1928. To establish this classification, Heitz relied on the use of stains.

In 1974 the biologist Roger Kornberg proposed a model for the organization of genetic material in structures known as nucleosomes, a hypothesis confirmed empirically by the experiments of Markus Noll.

What is chromatin?

Chromatin components: DNA and proteins

Chromatin is a nucleoprotein formed by the union of genetic material - DNA - with a heterogeneous set of proteins. This association is highly dynamic and acquires a complex three-dimensional conformation that allows it to carry out its regulatory and structural functions.

One of the most important proteins in chromatin are histones, which are found in almost the same proportion as DNA.

Histones are basic proteins, remarkably conserved throughout the evolutionary history of organic beings - that is, our histones do not vary much compared to those of other mammals, even another animal more distant phylogenetically.

The charge of histones is positive, so they can interact through electrostatic forces with the negative charge of the phosphate backbone present in DNA. There are five types of histones, namely: H1, H2A, H2B, H3, and H4.

There are also a number of proteins of a different nature than histones that participate in the compaction of DNA.

DNA compaction: nucleosomes

The basic unit of chromatin are nucleosomes - repetitive structures made up of DNA and histones, a conformation found throughout genetic material.

The DNA double helix is ​​wound into a complex of eight histones known as a histone octamer. The molecule coils in about two turns, followed by a short region (between 20 and 60 base pairs) that separates the nucleosomes from each other.

To understand this organization, we must take into account that the DNA molecule is extremely long (about 2 meters) and must be wound in an orderly way to establish itself in the nucleus (whose diameter is 3 to 10 µm). Furthermore, it must be made available for replication and transcription.

This objective is achieved with different levels of DNA compaction, the first being the aforementioned nucleosomes. These resemble the beads on a pearl necklace. Approximately 150 base pairs of DNA are wound at home "counts."

There are no true stories in bacteria. In contrast, there are a series of proteins that are reminiscent of histones and these are presumed to contribute to the packaging of bacterial DNA.

Top organization

The organization of chromatin is not restricted at the nucleosome level. This association of proteins and DNA are grouped together in a thicker structure of about 30 nm - because of this thickness it is called the "30 nm fiber" level.

The chromatin organized in the thickness of 30 nm is organized in the form of loops that extend in a kind of scaffold of protein nature (not histones).

This is the model currently being used, although more complex compaction mechanisms can be expected. The final organization consists of the chromosome.

Errors in the organization of chromatin

The compaction and organization of genetic material is vital for multiple biological functions. Different medical conditions have been associated with errors in the chromatin structure, among them X-linked alpha thalassemia, Rubinstein-Taybi syndrome, Coffin-Lowry syndrome, Rett syndrome, among others.

Chromatin types

There are two types of chromatin in the cell, revealed by the application of stains: euchromatin ("true" chromatin) and heterochromatin. In the first case the staining is weakly observed, while in the second the staining is intense.

This structural organization of DNA is unique to eukaryotic organisms and is crucial for the behavior of chromosomes and the regulation of gene expression.

If we evaluate the proportions of both types of chromatins in a cell that is in interface, we find that approximately 90% of the chromatin is euchromatin and the remaining 10% corresponds to heterochromatin. We will describe each type in detail below:

I. Heterochromatin

characteristics

The main difference that exists between both types of chromatin is related to the degree of compaction or "packing" of the molecule during specific stages in cell division.

Although the genetic material appears to be randomly scattered at the interface, it is not in this way.

There is a significant organization at this stage, where a differential partition of the chromosomal material can be seen within the nucleus.

The DNA of chromosomes does not intertwine with the DNA strand of other chromosomes and remains in specific regions called chromosomal territories. This organization seems to contribute to gene expression.

Heterochromatin is strongly condensed, making it inaccessible to the transcription machinery - so it is not transcribed. Also, it is poor in terms of the number of genes it has.

Types of heterochromatin

Certain regions of heterochromatin are persistent throughout cell lineages - that is, it will always behave like heterochromatin. This type of heterochromatin is known as constitutive. An example of this are the condensed regions of chromosomes called centromeres and telomeres.

In contrast, there are portions of heterochromatin that can vary the level of compaction in response to changes in developmental patterns or environmental variables.

Thanks to new studies, this view is being reformulated and now there is evidence that constitutive heterochromatin is also dynamic and capable of responding to stimuli.

Structure

One of the factors that determines the structure of chromatin is the chemical modifications of histones. In the case of chromatin that is transcriptionally inactive, they exhibit hypoacetylated histones.

The decrease in the number of acetyl groups is associated with gene silencing, since it does not mask the positive charge of lysines, allowing a strong electrostatic interaction between DNA and histones.

Another epigenetic brand is methylation. However, as the addition of a methyl group does not modify the charge of the protein, its consequence (activating or deactivating genes) is not so obvious and will depend on the region of the histone where the mark is found.

Empirically it has been found that the methylation of H3K4me3 and H3K36me3 is associated with gene activation and that of H3K9me3 and H3K27me3.

Features

In the example of constitutive heterochromatin we mention the centromere. This chromosomal region has a structural role and contributes to the movement of chromosomes during both mitotic and meiotic cell division events.

II. Euchromatin

characteristics

Contrary to heterochromatin, euchromatin is a less compact molecule, so the transcription machinery has easy access (specifically the RNA polymerase enzyme) and can be actively expressed through genetic pathways.

Structure

The structure of a nucleosome of a transcriptionally active chromatin is characterized by acetylated histones. The presence of monomethylated lysines is also associated with gene activation.

The addition of an acetyl group to these histone lysine residues neutralizes the positive charge of said amino acid. The immediate consequence of this change is the reduction of electrostatic interactions between histone and DNA, generating a looser chromatin.

This structural modification allows the interactions of the genetic material with the transcriptional machinery, which is characterized by being particularly bulky.

Features

Euchromatin encompasses all genes that are active and manage to be reached by the enzymatic machinery related to transcription. Therefore, the functions are as broad as the functions of the genes involved.

References

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