Chromosome structure. Chromosomes. Levels of chromosomal DNA compaction

). Chromatin is heterogeneous, and some types of such heterogeneity are visible under a microscope. The fine structure of chromatin in the interphase nucleus, determined by the nature of DNA folding and its interaction with proteins, plays an important role in the regulation of gene transcription and DNA replication and, possibly, cellular differentiation.

The sequences of DNA nucleotides that form genes and serve as a template for the synthesis of mRNA are distributed along the entire length of the chromosomes (individual genes, of course, are too small to be seen under a microscope). By the end of the 20th century, for approximately 6,000 genes, it was established on which chromosome and in which part of the chromosome they are located and what the nature of their linkage is (that is, their position relative to each other).

The heterogeneity of metaphase chromosomes, as already mentioned, can be seen even with light microscopy. Differential staining of at least 12 chromosomes revealed differences in the width of some bands between homologous chromosomes (Fig. 66.3). Such polymorphic regions consist of non-coding highly repetitive DNA sequences.

The methods of molecular genetics have made it possible to identify a huge number of smaller polymorphic DNA regions that are therefore undetectable by light microscopy. These regions are identified as restriction fragment length polymorphism, tandem repeats varying in number, and short tandem repeat polymorphism (mono-, di-, tri-, and tetranucleotide). Such variability usually does not manifest itself phenotypically.

However, polymorphism serves as a convenient tool for prenatal diagnosis due to the linkage of certain markers with mutant genes that cause diseases (for example, in Duchenne myopathy), as well as in establishing the zygosity of twins, establishing paternity, and predicting transplant rejection.

It is difficult to overestimate the importance of such markers, especially highly polymorphic short tandem repeats that are widespread in the genome, for mapping the human genome. In particular, they make it possible to establish the exact order and nature of the interaction of loci that play an important role in ensuring normal ontogenesis and cell differentiation. This also applies to those loci in which mutations lead to hereditary diseases.

Microscopically visible regions on the short arm of acrocentric autosomes (Fig. 66.1) provide rRNA synthesis and the formation of nucleoli, which is why they are called nucleolar organizer regions. In metaphase they are not condensed and do not stain. The regions of the nucleolar organizer are adjacent to the condensed sections of chromatin - satellites - located at the end of the short arm of the chromosome. Satellites do not contain genes and are polymorphic regions.

In a small proportion of cells, it is possible to identify other areas decondensed in metaphase, the so-called fragile areas, where “complete” chromosome breaks can occur. Abnormalities in the only such region located at the end of the long arm of the X chromosome are of clinical significance. Such disorders cause fragile X syndrome.

Other examples of specialized regions of chromosomes are telomeres and centromeres.

The role of heterochromatin, which accounts for a significant part of the human genome, has not yet been precisely established. Heterochromatin is condensed throughout almost the entire cell cycle, it is inactive and replicates late. Most regions are condensed and inactive in all cells (), although others, such as the X chromosome, can be either condensed and inactive or decondensed and active (facultative heterochromatin). If, due to chromosomal aberrations, genes end up close to heterochromatin, then the activity of such genes can change or even be blocked. Therefore, the manifestations of chromosomal aberrations, such as duplications or deletions, depend not only on the affected loci, but also on the type of chromatin in them. Many chromosomal abnormalities that are not lethal affect inactive or inactivated regions of the genome. This may explain that trisomy on some chromosomes or monosomy on the X chromosome are compatible with life.

Manifestations of chromosomal abnormalities also depend on the new arrangement of structural and regulatory genes in relation to each other and to heterochromatin.

Fortunately, many structural features of chromosomes can be reliably detected by cytological methods. Currently, there are a number of methods for differential chromosome staining (Fig. 66.1 and Fig. 66.3). The location and width of the bands are identical in each pair of homologous chromosomes, with the exception of polymorphic regions, so staining can be used in clinical cytogenetics to identify chromosomes and detect structural abnormalities in them.

Chromosome is a thread-like structure containing DNA in the cell nucleus, which carries genes, units of heredity, arranged in a linear order. Humans have 22 pairs of regular chromosomes and one pair of sex chromosomes. In addition to genes, chromosomes also contain regulatory elements and nucleotide sequences. They house DNA-binding proteins that control DNA functions. Interestingly, the word "chromosome" comes from the Greek word "chrome", meaning "color". Chromosomes received this name because they have the ability to be colored in different tones. The structure and nature of chromosomes vary from organism to organism. Human chromosomes have always been a subject of constant interest to researchers working in the field of genetics. The wide range of factors that are determined by human chromosomes, the abnormalities for which they are responsible, and their complex nature have always attracted the attention of many scientists.

