What is a chromosome. The structure and functions of chromosomes. reproduction in the organic world. The structure of germ cells. Lampbrush chromosomes

Chromosome is the organized structure of DNA and protein found in cells. This is one piece of DNA coiled into a spiral, containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins that serve to package DNA and control its functions. Chromosomal DNA codes for all or most of an organism's genetic information; some species also contain plasmids or other extrachromosomal genetic elements.

Or Down's disease, also known as trisomy 21 is an inherited disorder caused by the presence of part or all of 3 copies of 21 chromosomes. Usually, it is associated with delayed physical development, facial features, or mild to moderate intellectual...

Chromosomes vary widely between different organisms. The DNA molecule can be round or linear, and it can contain from 100,000 to more than 375,000,000 nucleotides in a long chain. Typically, eukaryotic cells (cells with nuclei) have large linear chromosomes, while prokaryotic cells (cells without defined nuclei) have smaller round chromosomes, although there are many exceptions to this rule. In addition, cells may contain chromosomes of several types; for example, mitochondria in most eukaryotes and chloroplasts in plants have their own little chromosomes.

In eukaryotes, nuclear chromosomes are packed with proteins into a dense structure called chromatin. This allows very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies throughout the cell cycle. Chromosomes are an essential building block for cell division and must reproduce, divide and pass successfully to their daughter cells to ensure genetic diversity and the survival of their offspring. Chromosomes can be either duplicated or non-duplicated. Non-duplicated chromosomes are single linear strands in which the duplicated chromosomes contain two identical copies (called chromatids) united by a centromere.

Compaction of duplicated chromosomes during mitosis and meiosis results in the classic four-arm structure. Chromosomal recombination plays a vital role in genetic diversity. If these structures are mishandled through processes known as chromosomal instability and translocation, the cell may undergo a mitotic catastrophe and die, or it may unexpectedly escape apoptosis, leading to cancer progression.

In practice, "chromosome" is a rather vague term. For prokaryotes and viruses where there is no chromatin, the term genophore is more appropriate. In prokaryotes, DNA is usually organized in a loop that coils tightly around itself, sometimes accompanied by one or smaller circular DNA molecules called plasmids. These small, round genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origin. The simplest genophores are found in viruses: these are DNA or RNA molecules - short linear or round genophores, which are often devoid of structural proteins.

Word " chromosome” is formed by the Greek words “χρῶμα” ( chroma, color) and "σῶμα" ( soma, body) due to the property of chromosomes to undergo very strong staining with certain dyes.

History of the study of chromosomes

In a series of experiments begun in the mid-1880s, Theodore Boveri definitely demonstrated that chromosomes are the vectors of heredity. His two principles were sequence chromosomes and individuality chromosomes. The second principle was very original. Wilhelm Roux suggested that each chromosome carries a different genetic load. Boveri was able to test and confirm this hypothesis. With the help of a rediscovery made in the early work of Gregor Mendel, in the early 1900s, Boveri was able to note the connection between the rules of inheritance and the behavior of chromosomes. Boveri influenced two generations of American cytologists: among them Edmund Beecher Wilson, Walter Sutton, and Theophilus Painter (Wilson and Painter actually worked with him).

In his famous book The cell in development and heredity Wilson linked together the independent work of Boveri and Sutton (circa 1902), calling the chromosome theory of heredity the "Sutton-Boveri Theory" (the names are sometimes interchanged). Ernst Mair notes that the theory has been hotly contested by some famous geneticists, such as William Bateson, Wilhelm Johansen, Richard Goldschmidt and T.H. Morgan, they all had a rather dogmatic mindset. In the end, full evidence was obtained from chromosome maps in Morgan's own laboratory.

Prokaryotes and chromosomes

Prokaryotes - bacteria and archaea - usually have one round chromosome, but there are many variations.

