Chromatin (chromosomes) are the structural components of the nucleus. The concept of a karyotype
Biochemical research in genetics is an important way to study its basic elements - chromosomes and genes. In this article we will look at what chromatin is, find out its structure and function in the cell.
Heredity is the main property of living matter
The main processes that characterize organisms living on Earth include respiration, nutrition, growth, excretion and reproduction. The latter function is the most important 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 (the constituent substance of chromosomes). These structures will be considered by us later.
Let us also add that the preservation and transmission of hereditary information to descendants is carried out according to a single mechanism that does not depend at all on the level of organization of an individual, that is, for a virus, 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, the Swiss scientist Miescher discovered compounds exhibiting the properties of acids in the nuclei of cells of the immune system, which he called first nuclein, and then nucleic acids. 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 residue. Scientists have established that two types of RNA can be present in cells. They enter into a complex with proteins and form the substance of the chromosomes. Like proteins, nucleic acids have several levels of spatial organization.
In 1953, the structure of DNA was deciphered by Nobel laureates Watson and Crick. It is a molecule consisting of two chains interconnected by hydrogen bonds that arise between nitrogenous bases according to the principle of complementarity (opposite to adenine is thymine base, opposite to cytosine - guanine). Chromatin, the structure and function of which we are studying, contains molecules of deoxyribonucleic and ribonucleic acids 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 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: as well as the synthesis of glucose and the formation of oxygen in plant cells. At the synthetic stage of the life cycle, the parent organelles double. Thus, as a result of mitosis (division of somatic cells) or meiosis (formation of eggs and sperm), daughter cells receive the necessary arsenal of cellular structures that provide cells with nutrients and energy.
Ribonucleic acid consists of a single strand and has a lower molecular weight than DNA. It is contained 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 part of plasmids - circular 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 an electron microscope looks like granules or threadlike 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 are nucleosomes. They are found in the chromosomes of the nucleus and are called fibrils (solenoid filaments). Summarizing all of the above, let's define what chromatin is. It is a complex compound and special proteins - histones. Double-stranded DNA molecules are wound on them, like on coils, to form nucleosomes.
Levels of chromatin organization
The substance of heredity has a different structure, which depends on many factors. For example, on what stage of the life cycle the cell is going through: the period of division (meiosis or meiosis), the presynthetic or synthetic period of the interphase. From the form of the solenoid, or fibril, as the simplest form, further chromatin compaction occurs. Heterochromatin is a denser state; it is formed in intron regions of the chromosome where 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 the nuclear contents occurs in the post-synthetic stage of the cell's life cycle, that is, immediately before division.
What determines the condensation of the substance of heredity
Continuing to study the question of "what is chromatin", scientists have found that its compaction depends on the histone proteins, which, along with DNA and RNA molecules, are part of the nucleosomes. They are composed of four types of proteins called core and linker proteins. At the time of transcription (reading information from genes using RNA), the heredity substance is weakly condensed and is called euchromatin.
Currently, the features of the distribution of DNA molecules associated with 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 filaments of the division spindle, called centromeres, it is denser than in telomeric regions - terminal loci.
Regulatory genes and chromatin composition
The concept of the regulation of gene activity, created by the French geneticists Jacob and Monod, gives an idea of the existence of deoxyribonucleic acid regions in which there is no information about the structures of proteins. They perform purely bureaucratic - managerial functions. 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 found that nucleotide sequences are located in these loci that prevent protein particles from attaching to DNA molecules. Such sites contain regulatory genes: promoters, enhancers, activators. Compaction of chromatin in them is high, and the length of these regions is on average about 300 nm. There is a definition of open chromatin in isolated nuclei, in which the enzyme DNase is used. It very quickly cleaves chromosome loci lacking histone proteins. Chromatin in these areas has been called hypersensitive.
The role of the substance of heredity
Complexes, including DNA, RNA and protein, called chromatin, are involved 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 an enhancer and a promoter are blocked by repressor proteins, and the same regulatory genes in secretory cells of the intestinal epithelium are active and are located in the zone of open chromatin. Genetic scientists have found that DNA, which does not code for proteins, accounts for more than 95% of the entire human genome. This means that there are many more control genes than those responsible for the synthesis of peptides. The introduction of methods such as DNA chips and sequencing made it possible to find out what chromatin is, and, as a result, to carry out mapping of the human genome.
