What is chromatin: definition, structure and functions

Biochemical research in genetics is an important way of studying its basic elements - chromosomes and genes. In this article we will look at what chromatin is, find out its structure and functions in the cell.

Heredity is the main property of living matter

The main processes that characterize the organisms living on Earth include respiration, nutrition, growth, excretion and reproduction. The latter function is the most significant for preserving life on our planet. How not to remember that the first commandment given by God to Adam and Eve was the following: "Be fruitful and multiply". At the cell level, the generative function is performed by nucleic acids (making up the substance of the chromosomes). These structures will be considered by us in the future.

We also add that the preservation and transfer of hereditary information to descendants is carried out by a single mechanism, which is completely independent of the level of organization of the individual, that is, for the virus, for bacteria, and for humans, it is universal.

What is the substance of heredity

In this paper, we study chromatin, the structure and functions of which directly depend on the organization of nucleic acid molecules. The Swiss scientist Micher in 1869 in the nuclei of the cells of the immune system, compounds showing acid properties, which he named first a nuclein, and then nucleic acids, were found. From the point of view of chemistry, these are high-molecular compounds - polymers. Their monomers are nucleotides having the following structure: purine or pyrimidine base, pentose and orthophosphoric acid residue . Scientists have established that in the cells there can be two kinds of nucleic acids: DNA and RNA. They enter the complex with proteins and form the substance of chromosomes. Just like proteins, nucleic acids have several levels of spatial organization.

In 1953, Nobel laureates Watson and Crick deciphered the structure of DNA. It is a molecule consisting of two chains connected by hydrogen bonds between the nitrogenous bases on the principle of complementarity (opposite the adenine is the thymine base, opposite the cytosine is guanine). Chromatin, the structure and functions of which we are studying, contains molecules of deoxyribonucleic and ribonucleic acid of various configurations. On this issue, we will dwell in more detail in the section "Levels of chromatin organization."

Localization of the substance of heredity in the cell

DNA is present in such cytostructures as the nucleus, as well as in organelles capable of division - mitochondria and chloroplasts. This is due to the fact that these organelles perform the most important functions in the cell: the synthesis of ATP, as well as the synthesis of glucose and the formation of oxygen in plant cells. At the synthetic stage of the life cycle, the maternal organelles are doubled. Thus, daughter cells as a result of mitosis (division of somatic cells) or meiosis (the formation of eggs and spermatozoa) receive the necessary arsenal of cellular structures that provide cells with nutrients and energy.

Ribonucleic acid is composed of one chain and has a lower molecular weight than DNA. It is contained both in the nucleus and in the hyaloplasm, and also forms part of many cellular organelles: ribosomes, mitochondria, endoplasmic reticulum, plastids. Chromatin in these organelles is associated with histone proteins and is part of plasmids - ring closed DNA molecules.

Chromatin and its structure

So, we have established that nucleic acids are contained in the substance of chromosomes - the structural units of heredity. Their chromatin under the electron microscope has the form of granules or filamentous formations. It contains, in addition to DNA, also RNA molecules, as well as proteins that exhibit basic properties and are called histones. All of the above structures are part of the nucleosomes. They are contained in the chromosomes of the nucleus and are called fibrils (filaments-solenoids). Summarizing all of the above, we will determine what is chromatin. It is a complex compound of deoxyribonucleic acid and special proteins - histones. Two-stranded DNA molecules are wound on them, like coils, forming nucleosomes.

Levels of chromatin organization

The substance of heredity has a different structure, which depends on many factors. For example, from what stage of the life cycle the cell experiences: the period of division (metoz or meiosis), the presynthetic or synthetic period of the interphase. From the shape of the solenoid, or fibril, as the simplest, further chromatin compactification occurs. Heterochromatin - a denser state, is formed in the intron sections of the chromosome, on which transcription is impossible. In the period of cell rest - interphases, when there is no fission process, - heterochromatin is located in the karyoplasm of the nucleus around the periphery, near its membrane. The compaction of nuclear contents occurs in the post-synthetic stage of the life cycle of the cell, that is, just before division.

What determines the condensation of the substance of heredity

Continuing to study the question "what is chromatin," scientists have established that its compaction depends on the histone proteins that enter the nucleosomes along with the DNA and RNA molecules. They consist of proteins of four kinds, called crustaceans and linkers. At the time of transcription (reading information from genes with RNA), the heredity substance is slightly condensed and is called euchromatin.

At present, the distribution of DNA molecules associated with histone proteins continues to be studied. For example, scientists have found that the chromatin of different loci of the same chromosome differs by the level of condensation. For example, at the points of attachment to the chromosome, the filaments of the fission spindle, called centromeres, are denser than in the telomeric regions-the end loci.

Genes-regulators and chromatin composition

In the concept of regulation of gene activity, created by French geneticists Jacob and Mono, an idea is given about the existence of deoxyribonucleic acid sites in which there is no information on protein structures. They perform purely bureaucratic - managerial functions. Called the regulator genes, these chromosome parts, as a rule, are devoid of histone proteins in their structure. Chromatin, the determination of which was carried out by the method of sequencing, was called open.

In the course of further studies, it was found that these loci contain nucleotide sequences that prevent the attachment of protein particles to DNA molecules. Such areas contain regulatory genes: promoters, enhancers, activators. The chromatin compacting in them is high, and the length of these sections on the average is about 300 nm. There is a biochemical method for the determination of open chromatin in isolated nuclei, in which a DNA-ase enzyme is used. He very quickly splits the loci of chromosomes, devoid of histone proteins. Chromatin in these areas was called supersensitive.

The role of the substance of heredity

Complexes including DNA, RNA and protein called chromatin participate in the ontogenesis of cells and change their composition depending on the type of tissue, as well as on the stage of development of the organism as a whole. For example, in epithelial cells of the skin, genes such as the enhancer and promoter are blocked by repressor proteins, and these regulatory genes in the secretory cells of the intestinal epithelium are active and located in the open chromatin zone. Genetic scientists have established that the proportion of DNA that does not encode proteins accounts for more than 95% of the entire human genome. This means that the controlling genes are much larger than those responsible for the synthesis of peptides. The introduction of such methods as DNA chips and sequencing made it possible to find out what chromatin is and, as a consequence, to map the human genome.

Studies of chromatin are very important in such branches of science as human genetics and medical genetics. This is due to the sharply increased level of the appearance of hereditary diseases, both genes and chromosomes. Early detection of these syndromes increases the percentage of positive predictions in their treatment.