In most cases, the chromosome size of bacteria can vary from 160,000 base pairs in an endosymbiotic bacterium Candidatus Carsonella ruddii up to 12,200,000 base pairs in a soil-dwelling bacterium Sorangium cellulosum. Spirochetes of the genus Borrelia are a notable exception to this classification, along with bacteria such as Borrelia burgdorferi(cause of Lyme disease) containing one linear chromosome.

Structure in Sequences

Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria usually have a single point (origin of duplication) from where duplication begins, while some archaea contain multiple points of origin of duplication. Genes in prokaryotes are often organized into operons and usually do not contain introns, unlike eukaryotes.

DNA packaging

Prokaryotes do not have nuclei. Instead, their DNA is organized into a structure called a nucleoid. A nucleoid is a separate structure that occupies a specific area of ​​a bacterial cell. However, this structure is dynamic, maintained and transformed by the actions of histone-like proteins that bind to the bacterial chromosome. In archaea, DNA in chromosomes is even more organized, with DNA packaged in structures similar to eukaryotic nucleosomes.

Bacterial chromosomes tend to bind to the bacterial plasma membrane. In molecular biology applications, this allows its isolation from the plasmid DNA by centrifugation of the lysed bacterium and sedimentation of the membranes (and attached DNA).

Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally supercoiled. The DNA must first be isolated in a weakened state in order to access transcription, regulation, and duplication.

in eukaryotes

Eukaryotes (cells with nuclei found in plants, yeast, and animals) have large, linear chromosomes contained in the cell nucleus. Each chromosome has one centromere, with one or two arms protruding from the centromere, although in most circumstances these arms are not visible as such. In addition, most eukaryotes have a single round mitochondrial genome, and some eukaryotes may have additional small round or linear cytoplasmic chromosomes.

In the nuclear chromosomes of eukaryotes, uncompacted DNA exists in a semi-ordered structure where it is wrapped around histones (structural proteins) to form a composite material called chromatin.

Chromatin

Chromatin is a complex of DNA and protein found in the nucleus of a eukaryote that packages chromosomes. The structure of chromatin varies greatly between different stages of the cell cycle, as required by DNA.

Interfacial chromatin

During interphase (the period of the cell cycle when the cell does not divide), two types of chromatin can be distinguished:

• Euchromatin, which consists of active DNA, that is, expressed as a protein.
• Heterochromatin, which consists mostly of inactive DNA. It appears to serve structural purposes during chromosome stages. Heterochromatin can be further divided into two types:
• Constitutive heterochromatin, never expressed. It is located around the centromere and usually contains repeat sequences.
• Facultative heterochromatin, sometimes expressed.

Metaphase chromatin and division

During the early stages of mitosis or meiosis (cell division), the strands of chromatin become increasingly dense. They cease to function as accessible genetic material (transcription stops) and become a compact transportable form. This compact shape makes the individual chromosomes visible, and they form a classic four-arm structure, with a pair of sister chromatids attached to each other at the centromere. The shorter arms are called " p shoulders" (from the French word " petite"- small), and longer shoulders are called " q shoulders" (letter " q' follows the letter ' p» in the Latin alphabet; q-g "grande" - large). This is the only natural context in which individual chromosomes are visible with an optical microscope.

During mitosis, microtubules grow from centrosomes located at opposite ends of the cell and also attach to the centromere in specialized structures called kinetochores, one of which is present on each sister chromatid. A special DNA base sequence in the region of kinetochores, together with special proteins, ensures long-term attachment to this region. The microtubules then pull the chromatids towards the centrosomes so that each daughter cell inherits one set of chromatids. When the cells divide, the chromatids unwind and the DNA can be transcribed again. Despite their appearance, chromosomes are structurally highly compacted, which allows these giant DNA structures to fit into cell nuclei.

human chromosomes

Chromosomes in humans can be divided into two types: autosomes and sex chromosomes. Certain genetic traits are associated with a person's sex and are transmitted through the sex chromosomes. Autosomes contain the rest of the genetic information that is inherited. All act in the same way during cell division. Human cells contain 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell. In addition to these, there are many hundreds of copies of the mitochondrial genome in human cells. Sequencing the human genome provided a lot of information about each chromosome. Below is a table that compiles statistics for chromosomes based on the Sanger Institute's human genome information in the VEGA (Vertebrate Genome Comments) database. The number of genes is a rough estimate, as it is partly based on gene prediction. The total length of chromosomes is also a rough estimate based on the estimated size of regions of inconsistent heterochromatins.