Chromatin research is very important in branches of science such as human genetics and medical genetics. This is due to the sharply increased level of occurrence of hereditary diseases, both genetic and chromosomal. Early detection of these syndromes increases the percentage of positive prognosis in their treatment.
Chromatin (from the Greek. Chroma - paint color) is the main structure of the interphase nucleus, which is very well stained with basic dyes and determines the chromatin pattern of the nucleus for each type of cell.
Due to the ability to stain well with various dyes and especially the main ones, this component of the nucleus is called "chromatin" (Flemming 1880).
Chromatin is a structural analogue of chromosomes and in the interphase nucleus is DNA-bearing bodies.
Two types of chromatin are morphologically distinguished:
1) heterochromatin;
2) euchromatin.
Heterochromatin(heterochromatinum) corresponds to chromosome regions partially condensed in the interphase and is functionally inactive. This chromatin stains very well and can be seen on histological preparations.
Heterochromatin, in turn, is divided into:
1) structural; 2) optional.
Structural Heterochromatin represents regions of chromosomes that are constantly in a condensed state.
Optional heterochromatin is a heterochromatin capable of decondensing and converting to euchromatin.
Euchromatin- these are chromosome regions decondensed in interphase. It is a working, functionally active chromatin. This chromatin is not stained and is not detected on histological preparations.
During mitosis, all euchromatin condenses as much as possible and is part of chromosomes. During this period, the chromosomes do not perform any synthetic functions. In this regard, the chromosomes of cells can be in two structural and functional states:
1) active (working), sometimes they are partially or completely decondensed and with their participation in the nucleus the processes of transcription and reduplication take place;
2) inactive (non-working, metabolic dormancy), when they are maximally condensed, they perform the function of distributing and transferring genetic material to daughter cells.
Sometimes, in some cases, the whole chromosome during the interphase period can remain in a condensed state, while it looks like a smooth heterochromatin. For example, one of the X chromosomes of somatic cells of a female body is subject to heterochromatization at the initial stages of embryogenesis (during cleavage) and does not function. This chromatin is called sex chromatin or Barr's bodies.
In different cells, sex chromatin has a different appearance:
a) in neutrophilic leukocytes - a type of drumstick;
b) in the epithelial cells of the mucous membrane - a kind of hemispherical lump.
Determination of sex chromatin is used to establish the genetic sex, as well as to determine the number of X chromosomes in the karyotype of an individual (it is equal to the number of sex chromatin bodies + 1).
Electron microscopic studies revealed that preparations of isolated interphase chromatin contain elementary chromosomal fibrils 20-25 nm thick, which consist of 10 nm thick fibrils.
Chemically, chromatin fibrils are complex complexes of deoxyribonucleoproteins, which include:
b) special chromosomal proteins;
The quantitative ratio of DNA, protein and RNA is 1: 1.3: 0.2. The share of DNA in the chromatin preparation is 30-40%. The length of individual linear DNA molecules varies in an indirect range and can reach hundreds of micrometers and even centimeters. The total length of DNA molecules in all chromosomes of one human cell is about 170 cm, which corresponds to 6x10 -12 g.
Chromatin proteins make up 60-70% of its dry weight and are presented in two groups:
a) histone proteins;
b) non-histone proteins.
Yo Histone proteins (histones) - alkaline proteins containing basic amino acids (mainly lysine, arginine) are located unevenly in the form of blocks along the length of the DNA molecule. One block contains 8 histone molecules that form a nucleosome. The size of the nucleosome is about 10 nm. The nucleosome is formed by compaction and supercoiling of DNA, which leads to a shortening of the length of the chromosomal fibril by about 5 times.
Yo Non-histone proteins make up 20% of the amount of histones and in the interphase nuclei form a structural network inside the nucleus, which is called the nuclear protein matrix. This matrix represents the backbone that determines the morphology and metabolism of the nucleus.
Perichromatin fibrils have a thickness of 3-5 nm, granules have a diameter of 45 nm and interchromatin granules have a diameter of 21-25 nm.
Chromatin represents are proteins (non-histone and histone) and a complex of nucleic acids (RNA and DNA), which together form highly ordered structures in space - eukaryotic chromosomes.
In chromatin, the ratio of protein to DNA is approximately 1: 1, the bulk of the protein is represented by histones.
