The nucleic acid DNA, which enters the nuclei of eukaryotic cells, is compactly packed thanks to its 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 considered in this article.
How DNA is organized in the nucleus
In order to “squeeze” a long polynucleotide DNA strand into the micro-space of the cell nucleus, it contains peculiar “coils” - histone proteins. A double-stranded thread of deoxyribonucleic acid is wound on them . This 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 cow. It is these histone proteins that form the nucleosome.
Structural Features of Nucleosome Peptides
Chemical analysis established the fact that the content of molecules of amino acids such as lysine and arginine in the core histones is excessive. The first is irreplaceable, and the other is partially interchangeable and is present in almost all peptides. Histone proteins 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 structural feature of these proteins is that it is practically identical in organisms belonging to the kingdom of Plants, Animals and Mushrooms.
Since histones are proteins of the nucleus, due to their structure, they can take part in the processes occurring in the karyoplasm. For example, the H1 peptide, the histone protein that holds the nucleosomes that are part of chromatin in an orderly-compact nucleus, is most important for the transcription process. Also, in case of damage to DNA loci, the so-called variant molecules of core peptides are involved in the repair of these sites.
Core peptides
They determine the structure of the nucleosome, which consists of four types of molecules called H2A, H2B and H3 and H4. There are two molecules of each type in the nucleosomes, this structure is called an octamer. The deoxyribonucleic molecule and core proteins form hydrophobic, hydrogen and covalent bonds between themselves. Histone proteins are the core of the nucleosome. They also contain unstructured NC tails. These parts consist of 15-30 amino acid residues and are involved in epigenetic processes that control gene expression. Crustal histones of the central part of the nucleosome have small molecular weights; in their sections, in contrast to the tail parts, islands of hydrophobic protein monomers are contained: valine, proline, lezin, methionine.
Recent scientific research in biochemistry has led to the emergence of the histone code hypothesis. Unlike the genetic code, which is universal for all forms of cell life on Earth, the histone code is volatile. By this term is understood modifications of the tail regions of peptides as a result of acetylation, methylation, phosphorylation reactions. All of the above chemical processes occur in the presence of multienzyme complexes. Thanks to such biochemical processes that modify core histones, the expression correction of genes that control intranuclear reactions involving DNA takes place: repair, transcription, replication. Chromatin itself, under the influence of changes in the histone code, undergoes remodeling, that is, changes its packing in the nucleosome (compacts it or, conversely, loosens it).
Linker protein
Histone H1, located in chromatin, binds to the outer part of the nucleosome and holds the deoxyribonucleic acid supercoil on it. Its fixation occurs at the location of the tetramer, consisting of two molecules of the peptide H3 and two molecules of H4. In representatives of the class of birds and the class of reptiles in erythrocytes, instead of histone H1, another linker protein H5 was found.
Peptide H1 contains an HMJB domain, a structural site with about 80 amino acid residues. It is almost the same in most organisms, including plants, animals, and humans. This domain is not modified and is conservative. Peptide H1 has two forms of spatial configuration: folded in the form of a globule and unfolded in a tertiary form. The latter occurs when the connection of the C-terminal portion of the histone with DNA-binding domains is broken. The linker peptide is actively involved in transcribing information from a gene to an mRNA molecule, in the processes of DNA self-doubling, 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 the H1 peptide, other types of histones, called cortical, 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 sequences of amino acid residues.
- Location in the genome.
For example, variant histone H2ABbd is interconnected with chromatin, in the DNA of which transcription occurs. The MACROH2A peptide is located on interphase chromosomes. Cytological studies found that histone H4 did not reveal variant forms, but it is able to form a large number of covalent bonds with other proteins that are part of the nucleosome octamer. Thus, scientists believe that histones are a group of special proteins that practically enter the chromatin of all cellular life forms.
How is histone information stored in the genome?
It can be argued that core, linker, and variant histones are encoded in gene clusters expressed in the synthetic phase of the cell's life cycle. For example, for humans, a group of hereditary inclinations called HIST1 consists of 35 genes located in the sixth somatic pair of chromosomes. The HIST2 cluster contains six genes encoding 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. It is interesting that the genes of core 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 the folding of the DNA helix in the nucleus, as well as in the processes of regulation, repair, and transcription in it.