Chromosomes	Genes	Total number of complementary base pairs of nucleic acids	Ordered complementary base pairs of nucleic acids
X (sex chromosome)
Y (sex chromosome)
Total		3079843747	2857698560

Number of chromosomes in different organisms

eukaryotes

These tables give the total number of chromosomes (including sex chromosomes) in the cell nuclei. For example, diploid human cells contain 22 different kinds of autosomes, each present in two copies, and two sex chromosomes. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosomes, such as hexaploid bread wheat contains six copies of seven different chromosomes, for a total of 42 chromosomes.

The number of chromosomes in some plants
plant species
Arabidopsis thaliana(diploid)
garden snail
Tibetan fox
domestic pig
laboratory rat
Syrian hamster
domestic sheep
Kingfisher
Silkworm
Number of chromosomes in other organisms
Kinds		Large chromosomes		Intermediate chromosomes	microchromosomes
Trypanosoma brucei
domestic pigeon ( Columba livia domestics)
				2 sex chromosomes

Normal members of individual eukaryotic species have the same number of nuclear chromosomes (see table). Other eukaryotic chromosomes, that is, mitochondrial and plasmid-like small chromosomes, vary greatly in number, and there can be a thousand copies per cell.

Species with asexual reproduction have one set of chromosomes, the same as in the cells of the organism. However, asexual species can be haploid and diploid.

Sexually reproducing species have somatic cells (body cells) that are diploid, having two sets of chromosomes, one from the mother and one from the father. Gametes, the reproductive cells, are haploid [n]: they have one set of chromosomes. Gametes are obtained by meiosis of a diploid germline cell. During meiosis, the corresponding chromosomes of the father and mother can exchange small parts of each other (crossover), and thereby form new chromosomes that are not inherited only from one or the other parent. When male and female gametes combine (fertilization), a new diploid organism is formed.

Some animal and plant species are polyploid: they have more than two sets of homologous chromosomes. Agriculturally important plants such as tobacco or wheat are often polyploid compared to hereditary species. Wheat has a haploid number of seven chromosomes found in some cultivated plants as well as in wild ancestors. The more common pasta and bread wheats are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to 14 (diploid) chromosomes in wild wheat.

prokaryotes

Prokaryotic species as a whole have one copy of each major chromosome, but most cells can easily survive with multiple copies. For instance, Buchnera, an aphid symbiont, has many copies of its chromosome, ranging from 10 to 400 copies per cell. However, in some large bacteria such as Epulopiscium fishelsoni, up to 100,000 copies of a chromosome may be present. The number of copies of plasmids and plasmid-like small chromosomes, as in eukaryotes, varies considerably. The number of plasmids in a cell is almost entirely determined by the rate of plasmid division - rapid division generates a high number of copies.

Karyotype

Generally karyotype is a characteristic chromosomal complement of eukaryotic species. The preparation and study of karyotypes is part of cytogenetics.

Although DNA duplication and transcription are highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are usually highly variable. The types of chromosome number and their detailed organization may vary. In some cases, there can be significant variation between species. Often there is:

1. fluctuation between the two sexes;
2. fluctuation between the germ line and the soma (between the gametes and the rest of the organism);
3. fluctuation between members of a population due to balanced genetic polymorphism;
4. geographical fluctuation between races;
5. mosaic or other abnormalities

Also, fluctuations in the karyotype can occur during development from a fertilized egg.

