Histones are ... The role of histones in DNA

Nucleic acid DNA, which enters the nuclei of eukaryotic cells, is compactly packed due to special structures. In cytology they have a special name - histones. These are peptides that exhibit basic chemical properties. Their structure and functions performed in the cell will be discussed in this article.

How DNA is organized in the nucleus

In order to "squeeze" a long polynucleotide chain of DNA into the microcosm of the cell nucleus, there are original "coils" - proteins-histones. They double-stranded strand deoxyribonucleic acid. Such a structure, located in the karyoplasm, is called the nucleosome. Biochemical studies have established that the histone protein is organized in the form of several modifications: histone H1 / H5, H2A, H2B, H3, H4. The first peptide from this list is usually called linker, the rest are crustaceans. It is these protein-histones that form the nucleosome.

Peculiarities of the structure of nucleosome peptides

The chemical analysis established the fact of the excess content in the core histones of molecules of such amino acids as lysine and arginine. The first is irreplaceable, and the other is partially replaceable and is present in virtually all peptides. Proteins-histones accumulate excess positive charges on amino acid residues. They neutralize the total negative charges of the PO ₄ ^3- anions that make up the DNA. Another feature of the structure of these proteins is that it is practically identical in organisms belonging to the kingdom of the plant, animals and mushrooms.

Since histones are the proteins of the nucleus, they, due to their structure, can participate in the processes occurring in the karyoplasm. For example, the most important for the transcription process is the peptide H1, a histone protein that holds the nucleosomes that are part of the chromatin in an ordered-compact nucleus. Also, in the case of damage to DNA loci, the so-called variant molecules of the core peptides participate in the repair of these sites.

Cow peptides

They determine the structure of the nucleosome, which consists of four kinds of molecules called H2A, H2B and H3 and H4. In nucleosomes there are two molecules of each type, this structure is called an octamer. The molecule deoxyribonucleic and core proteins form hydrophobic, hydrogen and covalent bonds among themselves. Proteins-histones are the core of the nucleosome. They also contain unstructured NC-tails. These parts consist of 15-30 amino acid residues and participate in epigenetic processes controlling the expression of genes. The core histones of the central part of the nucleosome have small molecular weights; in their parts, unlike the tail portions, there are islands of hydrophobic protein monomers: valine, proline, lezine, methionine.

The latest scientific research in the field of biochemistry led to the emergence of a hypothesis of the histone code. Unlike the genetic code, which is universal for all forms of cellular life on Earth, the histone code is variable. This term refers to the modification of tail sections of peptides as a result of the reactions of acetylation, methylation, phosphorylation. All of the above chemical processes occur in the presence of multienzyme complexes. Due to such biochemical processes modifying the core histones, the expression correction of the genes that control the intranuclear reactions involving DNA: reparation, transcription, replication occurs. The chromatin itself under the influence of changes in the histone code undergoes remodeling, that is, changes its packaging in the nucleosome (condenses it or, conversely, loosens).

Linker protein

Histone H1, which is in chromatin, combines with the outer part of the nucleosome and retains on it the super-helix of deoxyribonucleic acid. Its fixation takes place at the location of a tetramer consisting of two molecules of peptide H3 and two molecules of H4. In representatives of the class of birds and the class of reptiles in erythrocytes, another linker protein H5 was found instead of histone H1.

The H1 peptide contains an HMJB domain, a structural region of about 80 amino acid residues. It is almost the same in most organisms, including plants, animals and humans. This domain is not subject to modification and is conservative. Peptide H1 has two forms of spatial configuration: folded in the form of a globule and deployed - in a tertiary form. The latter occurs when the connection of the C-terminal region of histone with the DNA-binding domains is broken. The linker peptide actively participates in rewriting information from the gene to the mRNA molecule, in the processes of DNA self-duplication, as well as in the repair of its damaged loci. This is the biological role of histones in DNA.

How proteins form an octamer

Unlike peptide H1, other types of histones, called crustaceans, are characterized by sufficient plasticity and form variant forms. For example, H2A has the largest number of modifications: H2AZH2AX MACROH2A. They differ among themselves:

• C-terminal amino acid residue sequences.
• Location in the genome.

For example, variant histone H2ABbd is interrelated with chromatin, in the DNA of which transcription occurs. The MACROH2A peptide is located in interphase chromosomes. Cytological studies have shown that histone H4 does not have variant forms, but it is capable of forming a large number of covalent bonds with other proteins entering the octamer of the nucleosome. Thus, scientists believe that histones are a group of special proteins that practically enter the chromatin of all cellular forms of life.

How is information on histones in the genome stored

It can be argued that crustal, linker and variant histones are encoded in clusters of genes that are expressed in the synthetic phase of the cell's life cycle. For example, for a human, the hereditary deposit group, called HIST1, consists of 35 genes localized in the sixth somatic pair of chromosomes. The HIST2 cluster contains six genes that encode histones and is located in the first chromosome pair. It also contains the HIST3 locus, which includes three genes. In the twelfth pair, there is one gene encoding histone H4. Interestingly, the genes of crustal proteins do not have introns, and the genes of variant histones, on the contrary, contain them and are scattered throughout the genome.

To summarize, we were convinced that histones are proteins involved in laying the DNA helix in the nucleus, as well as in the regulation, repair and transcription processes occurring in it.