Interesting facts about human chromosomes

Human cells contain 23 pairs of nuclear chromosomes. Chromosomes are made up of DNA molecules that contain genes. The chromosomal DNA molecule contains three nucleotide sequences required for replication. When chromosomes are stained, the banded structure of mitotic chromosomes becomes apparent. Each strip contains numerous DNA nucleotide pairs.

Humans are a sexually reproducing species with diploid somatic cells containing two sets of chromosomes. One set is inherited from the mother, while the other is inherited from the father. Reproductive cells, unlike body cells, have one set of chromosomes. Crossing over between chromosomes leads to the creation of new chromosomes. New chromosomes are not inherited from either parent. This accounts for the fact that not all of us exhibit traits that we receive directly from one of our parents.

Autosomal chromosomes are assigned numbers from 1 to 22 in descending order as their size decreases. Each person has two sets of 22 chromosomes, an X chromosome from the mother and an X or Y chromosome from the father.

An abnormality in the contents of a cell's chromosomes can cause certain genetic disorders in people. Chromosomal abnormalities in people are often responsible for the occurrence of genetic diseases in their children. Those who have chromosomal abnormalities are often only carriers of the disease, while their children develop the disease.

Chromosomal aberrations (structural changes in chromosomes) are caused by various factors, namely deletion or duplication of part of a chromosome, inversion, which is a change in the direction of a chromosome to the opposite, or translocation, in which part of a chromosome is torn off and attached to another chromosome.

An extra copy of chromosome 21 is responsible for a very well known genetic disorder called Down syndrome.

Trisomy 18 results in Edwards syndrome, which can cause death in infancy.

Deletion of part of the fifth chromosome results in a genetic disorder known as Cri-Cat Syndrome. People affected by this disease often have mental retardation and their crying in childhood resembles that of a cat.

Disorders caused by sex chromosome abnormalities include Turner syndrome, in which female sexual characteristics are present but characterized by underdevelopment, as well as XXX syndrome in girls and XXY syndrome in boys, which cause dyslexia in affected individuals.

Chromosomes were first discovered in plant cells. Van Beneden's monograph on fertilized roundworm eggs led to further research. August Weissman later showed that the germ line was distinct from the soma and discovered that cell nuclei contained hereditary material. He also suggested that fertilization leads to the formation of a new combination of chromosomes.

These discoveries became cornerstones in the field of genetics. Researchers have already accumulated a significant amount of knowledge about human chromosomes and genes, but much remains to be discovered.

Video

Chromosomes are the nucleoprotein structures of a eukaryotic cell in which most of the hereditary information is stored. Due to their ability to self-reproduce, it is chromosomes that provide the genetic connection of generations. Chromosomes are formed from a long DNA molecule, which contains a linear group of many genes, and all the genetic information be it about a person, animal, plant or any other living creature.

The morphology of chromosomes is related to the level of their spiralization. So, if during the interphase stage the chromosomes are maximized, then with the onset of division the chromosomes actively spiral and shorten. They reach their maximum shortening and spiralization during the metaphase stage, when new structures are formed. This phase is most convenient for studying the properties of chromosomes and their morphological characteristics.

History of the discovery of chromosomes

Back in the middle of the 19th century before last, many biologists, studying the structure of plant and animal cells, drew attention to thin threads and tiny ring-shaped structures in the nucleus of some cells. And so the German scientist Walter Fleming used aniline dyes to treat the nuclear structures of the cell, which is called “officially” opens the chromosomes. More precisely, he named the discovered substance “chromatid” for its ability to stain, and the term “chromosomes” was introduced into use a little later (in 1888) by another German scientist, Heinrich Wilder. The word "chromosome" comes from the Greek words "chroma" - color and "somo" - body.

Chromosomal theory of heredity

Of course, the history of the study of chromosomes did not end with their discovery; in 1901-1902, American scientists Wilson and Saton, independently of each other, drew attention to the similarity in the behavior of chromosomes and Mendeleev’s factors of heredity - genes. As a result, scientists came to the conclusion that genes are located in chromosomes and it is through them that genetic information is transmitted from generation to generation, from parents to children.

In 1915-1920, the participation of chromosomes in gene transmission was proven in practice in a series of experiments carried out by the American scientist Morgan and his laboratory staff. They managed to localize several hundred hereditary genes in the chromosomes of the Drosophila fly and create genetic maps of the chromosomes. Based on these data, the chromosomal theory of heredity was created.