The technique for determining the karyotype is commonly referred to as karyotyping. Cells can be blocked partially through division (in metaphase) under artificial conditions (in a reaction tube) with colchicine. These cells are then stained, photographed, and ordered into a karyogram, with a set of ordered chromosomes, autosomes in length order, and sex chromosomes (here X/Y) at the end.

As with many sexually reproducing species, humans have special gonosomes (sex chromosomes, as opposed to autosomes). It is XX for women and XY for men.

Historical note

It took many years to study the human karyotype before the most basic question was answered: How many chromosomes are there in a normal diploid human cell? In 1912, Hans von Winivarter reported 47 chromosomes in spermatogonia and 48 in oogonia, including the XX/XO sex determination mechanism. Painter in 1922 was unsure about a person's diploid number - 46 or 48, initially leaning towards 46. He later revised his opinion from 46 to 48, and correctly insisted that a person has the XX/XY system.

To finally solve the problem, new techniques were needed:

1. Use of cells in culture;
2. Prepare cells in a hypotonic solution where they swell and spread chromosomes;
3. Delay of mitosis in metaphase with a solution of colchicine;
4. Crushing the drug on the subject holder, stimulating the chromosomes in a single plane;
5. Slicing the micrograph and arranging the results in an irrefutable karyogram.

It was not until 1954 that the human diploid number of 46 was confirmed. Given the techniques of Winivarter and Painter, their results were quite remarkable. The chimpanzee (the closest living relative of modern humans) has 48 chromosomes.

Delusions

Chromosomal abnormalities are disruptions in the normal chromosomal content of a cell and are a major cause of genetic conditions in humans such as Down syndrome, although most abnormalities have little or no effect. Some chromosomal disorders do not cause disease in carriers, such as translocations or chromosomal inversions, although they may lead to an increased chance of having a child with a chromosome disorder. An abnormal number of chromosomes or sets of chromosomes, called aneuploidy, can be lethal or give rise to genetic disorders. Families that may carry a chromosomal rearrangement are offered genetic counseling.

The gain or loss of DNA from chromosomes can lead to a variety of genetic disorders. Examples among people:

• Cat cry syndrome, caused by the division of part of the short arm of chromosome 5. The condition is so named because affected children make high-pitched, cat-like cries. People affected by this syndrome have wide-set eyes, a small head and jaw, moderate to severe mental health problems, and short stature.
• Down syndrome, the most common trisomy, is usually caused by an extra copy of chromosome 21 (trisomy 21). Characteristic features include decreased muscle tone, a stocky build, asymmetrical cheekbones, slanted eyes, and mild to moderate developmental disabilities.
• Edwards syndrome, or trisomy 18, is the second most common trisomy. Symptoms include slowness of movement, developmental disorders, and numerous congenital anomalies that cause serious health problems. 90% of patients die in infancy. They are characterized by clenched fists and overlapping fingers.
• Isodicentric chromosome 15, also called idic(15), partial tetrasomy of the long arm of chromosome 15 or reverse duplication of chromosome 15 (inv dup 15).
• Jacobsen's syndrome occurs very rarely. It is also called a terminal deletion disorder of the long arm of chromosome 11. Those affected have normal intelligence or mild developmental disabilities, with poor speech skills. Most have a bleeding disorder called the Paris-Trousseau syndrome.
• Klinefelter syndrome (XXY). Men with Klinefelter's syndrome are usually sterile, usually taller, their arms and legs are longer than their peers. Boys with the syndrome are usually shy and quiet, and are more likely to have slow speech and dyslexia. Without testosterone treatment, some may develop gynecomastia during adolescence.
• Patau syndrome, also called D-syndrome or trisomy 13 chromosomes. The symptoms are somewhat similar to trisomy 18, without the characteristic folded hand.
• Small extra marker chromosome. This means the presence of an extra abnormal chromosome. The properties depend on the origin of the additional genetic material. Cat's eye syndrome and isodicentric chromosome 15 (or idic15) syndrome are caused by an extra marker chromosome, like Pallister-Killian syndrome.
• Triple X syndrome (XXX). XXX girls tend to be taller, thinner and more likely to have dyslexia.
• Turner syndrome (X instead of XX or XY). With Turner syndrome, female sexual characteristics are present, but underdeveloped. Women with Turner syndrome have a short torso, low forehead, abnormal development of the eyes and bones, and a concave chest.
• Syndrome XYY. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are more likely to have learning difficulties.
• Wolf Hirschhorn syndrome, which is caused by partial destruction of the short arm of chromosome 4. It is characterized by severe growth retardation and severe mental health problems.