Types of chromatin
By its structure, chromatin is heterogeneous. All chromatin is conventionally divided into two functional categories:
1) inactive - heterochromatin - contains at the moment unreadable genetic information;
2) active - euchromatin - it is from him that the genetic information is read.
The ratio of the content of heterochromatin and euchromatin is constantly in a mobile stage. Mature cells, such as blood, have nuclei characterized by condensed, densest chromatin lying lumps.
In the nuclei of somatic female cells, lumps of chromatin are close to the membrane of the nucleus - this is the female chromatin of the germ cell.
Sexual male chromatin is represented by a lump in male somatic cells, glowing when stained with fluorochromes. Sex chromatin makes it possible to establish the sex of the unborn child from the cells obtained from the amniotic fluid of a pregnant woman.
Chromatin structure
Chromatin - nucleoprotein of the cell nucleus, which is the main constituent of chromosomes.
Chromatin composition:
Histones - 30-50%;
Non-histone proteins - 4-33%;
DNA - by weight 30-40%;
Depending on the nature of the object, as well as on the method of chromatin isolation, the sizes of DNA molecules, the number of RNA, non-histone proteins vary widely.
Chromatin functions
Chromatin and chromosome in chemical organization (DNA complex with proteins) do not differ from each other, they mutually pass into each other.
In the interphase, it is not possible to distinguish between individual chromosomes. They are weakly coiled and form loosened chromatin, which is distributed throughout the entire volume of the nucleus. It is precisely the loosening of the structure that is considered a required condition for transcription, the transmission of information of a hereditary nature that is present in DNA.
Karyotype
Karyotype (from karyo ... and Greek tepos - sample, shape, type), chromosome set, a set of characteristics of chromosomes (their number, size, shape and details of the microscopic structure) in the cells of the body of an organism of one type or another. The concept of a karyotype was introduced by the Sov. geneticist G. A. Levitsky (1924). The karyotype is one of the most important genetic characteristics of the species, because each species has its own karyotype, which differs from the karyotype of closely related species (this is the basis for a new branch of taxonomy - the so-called karyosystematics).
8. Features of the morphological and functional structure of chromosomes. Hetero- and euchromatin. (one answer to 2 questions).
Chromosomes: structure and classification
Chromosomes(Greek - chromo- Colour, soma- body) is a spiralized chromatin. Their length is 0.2 - 5.0 μm, diameter is 0.2 - 2 μm.
Metaphase chromosome consists of two chromatids that connect centromere (primary constriction). She divides the chromosome in two shoulder... Individual chromosomes have secondary constrictions... The area that they separate is called companion, and such chromosomes are satellite. The ends of the chromosomes are called telomeres... Each chromatid contains one continuous DNA molecule in conjunction with histone proteins. Intensively stained areas of chromosomes are areas of strong spiralization (heterochromatin). Lighter areas are areas of weak spiralization (euchromatin).
The types of chromosomes are distinguished by the location of the centromere.
1. Metacentric chromosomes- the centromere is located in the middle, and the shoulders are the same length. The area of the shoulder near the centromere is called proximal, the opposite is called distal.
2. Submetacentric chromosomes- the centromere is displaced from the center and the shoulders have different lengths.
3. Acrocentric chromosomes- the centromere is strongly displaced from the center and one arm is very short, the second arm is very long.
In the cells of the salivary glands of insects (Drosophila flies), there are giant, polytene chromosomes(multi-filamentous chromosomes).
There are 4 rules for the chromosomes of all organisms:
1. The rule of constancy of the number of chromosomes... Normally, organisms of certain species have a constant, characteristic number of chromosomes. For example: a person has 46, a dog has 78, a fruit fly has 8.
2. Pairing chromosomes... In a diploid set, normally each chromosome has a paired chromosome - the same in shape and size.
3. Chromosome personality... Chromosomes of different pairs differ in shape, structure and size.
4. Continuity of chromosomes... When the genetic material is duplicated, the chromosome is formed from the chromosome.
The set of chromosomes of a somatic cell, characteristic of an organism of a given species, is called karyotype .
1. Chromosomes, which are the same in the cells of male and female organisms, are called autosomes
idiogram
The classification of chromosomes is carried out according to various criteria.
1. Chromosomes, which are the same in the cells of male and female organisms, are called autosomes... In humans, there are 22 pairs of autosomes in the karyotype. Chromosomes that are different in the cells of male and female organisms are called heterochromosomes, or sex chromosomes... In a man, these are the X and Y chromosomes, in a woman - X and X.