Chromosome structure

The structure of chromosomes varies depending on the species, so the metaphase chromosome (formed in the metaphase stage during cell division) consists of two longitudinal threads - chromatids, which connect at a point called the centromere. A centromere is a region of a chromosome that is responsible for the separation of sister chromatids into daughter cells. It also divides the chromosome into two parts, called the short and long arms, and is also responsible for the division of the chromosome, since it contains a special substance - the kinetochore, to which the spindle structures are attached.

Here the picture shows the visual structure of a chromosome: 1. chromatids, 2. centromere, 3. short chromatid arm, 4. long chromatid arm. At the ends of the chromatids there are telomeres, special elements that protect the chromosome from damage and prevent fragments from sticking together.

Shapes and types of chromosomes

The sizes of plant and animal chromosomes vary significantly: from fractions of a micron to tens of microns. The average lengths of human metaphase chromosomes range from 1.5 to 10 microns. Depending on the type of chromosome, its staining abilities also differ. Depending on the location of the centromere, the following forms of chromosomes are distinguished:

Metacentric chromosomes, which are characterized by a central location of the centromere.
Submetacentric, they are characterized by an uneven arrangement of chromatids, when one arm is longer and the other is shorter.
Acrocentric or rod-shaped. Their centromere is located almost at the very end of the chromosome.

Functions of chromosomes

The main functions of chromosomes, both for animals and plants and all living beings in general, are the transfer of hereditary, genetic information from parents to children.

Set of chromosomes

The importance of chromosomes is so great that their number in cells, as well as the characteristics of each chromosome, determine the characteristic feature of a particular biological species. So, for example, the Drosophila fly has 8 chromosomes, the y has 48, and the human chromosome set is 46 chromosomes.

In nature, there are two main types of chromosome sets: single or haploid (found in germ cells) and double or diploid. The diploid set of chromosomes has a pair structure, that is, the entire set of chromosomes consists of chromosome pairs.

Human chromosome set

As we wrote above, the cells of the human body contain 46 chromosomes, which are combined into 23 pairs. All together they make up the human chromosome set. The first 22 pairs of human chromosomes (they are called autosomes) are common to both men and women, and only 23 pairs - sex chromosomes - vary between sexes, which also determines a person’s gender. The set of all pairs of chromosomes is also called a karyotype.

The human chromosome set has this type, 22 pairs of double diploid chromosomes contain all our hereditary information, and the last pair differs, in men it consists of a pair of conditional X and Y sex chromosomes, while in women there are two X chromosomes.

All animals have a similar structure of the chromosome set, only the number of non-sex chromosomes in each of them is different.

Genetic diseases associated with chromosomes

A malfunction of chromosomes, or even their incorrect number itself, is the cause of many genetic diseases. For example, Down syndrome appears due to the presence of an extra chromosome in the human chromosome set. And such genetic diseases as color blindness and hemophilia are caused by malfunctions of existing chromosomes.

Chromosomes, video

And finally, an interesting educational video about chromosomes.

This article is available in English - .

Eukaryotic chromosomes

Centromere

Primary constriction

X. p., in which the centromere is localized and which divides the chromosome into arms.

Secondary constrictions

A morphological feature that allows the identification of individual chromosomes in a set. They differ from the primary constriction by the absence of a noticeable angle between the chromosome segments. Secondary constrictions are short and long and are localized at different points along the length of the chromosome. In humans, these are chromosomes 13, 14, 15, 21 and 22.

Types of chromosome structure

There are four types of chromosome structure:

telocentric(rod-shaped chromosomes with a centromere located at the proximal end);
acrocentric(rod-shaped chromosomes with a very short, almost invisible second arm);
submetacentric(with shoulders of unequal length, resembling the letter L in shape);
metacentric(V-shaped chromosomes with arms of equal length).

The chromosome type is constant for each homologous chromosome and may be constant in all members of the same species or genus.

Satellites

Satellite- this is a round or elongated body, separated from the main part of the chromosome by a thin chromatin thread, with a diameter equal to or slightly smaller than the chromosome. Chromosomes with a satellite are usually referred to as SAT chromosomes. The shape, size of the satellite and the thread connecting it are constant for each chromosome.

Nucleolar zone

Zones of the nucleolus ( nucleolar organizers) - special areas with which the appearance of some secondary constrictions is associated.

Chromonema

Chromonema is a helical structure that can be seen in decompacted chromosomes through an electron microscope. It was first observed by Baranetsky in 1880 in the chromosomes of Tradescantia anther cells, the term was introduced by Veidovsky. Chromonema can consist of two, four or more threads, depending on the object being studied. These threads form two types of spirals:

paranemic(spiral elements are easy to separate);
plectonemic(the threads are tightly intertwined).