eukaryotic chromosomes

Centromere

Primary constriction

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

Secondary constrictions

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

Types of chromosome structure

There are four types of chromosome structure:

• telocentric (rod chromosomes with a centromere located at the proximal end);
• acrocentric(rod-shaped chromosomes with a very short, almost imperceptible 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 (satellites)

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

nucleolus zone

Zones of the nucleolus ( nucleolus organizers) are special areas associated with the appearance of some secondary constrictions.

Chromonema

A chromoneme 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 Veydovsky. Chromonema may consist of two, four or more threads, depending on the object under study. These threads form spirals of two types:

• paranemic(elements of the spiral 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 with a light microscope):

giant chromosomes

Such chromosomes, which are characterized by huge sizes, 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 it was possible to reveal signs of gene activity.

Polytene chromosomes

The Balbiani were first discovered in th, but their cytogenetic role was identified by Kostov, Paynter, Geitz, and Bauer. Contained in the cells of the salivary glands, intestines, trachea, fat body and malpighian vessels of Diptera larvae.

Lampbrush chromosomes

Bacterial chromosomes

There is evidence of the presence of proteins associated with nucleoid DNA in bacteria, but no histones have been found in them.

Literature

• E. de Robertis, V. Novinsky, F. Saez Biology of the cell. - M.: Mir, 1973. - S. 40-49.

see also

Wikimedia Foundation. 2010 .

• Khromchenko Matvey Solomonovich
• Chronicle

See what "Chromosomes" are in other dictionaries:

CHROMOSOMES- (from chromo ... and soma), organelles of the cell nucleus, which are carriers of genes and determine inheritances, properties of cells and organisms. They are capable of self-reproduction, have a structural and functional individuality and keep it in a row ... ... 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 a linear order on chromosomes. Self-duplication and regular distribution of chromosomes along ... ... Big Encyclopedic Dictionary

CHROMOSOMES- CHROMOSOMES, structures that carry genetic information about the body, 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 the nucleus of a cell containing DNA, which contains the hereditary information of an organism. Genes are arranged in a 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 their structural and functional identity and stain 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. H. cell set. Chromosomal theory of heredity. ... ... Dictionary Ozhegov

Chromosomes - self-reproducing structures of the cell nucleus. In both prokaryotic and eukaryotic organisms, genes are arranged in groups on separate DNA molecules, which, with the participation of proteins and other cell macromolecules, are organized into chromosomes. Mature cells of the germ line (gametes - eggs, sperm) of multicellular organisms contain one (haploid) set of chromosomes of the organism.

After complete sets of chromatids have moved to the poles, they are called chromosomes. Chromosomes are structures in the nucleus of eukaryotic cells that spatially and functionally organize DNA in the genome of individuals.

Chemical composition of chromosomes. The chromosome is a deoxyribonucleoprotein (DNP), that is, a complex formed from one continuous double-stranded DNA molecule and proteins (histones and non-histones). Chromosomes also contain lipids and minerals (for example, Ca 2+, Mg 2+ ions).

Each chromosome is complex supramolecular structure formed as a result of chromatin compaction.