2. The arrangement of chromosomes in decreasing size is called idiogram... This is a classified karyotype. Chromosomes are arranged in pairs (homologous chromosomes). The first pair are the largest, the 22nd pair are the smallest and the 23rd pair are the sex chromosomes.
3. In 1960. was offered Denver classification chromosomes. It is built on the basis of their shape, size, centromere position, the presence of secondary constrictions and satellites. An important indicator in this classification is centromeric index(QI). This is the ratio of the length of the short arm of the chromosome to its entire length, expressed as a percentage. All chromosomes are divided into 7 groups. Groups are designated by Latin letters from A to G.
Group A includes 1 - 3 pairs of chromosomes. These are large metacentric and submetacentric chromosomes. Their QI is 38-49%.
Group B... The 4th and 5th pairs are large metacentric chromosomes. QI 24-30%.
Group C... Pairs of chromosomes 6 - 12: medium size, submetacentric. QI 27-35%. This group also includes the X chromosome.
Group D... 13 - 15th pairs of chromosomes. Chromosomes are acrocentric. QI is about 15%.
Group E... Pairs of chromosomes 16 - 18. Relatively short, metacentric or submetacentric. QI 26-40%.
Group F... 19th - 20th pairs. Short, submetacentric chromosomes. QI 36-46%.
Group G... 21-22nd pairs. Small, acrocentric chromosomes. QI 13-33%. The Y chromosome also belongs to this group.
4. Paris classification human chromosomes created in 1971. With the help of this classification, it is possible to determine the localization of genes in a particular pair of chromosomes. Using special staining methods, a characteristic order of alternation of dark and light stripes (segments) is revealed in each chromosome. Segments are designated by the name of the methods that identify them: Q - segments - after staining with acriquine-mustard gas; G - segments - staining with Giemsa dye; R - segments - staining after heat denaturation and others. The short arm of the chromosome is denoted by the letter p, the long one by the letter q. Each arm of the chromosome is divided into regions and numbered from the centromere to the telomere. The stripes within the regions are numbered in order from the centromere. For example, the location of the esterase D gene - 13p14 - is the fourth band of the first region of the short arm of chromosome 13.
Chromosome function: storage, reproduction and transmission of genetic information during the reproduction of cells and organisms.
As a rule, a eukaryotic cell has one core, but there are binucleated (ciliates) and multinucleated cells (opaline). Some highly specialized cells lose their nucleus again (erythrocytes of mammals, sieve tubes of angiosperms).
The shape of the nucleus is spherical, elliptical, less often lobed, bean-shaped, etc. The diameter of the nucleus is usually from 3 to 10 microns.
1 - outer membrane; 2 - inner membrane; 3 - pores; 4 - nucleolus; 5 - hetero-chromatin; 6 - euchro-matin.
The nucleus is delimited from the cytoplasm by two membranes (each of them has a typical structure). Between the membranes there is a narrow gap filled with a semi-liquid substance. In some places, the membranes merge with each other, forming pores (3), through which the exchange of substances between the nucleus and the cytoplasm takes place. The outer nuclear (1) membrane from the side facing the cytoplasm is covered with ribosomes, which give it roughness, the inner (2) membrane is smooth. Nuclear membranes are part of the cell membrane system: the outgrowths of the outer nuclear membrane are connected to the channels of the endoplasmic reticulum, forming a single system of communicating channels.
Karyoplasm (nuclear juice, nucleoplasm)- the inner content of the nucleus, in which chromatin and one or more nucleoli are located. The composition of nuclear juice includes various proteins (including enzymes of the nucleus), free nucleotides.
Nucleolus(4) is a rounded dense body immersed in nuclear juice. The number of nucleoli depends on the functional state of the nucleus and varies from 1 to 7 or more. Nucleoli are found only in non-dividing nuclei; during mitosis, they disappear. The nucleolus is formed on certain parts of the chromosomes that carry information about the structure of rRNA. Such regions are called the nucleolar organizer and contain numerous copies of the genes encoding rRNA. Ribosome subunits are formed from rRNA and proteins coming from the cytoplasm. Thus, the nucleolus is an accumulation of rRNA and ribosomal subunits at different stages of their formation.
