Cell nucleus

The nucleus (nucleus, s. Karyon) is present in all human cells, except for erythrocytes and platelets. Kernel functions - storage and transfer to the new (child) cells of hereditary information. These functions are related to the presence of DNA in the nucleus. In the nucleus there is also a synthesis of proteins - ribonucleic acid RNA and ribosomal materials.

In most cells, the nucleus is spherical or ovoid; however, other forms of the nucleus (ring-shaped, rod-shaped, fusiform, clairvoid, bean-shaped, segmented, pyriform, polymorphous) are also found. The dimensions of the core vary widely, from 3 to 25 μm. The largest nucleus is an ovum. Most human cells are single-nucleated, but there are dual-core cells (some neurons, hepatocytes, cardiomyocytes). Some structures are multinucleated (muscle fibers). The nucleus distinguishes between the nuclear envelope, chromatin, nucleolus and nucleoplasm.

The nuclear envelope, or caryotheca, separating the contents of the nucleus from the cytoplasm, consists of internal and external nuclear membranes 8 nm thick each. The membranes are separated by a perinuclear space (karyoteca cistern) 20-50 nm wide, which contains fine-grained material of moderate electron density. The outer nuclear membrane passes into a granular endoplasmic reticulum. Therefore, the perinuclear space forms a single cavity with an endoplasmic reticulum. The inner nuclear membrane is internally connected to a branched network of protein fibrils consisting of individual subunits.

In the nuclear shell, there are many rounded nuclear pores with a diameter of 50-70 nm each. Nuclear pores occupy as much as 25% of the core surface in general. The amount of pore per core reaches 3000-4000. At the edges of the pores, the outer and inner membranes join one another and form a so-called pore ring. Each pore is closed by a diaphragm, which is also called the pore complex. The pore apertures have a complex structure, they are formed by interconnected protein granules. Through nuclear pores selective transport of large particles, as well as the exchange of substances between the nucleus and the cytosode of the cell.

Under the nuclear envelope are nucleoplasm (karyoplasm) (nucleoplasma, S. Karyoplasma), which has a homogeneous structure, and nucleolus. In the nucleoplasm of a non-fissile core, in its nuclear protein matrix, osmiophil granules (lumps) of the so-called heterochromatin are located. The areas of more loose chromatin located between the granules are called euchromatin. Loose chromatin is also called decondensed chromatin, in which synthetic processes occur most intensely. During cell division, chromatin thickens, condenses, forms chromosomes.

The chromatinum of the non-fading nucleus and the fissioning chromosome are formed by molecules of deoxyribonucleic acid (DNA) bound to ribonucleic acid (RNA) and histone and non-histone proteins. It should be emphasized the chemical identity of chromatin and chromosomes.

Each DNA molecule consists of two long right-wound polynucleotide chains (double helices), and each nucleotide is made up of a nitrogenous base, glucose and a phosphoric acid residue. The base is located inside the double helix, and the sugar-phosphate skeleton is outside.

Hereditary information in DNA molecules is recorded in a linear sequence of its nucleotides. An elementary part of heredity is the gene. A gene is a region of DNA that has a specific sequence of nucleotides responsible for the synthesis of one particular specific protein.

The DNA molecule in the nucleus is compactly packed. Thus, one DNA molecule containing 1 million nucleotides, with their linear arrangement, would occupy a length of only 0.34 mm. The length of one human chromosome in the stretched form is about 5 cm, but in the compacted state the chromosome has a volume of about 10 ^-15 cm ³.

DNA molecules associated with histone proteins form nucleosomes, which are the structural units of chromatin. The nucleosome has the form of a bead with a diameter of 10 nm. Each nucleosome consists of gynae, around which a DNA segment is wrapped, comprising 146 pairs of nucleotides. Between nucleosomes are located linear sections of DNA, consisting of 60 pairs of nucleotides.

Chromatin is represented by fibrils, which form loops of about 0.4 μm in length, containing from 20,000 to 30,000 pairs of nucleotides.

As a result of condensation (condensation) and twisting (superpecialization) of deoxyribonucleoproteins (DNP) in the fissioning nucleus, chromosomes become prominent. These structures - the chromosomes (chromasomae, from the Greek chroma - the paint, soma - the body) - are elongated rod-like formations having two arms separated by a so-called constriction-centromere. Depending on the location of the centromere and the relative location and length of the arms (legs), three types of chromosomes are distinguished: metacentric, having approximately the same shoulders; submetacentric, in which the length of the arms is different; acrocentric, with one shoulder long, and the other very short, barely noticeable. In the chromosome there are eu and heterochromatic regions. The latter in the nascent nucleus and in the early prophase of mitosis remain compact. The alternation of eu- and heterochromatin sections is used to identify chromosomes.

The surface of chromosomes is covered by various molecules, mainly ribonucleoproteins (RNP). In somatic cells there are 2 copies of each chromosome, they are called homologous. They are the same in length, shape, structure, carry the same genes, which are located the same. Features of the structure, the number and size of chromosomes are called karyotypes. A normal human karyotype consists of 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Human somatic cells (diploid) have twice the number of chromosomes - 46. Sex cells contain a haploid (single) set - 23 chromosomes. Therefore, in DNA, DNA is 2 times less than in diploid somatic cells.

The nucleolus (nucleolus), one or more, is detected in all nondividing cells. It has the form of an intensely colored round body, the magnitude of which is proportional to the intensity of protein synthesis. The nucleolus consists of an electronically dense nucleolone (from the Greek peta-thread), in which a filamentous (fibrillar) part is distinguished, consisting of a set of interlacing RNA strands about 5 nm thick and a granular part. The granular (granular) part is formed by grains with a diameter of about 15 nm, which are particles of RNP - precursors of ribosomal subunits. Perianopercular chromatin is introduced into the depressions of the nucleolone. In the nucleolus, ribosomes are formed.

[1], [2], [3], [4], [5]