The structure of chromosomes. In most cases, chromosomes are clearly visible only in dividing cells from the metaphase stage, when they can be seen even with a light microscope. During this period, it is possible to determine the number of chromosomes in the nucleus, their size, shape and structure. These chromosomes are called metaphase. Interphase chromosomes are often called simply chromatin.

The number of chromosomes is usually constant for all cells of an individual of any kind of plants, animals and humans. But in different species, the number of chromosomes is not the same (from two to several hundred). The horse roundworm has the smallest number of chromosomes, the largest is found in protozoa and ferns, which are characterized by high levels polyploidy. Typically, diploid sets contain from one to several dozen chromosomes.

The number of chromosomes in the nucleus is not related to the level of evolutionary development of living organisms. In many primitive forms, it is large, for example, in the nuclei of some types of protozoa there are hundreds of chromosomes, while in chimpanzees there are only 48.

Each chromosome is made up of one DNA molecule. elongated rod-shaped structure - chromatid, which has two "shoulders" separated by a primary constriction, or centromere. The metaphase chromosome consists of two sister chromatids connected by a centromere, each of which contains one DNA molecule, arranged in a spiral.

Centromere- This is a small fibrillar body that carries out the primary constriction of the chromosome. It is the most important part of the chromosome, as it determines its movement. The centromere to which the spindle fibers are attached during division (during mitosis and meiosis) is called kinetochore(from the Greek kinetos - mobile and choros - place). It controls the movement of divergent chromosomes during cell division. A chromosome lacking a centromere is unable to perform orderly movement and may be lost.

Usually the centromere of a chromosome occupies a certain place, and this is one of the species characteristics by which chromosomes are distinguished. A change in the position of the centromere in a particular chromosome serves as an indicator of chromosomal rearrangements. The arms of chromosomes end in regions that are unable to connect with other chromosomes or their fragments. These ends of chromosomes are called telomeres. Telomeres protect the ends of chromosomes from sticking together and thus ensure the preservation of their integrity. For the discovery of the mechanism of protection of chromosomes by telomeres and the enzyme telomerase, American scientists E. Blackburn, K. Greider and D. Shostak were awarded the Nobel Prize in Medicine and Physiology in 2009. The ends of chromosomes are often enriched heterochromatin.

Depending on the location of the centromere, three main types of chromosomes are determined: equal-armed (arms of equal length), unequal-armed (with arms of different lengths) and rod-shaped (with one, very long and another, very short, barely noticeable shoulder). Some chromosomes have not only one centromere, but also a secondary constriction that is not associated with the attachment of the spindle thread during division. This area - nucleolar organizer, which performs the function of synthesis of the nucleolus in the nucleus.

Chromosome replication

An important property of chromosomes is their ability to duplicate (self-reproduce). Chromosome duplication usually precedes cell division. Chromosome duplication is based on the process of replication (from Latin replicatio - repetition) of DNA macromolecules, which ensures the exact copying of genetic information and its transmission from generation to generation. Chromosome duplication is a complex process that involves not only the replication of giant DNA molecules, but also the synthesis of DNA-bound chromosomal proteins. The final stage is the packaging of DNA and proteins into special complexes that form a chromosome. As a result of replication, instead of one maternal chromosome, two identical daughter chromosomes appear.

Function of chromosomes is:

• in the storage of hereditary information. Chromosomes are carriers of genetic information;
• transmission of hereditary information. Hereditary information is transmitted by replication of the DNA molecule;
• implementation of hereditary information. Thanks to the reproduction of one or another type of i-RNA and, accordingly, one or another type of protein, control is exercised over all the vital processes of the cell and the whole organism.

Thus, chromosomes with genes enclosed in them cause a continuous series of reproduction.

Chromosomes carry out complex coordination and regulation of processes in the cell due to the genetic information contained in them, which ensures the synthesis of the primary structure of enzyme proteins.

