Cell nucleus

The nucleus (s. karyon) is present in all human cells except erythrocytes and thrombocytes. The functions of the nucleus are to store and transmit hereditary information to new (daughter) cells. These functions are associated with the presence of DNA in the nucleus. The synthesis of proteins - ribonucleic acid RNA and ribosomal materials - also occurs in the nucleus.

Most cells have a spherical or ovoid nucleus, but there are also other forms of the nucleus (ring-shaped, rod-shaped, spindle-shaped, bead-shaped, bean-shaped, segmented, pear-shaped, polymorphic). The size of the nucleus varies widely - from 3 to 25 µm. The largest nucleus is found in the egg cell. Most human cells are mononuclear, but there are binuclear ones (some neurons, hepatocytes, cardiomyocytes). Some structures are multinuclear (muscle fibers). The nucleus has a nuclear membrane, chromatin, nucleolus and nucleoplasm.

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

The nuclear membrane contains numerous round nuclear pores, each 50-70 nm in diameter. Nuclear pores occupy up to 25% of the nuclear surface in total. The number of pores in one nucleus reaches 3000-4000. At the edges of the pores, the outer and inner membranes are connected to each other and form the so-called pore ring. Each pore is closed by a diaphragm, which is also called a pore complex. The pore diaphragms have a complex structure; they are formed by protein granules connected to each other. Selective transport of large particles and the exchange of substances between the nucleus and the cytosodeme of the cell are carried out through nuclear pores.

Under the nuclear membrane are the nucleoplasm (karyoplasm) (nucleoplasm, s. karyoplasma), which has a homogeneous structure, and the nucleolus. In the nucleoplasm of a non-dividing nucleus, in its nuclear protein matrix, are located osmiophilic granules (clumps) of the so-called heterochromatin. Areas of more loosened chromatin, located between the granules, are called euchromatin. Loose chromatin is also called decondensed chromatin, in which synthetic processes occur most intensively. During cell division, chromatin is compacted, condensed, and forms chromosomes.

Chromatin (chromatinum) of a non-dividing nucleus and chromosomes of a dividing nucleus are formed by molecules of deoxyribonucleic acid (DNA) associated with ribonucleic acid (RNA) and proteins - histones and non-histones. It should be emphasized that chromatin and chromosomes are chemically identical.

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

Hereditary information in DNA molecules is recorded in the linear sequence of its nucleotides. The elementary particle of heredity is the gene. A gene is a section of DNA that has a specific sequence of nucleotides responsible for the synthesis of one specific protein.

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

DNA molecules bound to histone proteins form nucleosomes, which are the structural units of chromatin. A nucleosome has the appearance of a bead with a diameter of 10 nm. Each nucleosome consists of histones around which a section of DNA is twisted, including 146 pairs of nucleotides. Between the nucleosomes are linear sections of DNA consisting of 60 pairs of nucleotides.

Chromatin is represented by fibrils that form loops about 0.4 µm long, containing from 20,000 to 30,000 nucleotide pairs.

As a result of compaction (condensation) and twisting (superspecialization) of deoxyribonucleoproteins (DNP) in the dividing nucleus, chromosomes become visible. These structures - chromosomes (chromasomae, from the Greek chroma - paint, soma - body) - are elongated rod-shaped formations with two arms separated by the so-called constriction - the centromere. Depending on the location of the centromere and the relative position and length of the arms (legs), three types of chromosomes are distinguished: metacentric, with approximately the same arms; submetacentric, in which the length of the arms varies; acrocentric, in which one arm is long and the other is very short, barely noticeable. The chromosome has eu- and heterochromatic regions. The latter remain compact in the non-dividing nucleus and in the early prophase of mitosis. The alternation of eu- and heterochromatic regions is used to identify chromosomes.

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

The nucleolus, one or more, is found in all non-dividing cells. It has the appearance of an intensely stained round body, the size of which is proportional to the intensity of protein synthesis. The nucleolus consists of an electron-dense nucleolonema (from the Greek peta - thread), in which a filamentous (fibrillar) part is distinguished, consisting of many intertwined RNA threads about 5 nm thick, and a granular part. The granular (granular) part is formed by grains about 15 nm in diameter, which are RNP particles - precursors of ribosomal subunits. Perinucleolar chromatin is embedded in the depressions of the nucleolonema. Ribosomes are formed in the nucleolus.

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