Chromatin- internal nucleoprotein structures of the nucleus, stained with some dyes and differing in shape from the nucleolus. Chromatin is in the form of lumps, granules and filaments. The chemical composition of chromatin: 1) DNA (30-45%), 2) histone proteins (30-50%), 3) non-histone proteins (4-33%), therefore, chromatin is a deoxyribonucleoprotein complex (DNP). Depending on the functional state of chromatin, there are: heterochromatin(5) and euchromatin(6). Euchromatin is genetically active, heterochromatin is genetically inactive regions of chromatin. Euchromatin under light microscopy is indistinguishable, weakly stained and represents decondensed (despiralized, untwisted) areas of chromatin. Heterochromatin under a light microscope looks like lumps or granules, intensely stains and represents condensed (spiralized, compacted) areas of chromatin. Chromatin is a form of existence of genetic material in interphase cells. During cell division (mitosis, meiosis), chromatin is converted into chromosomes.
Kernel functions: 1) storage of hereditary information and its transfer to daughter cells in the process of division, 2) regulation of cell life by regulating the synthesis of various proteins, 3) the place of formation of ribosome subunits.
Are cytological rod-shaped structures, which are condensed chromatin and appear in the cell during mitosis or meiosis. Chromosomes and chromatin are different forms of the spatial organization of the deoxyribonucleoprotein complex corresponding to different phases of the cell's life cycle. The chemical composition of chromosomes is the same as that of chromatin: 1) DNA (30-45%), 2) histone proteins (30-50%), 3) non-histone proteins (4-33%).
The basis of the chromosome is one continuous double-stranded DNA molecule; the DNA length of one chromosome can be up to several centimeters. It is clear that a molecule of this length cannot be located in the cell in an elongated form, but undergoes folding, acquiring a certain three-dimensional structure, or conformation. The following levels of spatial packing of DNA and DNP can be distinguished: 1) nucleosomal (winding of DNA onto protein globules), 2) nucleomeric, 3) chromomeric, 4) chromonemal, 5) chromosomal.
In the process of converting chromatin into chromosomes, DNP forms not only spirals and supercoils, but also loops and superloops. Therefore, the process of chromosome formation, which occurs in prophase of mitosis or prophase 1 of meiosis, is better called not spiralization, but condensation of chromosomes.
1 - metacentric; 2 - submetacentric; 3, 4 - acrocentric. Chromosome structure: 5 - centromere; 6 - secondary constriction; 7 - satellite; 8 - chromatids; 9 - telomeres.
The metaphase chromosome (chromosomes are studied in the metaphase of mitosis) consists of two chromatids (8). Any chromosome has primary constriction (centromere)(5), which divides the chromosome into the shoulders. Some chromosomes have secondary constriction(6) and satellite(7). Satellite is a section of a short arm separated by a secondary constriction. Chromosomes that have a satellite are called satellite chromosomes (3). The ends of the chromosomes are called telomeres(nine). Depending on the position of the centromere, there are: a) metacentric(equal shoulder) (1), b) submetacentric(moderately unequal) (2), c) acrocentric(sharply unequal) chromosomes (3, 4).
Somatic cells contain diploid(double - 2n) set of chromosomes, sex cells - haploid(single - n). The diploid set of roundworms is 2, Drosophila - 8, chimpanzees - 48, crayfish - 196. Chromosomes of the diploid set are divided into pairs; chromosomes of one pair have the same structure, size, set of genes and are called homologous.
Karyotype- a set of information about the number, size and structure of metaphase chromosomes. An idiogram is a graphic representation of a karyotype. Representatives of different species have different karyotypes, one species is the same. Autosomes- chromosomes, the same for male and female karyotypes. Sex chromosomes- chromosomes by which the male karyotype differs from the female.
The human chromosome set (2n = 46, n = 23) contains 22 pairs of autosomes and 1 pair of sex chromosomes. Autosomes are grouped and numbered:
| Group | Number of pairs | Number | The size | The form |
|---|---|---|---|---|
| A | 3 | 1, 2, 3 | Large | 1, 3 - metacentric, 2 - submetacentric |
| B | 2 | 4, 5 | Large | Submetacentric |
| C | 7 | 6, 7, 8, 9, 10, 11, 12 | Average | Submetacentric |
| D | 3 | 13, 14, 15 | Average | |
| E | 3 | 16, 17, 18 | Small | Submetacentric |
| F | 2 | 19, 20 | Small | Metacentric |
| G | 2 | 21, 22 | Small | Acrocentric, satellite (secondary constriction in the short shoulder) |
Sex chromosomes do not belong to any of the groups and do not have a number. Sex chromosomes of women - XX, men - XY. X-chromosome - middle submetacentric, Y-chromosome - small acrocentric.
