Biochemical research in genetics is an important way to study its main elements - chromosomes and genes. In this article we will look at what chromatin is, find out its structure and function in the cell.
To the main processes characterizing organisms,living on Earth include respiration, nutrition, growth, excretion and reproduction. The latter function is the most significant for the preservation of 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 (constituting the substance of chromosomes). These structures will be considered by us in the future.
We also add that saving and transferringhereditary 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, and for bacteria, and for man, it is universal.
In this paper, we study chromatin, structure andwhose functions are directly dependent on the organization of nucleic acid molecules. In 1869, a Swiss scientist, in the nuclei of cells of the immune system, discovered compounds showing the properties of acids, which he called first nuclein and then nucleic acids. From the point of view of chemistry, these are high-molecular compounds - polymers. Their monomers are nucleotides having the following structure: a purine or pyrimidine base, pentose, and orthophosphoric acid residue. Scientists have found that two types of nucleic acids can be present in cells: DNA and RNA. They enter into a complex with proteins and form a substance of chromosomes. Like proteins, nucleic acids have several levels of spatial organization.
In 1953, the Nobel Prize winnersWatson and Crick deciphered the structure of DNA. It is a molecule consisting of two chains interconnected by hydrogen bonds arising between nitrogen bases on the principle of complementarity (opposite adenine is thymine base, opposite cytosine - guanine). Chromatin, the structure and functions of which we study, contains deoxyribonucleic and ribonucleic acid molecules of various configurations. We will dwell on this question in more detail in the section “Levels of chromatin organization”.
DNA is present in cytostructures such asnucleus, as well as in organelles capable of division - mitochondria and chloroplasts. This is due to the fact that these organoids perform the most important functions in the cell: ATP synthesis, as well as glucose synthesis and the formation of oxygen in plant cells. At the synthetic stage of the life cycle maternal organelles double. Thus, the 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 consists of one chain andhas a lower molecular weight than DNA. It is contained both in the nucleus and in hyaloplasm, and also forms part of many cellular organoids: ribosomes, mitochondria, endoplasmic reticulum, and plastids. Chromatin in these organelles is associated with histone proteins and is part of the plasmids - closed circular DNA molecules.
So, we found that nucleic acidscontained in the substance of chromosomes - structural units of heredity. Their chromatin under an 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 the above, we define what chromatin. This is a complex compound of deoxyribonucleic acid and special proteins - histones. On them, as on coils, double-stranded DNA molecules are wound, forming nucleosomes.
The substance of heredity has a differentstructure that depends on many factors. For example, depending on what stage of the life cycle a cell experiences: the period of division (metosis or meiosis), the presynthetic or synthetic interphase period. From the form of the solenoid, or fibrils, as the most simple, further compaction of chromatin occurs. Heterochromatin - a denser state, is formed in the intron sites of the chromosome, on which transcription is impossible. During the resting period of the cell - interphase, when there is no division process - heterochromatin is located in the karyoplasm of the nucleus along the periphery, near its membrane. Compaction of nuclear contents occurs in the postsynthetic stage of the cell's life cycle, that is, immediately before division.
Continuing to study the question "what is chromatin,"Scientists have established that its compaction depends on histone proteins, which are part of nucleosomes along with DNA and RNA molecules. They are made up of four types of proteins called core and linker proteins. At the time of transcription (reading information from genes using RNA), the substance of heredity is weakly condensed and is called euchromatin.
Currently, the distribution featuresDNA molecules bound to histone proteins continue to be studied. For example, scientists have found that the chromatin of different loci of the same chromosome differs in the level of condensation. For example, in the places of attachment to the chromosome of the threads of the spindle of division, called centromere, it is more dense than in the telomeric regions - terminal loci.
In the concept of regulation of gene activity,created by French geneticists Jacob and Mono, it gives an idea of the existence of deoxyribonucleic acid sites in which there is no information about the structures of proteins. They perform purely bureaucratic - managerial functions. Being called regulator genes, these parts of chromosomes, as a rule, are devoid of histone proteins in their structure. Chromatin, the determination of which was carried out by sequencing, was called open.
In the course of further research it was foundthat in these loci nucleotide sequences are located that prevent protein particles from attaching to the DNA molecules. Such sites contain regulatory genes: promoters, echancers, activators. Compaction of chromatin in them is high, and the length of these areas on average is about 300 nm. There is a biochemical method for the determination of open chromatin in isolated nuclei using the DNA-ase enzyme. It very quickly cleaves chromosome loci lacking histone proteins. Chromatin in these areas has been identified as oversensitive.
Complexes, including DNA, RNA and protein,called chromatin, participate in the ontogeny 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 the epithelial cells of the skin, such genes as echancer and promoter are blocked by repressor proteins, and the same regulatory genes in the secretory cells of the intestinal epithelium are active and are located in the open chromatin zone. Genetic scientists have found that more than 95% of the entire human genome accounts for DNA that does not encode proteins. This means that there are much more gene control than those responsible for peptide synthesis. The introduction of methods such as DNA chips and sequencing made it possible to find out what chromatin is and, as a result, to map the human genome.
Chromatin studies are very important in suchbranches of science, like human genetics and medical genetics. This is due to the sharply increased incidence of hereditary diseases, both gene and chromosomal. Early detection of these syndromes increases the percentage of positive prognoses for their treatment.