Chromosomal rearrangements

Violation of the structure of chromosomes occurs as a result of spontaneous or provoked changes (for example, after irradiation).

Gene (point) mutations (changes at the molecular level);
Aberrations (microscopic changes visible using a light microscope):

Giant chromosomes

Such chromosomes, which are characterized by their enormous size, can be observed in some cells at certain stages of the cell cycle. For example, they are found in the cells of some tissues of dipteran insect larvae (polytene chromosomes) and in the oocytes of various vertebrates and invertebrates (lampbrush chromosomes). It was on preparations of giant chromosomes that signs of gene activity were revealed.

Polytene chromosomes

Balbiani were first discovered in 2010, but their cytogenetic role was revealed by Kostov, Paynter, Geitz and Bauer. Contained in the cells of the salivary glands, intestines, tracheas, fat body and Malpighian vessels of dipteran larvae.

Lamp brush chromosomes

Bacterial chromosomes

There is evidence that bacteria have proteins associated with nucleoid DNA, but histones have not been found in them.

Literature

E. de Robertis, V. Novinsky, F. Saez Cell biology. - M.: Mir, 1973. - P. 40-49.

CHROMOSOMES- (from chromo... and soma), organelles of the cell nucleus, which are carriers of genes and determine the inheritance, properties of cells and organisms. Capable of self-reproduction, have structural and functional individuality and maintain it in a series... ... Biological encyclopedic dictionary

CHROMOSOMES- [Dictionary of foreign words of the Russian language

CHROMOSOMES- (from chromo... and Greek soma body) structural elements of the cell nucleus containing DNA, which contains the hereditary information of the organism. Genes are arranged in linear order on chromosomes. Self-duplication and regular distribution of chromosomes along... ... Big Encyclopedic Dictionary

CHROMOSOMES- CHROMOSOMES, structures that carry genetic information about the organism, which is contained only in the nuclei of EUKARYOTIC cells. Chromosomes are thread-like, they consist of DNA and have a specific set of GENES. Each type of organism has a characteristic... ... Scientific and technical encyclopedic dictionary

Chromosomes- Structural elements of the cell nucleus containing DNA, which contains the hereditary information of the organism. Genes are arranged in linear order on chromosomes. Each human cell contains 46 chromosomes, divided into 23 pairs, of which 22... ... Great psychological encyclopedia

Chromosomes- * templesomes * chromosomes are self-reproducing elements of the cell nucleus that retain structural and functional individuality and are stained with basic dyes. They are the main material carriers of hereditary information: genes... ... Genetics. encyclopedic Dictionary

CHROMOSOMES- CHROMOSOMES, ohm, units. chromosome, s, female (specialist.). A permanent component of the nucleus of animal and plant cells, carriers of hereditary genetic information. | adj. chromosomal, oh, oh. X. cell set. Chromosomal theory of heredity.... ... Ozhegov's Explanatory Dictionary

Lecture No. 3

Topic: Organizing the flow of genetic information

Lecture outline

1. Structure and functions of the cell nucleus.

2. Chromosomes: structure and classification.

3. Cellular and mitotic cycles.

4. Mitosis, meiosis: cytological and cytogenetic characteristics, significance.

Structure and function of the cell nucleus

The main genetic information is contained in the cell nucleus.

Cell nucleus(lat. – nucleus; Greek – karyon) was described in 1831. Robert Brown. The shape of the nucleus depends on the shape and function of the cell. The size of the nuclei varies depending on the metabolic activity of the cells.

Interphase core shell (karyolemma) consists of outer and inner elementary membranes. Between them is perinuclear space. There are holes in the membranes - pores. Between the edges of the nuclear pore there are protein molecules that form pore complexes. The pore opening is covered with a thin film. During active metabolic processes in the cell, most of the pores are open. Through them there is a flow of substances - from the cytoplasm to the nucleus and back. Number of pores in one nucleus

Rice. Diagram of the structure of the cell nucleus

1 and 2 – outer and inner membranes of the nuclear envelope, 3

– nuclear pore, 4 – nucleolus, 5 – chromatin, 6 – nuclear juice

reaches 3-4 thousand. The outer nuclear membrane connects to the endoplasmic reticulum channels. It is usually located ribosomes. Proteins on the inner surface of the nuclear envelope form nuclear lamina. It maintains the constant shape of the nucleus and chromosomes are attached to it.

