Biochemical research in genetics is an important way to study its basic elements - chromosomes and genes. In this article, we will consider what chromatin is, and 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 saving 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 (the constituent substance of the chromosomes). These structures will be considered by us in the future.
We also add that the preservation and transmission 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, and for bacteria, and for humans it is universal.
What is the substance of heredity
In this work, we study chromatin, the structure and functions of which directly depend on the organization of nucleic acid molecules. In 1869, a Swiss scientist Mischer discovered compounds in the nuclei of the cells of the immune system that exhibit the properties of acids, first called nuclein and then nucleic acid. From the point of view of chemistry, these are high molecular weight compounds - polymers. Their monomers are nucleotides with the following structure: purine or pyrimidine base, pentose and the residue of phosphoric acid. Scientists have determined that two types of nucleic acids can be present in cells : DNA and RNA. They enter into a complex with proteins and form the substance of chromosomes. 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 interconnected by hydrogen bonds arising between nitrogenous bases according to the principle of complementarity (the thymine base is located opposite the adenine, the guanine opposite the cytosine). Chromatin, the structure and functions of which we are studying, contains deoxyribonucleic and ribonucleic acid molecules of various configurations. We will dwell on this issue in more detail in the section “Levels of chromatin organization”.
Localization of the substance of heredity in the cell
DNA is present in cytostructures such as the nucleus, 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 oxygen formation in plant cells. At the synthetic stage of the life cycle, maternal organelles double. Thus, daughter cells as a result of mitosis (division of somatic cells) or meiosis (the formation of eggs and sperm) receive the necessary arsenal of cell structures that provide cells with nutrients and energy.
Ribonucleic acid consists of a single chain and has a lower molecular weight than DNA. It is found both in the nucleus and in the hyaloplasm, and is also part of many cellular organelles: ribosomes, mitochondria, endoplasmic reticulum, plastids. Chromatin in these organelles is associated with histone proteins and is a part of plasmids - ring closed DNA molecules.
Chromatin and its structure
So, we found that nucleic acids are contained in the substance of chromosomes - structural units of heredity. Their chromatin under the electron microscope has the form of granules or filamentous formations. It contains, in addition to DNA, RNA molecules, as well as proteins that exhibit basic properties and are called histones. All of the above structures are part of nucleosomes. They are contained in the chromosomes of the nucleus and are called fibrils (filament-solenoids). To summarize all of the above, we will determine what chromatin is. This is a complex compound of deoxyribonucleic acid and special proteins - histones. Double-stranded DNA molecules are wound on them, like on coils, forming nucleosomes.
Chromatin Organization Levels
The substance of heredity has a different structure, which depends on many factors. For example, from what stage of the life cycle the cell undergoes: the period of division (metosis or meiosis), the presynthetic or synthetic period of interphase. From the form of the solenoid, or fibril, as the simplest, further chromatin compactification occurs. Heterochromatin is a more dense state, formed in intron regions of the chromosome on which transcription is impossible. During the resting period of the cell — interphase, when there is no process of division — heterochromatin is located in the karyoplasm of the nucleus at the periphery, near its membrane. Compaction of the nuclear contents occurs in the postsynthetic stage of the cell life cycle, that is, immediately before division.
What determines the condensation of a substance of heredity
Continuing to study the question of “what is chromatin,” scientists have found that its compaction depends on histone proteins, which are part of nucleosomes along with DNA and RNA molecules. They consist of four types of proteins called core and linker. At the moment of transcription (reading information from genes using RNA), the substance of heredity is weakly condensed and is called euchromatin.
Currently, the distribution of DNA molecules associated with histone proteins continues to be studied. For example, scientists have found that chromatin of different loci of the same chromosome differs in the level of condensation. For example, at the points of attachment to the chromosome of the strands of the division spindle, called centromeres, it is more dense than in telomeric regions - terminal loci.
Regulatory genes and chromatin composition
The concept of regulation of gene activity, created by French geneticists Jacob and Mono, 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 regulatory 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 studies, it was found that nucleotide sequences are located at these loci that prevent protein particles from attaching to DNA molecules. Such sites contain regulatory genes: promoters, enhancers, activators. The chromatin compaction in them is high, and the length of these sites averages about 300 nm. There is a biochemical method for determining open chromatin in isolated nuclei, in which the enzyme is used, DNA-ase. It very quickly breaks down chromosome loci lacking histone proteins. Chromatin in these areas was called hypersensitive.
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 skin cells, genes such as the enhancer 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 non-protein-encoding DNA accounts for more than 95% of the entire human genome. This means that there are many more control genes 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 research is very important in such fields of science as human genetics and medical genetics. This is due to a sharp increase in the occurrence of hereditary diseases - both gene and chromosomal. Early detection of these syndromes increases the percentage of positive predictions in their treatment.