Fine structure of the cell nucleus
Cell nucleus
Core(lat. nucleus) is one of the structural components of a eukaryotic cell containing genetic information (DNA molecules). In the nucleus, replication occurs - the doubling of DNA molecules, as well as transcription - the synthesis of RNA molecules on a DNA molecule. In the nucleus, the synthesized RNA molecules undergo a number of modifications, after which they enter the cytoplasm. The formation of ribosome subunits also occurs in the nucleus in special formations - the nucleoli.
Diagram of the structure of the cell nucleus.
The enormous length of eukaryotic DNA molecules predetermined the emergence of special mechanisms for storage, replication and implementation of genetic material. Chromatin called molecules of chromosomal DNA in a complex with specific proteins necessary for the implementation of these processes. The bulk is made up of "storage proteins", the so-called histones. These proteins are built nucleosomes, structures on which the strands of DNA molecules are wound. Nucleosomes are arranged quite regularly, so that the resulting structure resembles a bead. The nucleosome consists of four types of proteins: H2A, H2B, H3, and H4. One nucleosome contains two proteins of each type - a total of eight proteins. Histone H1, which is larger than other histones, binds to DNA where it enters the nucleosome. The nucleosome together with H1 is called chromosome.
Diagram showing the cytoplasm, together with its components (or organelles), in a typical animal cell. Organelles:
(1) Nucleolus
(2) Core
(3) ribosome (small dots)
(4) Vesicle
(5) rough endoplasmic reticulum (ER)
(6) Golgi apparatus
(7) Cytoskeleton
(8) Smooth endoplasmic reticulum
(9) Mitochondria
(10) Vacuole
(11) Cytoplasm
(12) Lysosome
(13) Centriole and Centrosome
A strand of DNA with nucleosomes forms an irregular solenoid-like structure about 30 nanometers thick, the so-called 30 nm fibril... Further packing of this fibril can have different densities. If chromatin is packed tightly, it is called condensed or heterochromatin, it is clearly visible under a microscope. DNA found in heterochromatin is not transcribed, usually this condition is characteristic of insignificant or silent areas. In the interphase, heterochromatin is usually located at the periphery of the nucleus (parietal heterochromatin). Complete condensation of chromosomes occurs before cell division. If chromatin is loosely packed, it is called eu- or interchromatin... This type of chromatin is much less dense when viewed under a microscope and is usually characterized by the presence of transcriptional activity. The packing density of chromatin is largely determined by histone modifications - acetylation and phosphorylation.
It is believed that there are so-called functional domains of chromatin(DNA of one domain contains about 30 thousand base pairs), that is, each chromosome region has its own "territory". Unfortunately, the issue of the spatial distribution of chromatin in the nucleus has not been sufficiently studied yet. It is known that telomeric (terminal) and centromeric (responsible for the binding of sister chromatids in mitosis) chromosome regions are fixed on the proteins of the nuclear lamina.
Nuclear envelope, nuclear lamina and nuclear pores (karyolemma)
The nucleus is separated from the cytoplasm nuclear envelope formed due to the expansion and fusion of the cisterns of the endoplasmic reticulum with each other in such a way that the nucleus has double walls due to the narrow compartments surrounding it. The cavity of the nuclear envelope is called lumen or perinuclear space... The inner surface of the nuclear envelope is underlain by a nuclear lamina, a rigid protein structure formed by lamina proteins, to which strands of chromosomal DNA are attached. Lamins are attached to the inner membrane of the nuclear envelope using transmembrane proteins anchored in it - lamina receptors... In some places, the inner and outer membranes of the nuclear envelope merge and form the so-called nuclear pores, through which material exchange takes place between the nucleus and the cytoplasm. The pore is not a hole in the nucleus, but has a complex structure organized by several dozen specialized proteins - nucleoporins. Under an electron microscope, it is visible as eight interconnected protein granules from the outer and the same amount from the inner side of the nuclear envelope.
Under an electron microscope, several subcompartments are isolated in the nucleolus. So called Fibrillar centers surrounded by plots dense fibrillar component, where rRNA synthesis occurs. Outside of the dense fibrillar component is located granular component, which is an accumulation of maturing ribosomal subunits.