Nuclear juice - karyolymph, a colloidal solution in a gel state that contains proteins, lipids, carbohydrates, RNA, nucleotides, and enzymes. Nucleolus– a non-permanent component of the nucleus. It disappears at the beginning of cell division and is restored at the end of it. Chemical composition of nucleoli: protein (~90%), RNA (~6%), lipids, enzymes. Nucleoli are formed in the area of secondary constrictions of satellite chromosomes. Function of nucleoli: assembly of ribosomal subunits.

X romatine nuclei are interphase chromosomes. They contain DNA, histone proteins and RNA in a ratio of 1:1.3:0.2. DNA combines with protein to form deoxyribonucleoprotein(DNP). During mitotic division of the nucleus, DNP spirals and forms chromosomes.

Functions of the cell nucleus:

1) stores the hereditary information of the cell;

2) participates in cell division (reproduction);

3) regulates metabolic processes in the cell.

Chromosomes: structure and classification

Chromosomes(Greek - chromo- color, soma– body) is a spiralized chromatin. Their length is 0.2 – 5.0 µm, diameter 0.2 – 2 µm.

Rice. Types of chromosomes

Metaphase chromosome consists of two chromatid, which connect centromere (primary constriction). It divides the chromosome into two shoulder. Individual chromosomes have secondary constrictions. The area they separate is called satellite, and such chromosomes are satellite. The ends of chromosomes are called telomeres. Each chromatid contains one continuous DNA molecule combined with histone proteins. Intensely stained areas of chromosomes are areas of strong spiralization ( heterochromatin). Lighter areas are areas of weak spiralization ( euchromatin).

Chromosome types are distinguished by the location of the centromere (Fig.).

1. Metacentric chromosomes– the centromere is located in the middle, and the arms have the same length. The section of the arm near the centromere is called proximal, the opposite is called distal.

2. Submetacentric chromosomes– the centromere is offset from the center and the arms 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-stranded chromosomes).

There are 4 rules for the chromosomes of all organisms:

1. Rule of constant number of chromosomes. Normally, organisms of certain species have a constant, species-specific number of chromosomes. For example: a person has 46, a dog has 78, a Drosophila fly has 8.

2. Chromosome pairing. In a diploid set, each chromosome normally has a paired chromosome - identical in shape and size.

3. Individuality of chromosomes. Chromosomes of different pairs differ in shape, structure and size.

4. Chromosome continuity. When genetic material is duplicated, a chromosome is formed from a chromosome.

The set of chromosomes of a somatic cell, characteristic of an organism of a given species, is called karyotype.

Chromosomes are classified according to different characteristics.

1. Chromosomes that are identical in the cells of male and female organisms are called autosomes. A person has 22 pairs of autosomes in their 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 they are the X and X chromosomes.

2. The arrangement of chromosomes in decreasing order of magnitude is called idiogram. This is a systematic karyotype. Chromosomes are arranged in pairs (homologous chromosomes). The first pair are the largest ones, the 22nd pair are the small ones, and the 23rd pair are the sex chromosomes.

3. In 1960 Denver classification of chromosomes was proposed. It is built on the basis of their shape, size, position of the centromere, the presence of secondary constrictions and satellites. An important indicator in this classification is centromeric index(CI). This is the ratio of the length of the short arm of a 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 CI is 38-49%.

Group B. The 4th and 5th pairs are large metacentric chromosomes. CI 24-30%.

Group C. Pairs of chromosomes 6 – 12: medium size, submetacentric. CI 27-35%. This group also includes the X chromosome.

Group D. 13 – 15th pairs of chromosomes. The chromosomes are acrocentric. CI is about 15%.

Group E. Pairs of chromosomes 16 – 18. Relatively short, metacentric or submetacentric. CI 26-40%.

Group F. 19th – 20th pairs. Short, submetacentric chromosomes. CI 36-46%.

Group G. 21-22nd pairs. Small, acrocentric chromosomes. CI 13-33%. The Y chromosome also belongs to this group.

4. The Paris classification of human chromosomes was created in 1971. Using this classification, it is possible to determine the localization of genes in a specific pair of chromosomes. Using special staining methods, a characteristic order of alternating dark and light stripes (segments) is identified in each chromosome. Segments are designated by the name of the methods that identify them: Q - segments - after staining with quinine mustard; G – segments – stained with Giemsa dye; R – segments – staining after heat denaturation and others. The short arm of the chromosome is designated by the letter p, the long arm by the letter q. Each chromosome arm is divided into regions and designated by numbers from centromere to telomere. Bands within regions are numbered in order from the centromere. For example, the location of the esterase D gene is 13p14 - the fourth band of the first region of the short arm of the 13th chromosome.

Function of chromosomes: storage, reproduction and transmission of genetic information during the reproduction of cells and organisms.

Related information.