Each species has a certain number of chromosomes in its cells. They are carriers of genes that determine the hereditary properties of cells and organisms of the species. Gene- this is a section of the DNA molecule of the chromosome, on which various RNA molecules (translators of genetic information) are synthesized.

Somatic, that is, bodily, cells usually contain a double, or diploid, set of chromosomes. It consists of pairs (2n) of almost identical in shape and size chromosomes. Such paired chromosome sets similar to each other are called homologous (from the Greek homos - equal, identical, common). They come from two organisms; one set from the mother and the other from the father. Such a paired set of chromosomes contains all the genetic information of the cell and the organism (individual). Homologous chromosomes are the same in shape, length, structure, location of the centromere and carry the same genes that have the same localization. They contain the same set of genes, although they may differ in their alleles. Thus, homologous chromosomes contain very close but not identical hereditary information.

The set of signs of chromosomes (their number, size, shape and details of the microscopic structure) in the cells of the body of an organism of one species or another is called karyotype. The shape of chromosomes, their number, size, location of the centromere, the presence of secondary constrictions are always specific for each species; they can be used to compare the relationship of organisms and establish their belonging to a particular species.

The constancy of the karyotype, characteristic of each species, developed in the process of its evolution and is due to the laws of mitosis and meiosis. However, during the existence of a species in its karyotype, due to mutations, changes in chromosomes can occur. Some mutations significantly change the hereditary qualities of the cell and the organism as a whole.

The constant characteristics of the chromosome set - the number and morphological features of chromosomes, determined mainly by the location of the centromeres, the presence of secondary constrictions, the alternation of euchromatic and heterochromatic regions, etc., make it possible to identify species. Therefore, the karyotype is called a "passport".

Chromosomes are self-reproducing structures of the cell nucleus. In both prokaryotic and eukaryotic organisms, genes are arranged in groups on separate DNA molecules, which, with the participation of proteins and other cell macromolecules, are organized into chromosomes. Mature cells of the germ line (gametes - eggs, sperm) of multicellular organisms contain one (haploid) set of chromosomes of the organism.

After complete sets of chromatids have moved to the poles, they are called chromosomes (chromosomes). Chromosomes are structures in the nucleus of eukaryotic cells that spatially and functionally organize DNA in the genome of individuals.

Each DNA molecule is packed into a separate chromosome, and all the genetic information stored in the chromosomes of one organism makes up its genome. It should be noted that chromosomes in a cell change their structure and activity in accordance with the stage of the cell cycle: in mitosis they are more condensed and transcriptionally inactivated; in interphase, on the contrary, they are active in relation to RNA synthesis and are less condensed.

To form a functional chromosome, a DNA molecule must be able not only to direct the synthesis of RNA, but also, by multiplying, be transmitted from one generation of cells to the next. This requires three types of specialized nucleotide sequences (these have been identified on the chromosomes of the yeast Saccharomyces cerevisiae).

1. For normal replication, a DNA molecule needs a specific sequence that acts as a replication origin (DNA replication origin).

2. The second necessary element - the centromere - holds two copies of the duplicated chromosome together and attaches any DNA molecule containing this sequence through a protein complex - the kinetochore to the mitotic spindle (during cell division so that each daughter cell receives one copy.

3. The third essential element that every linear chromosome needs is the telomere. The telomere is a special sequence at the end of each chromosome. This simple repeating sequence is periodically extended by a special enzyme, telomerase, and thus the loss of several nucleotides of telomere DNA that occurs in each replication cycle is compensated. As a result, the linear chromosome is completely replicated. All of the elements described above are relatively short (typically less than 1,000 base pairs each). Apparently, similar three types of sequences should also work in human chromosomes, but so far only telomeric sequences of human chromosomes have been well characterized.

In diploid (polyploid) organisms, whose cells contain one (several) set of chromosomes from each of the parents, identical chromosomes are called homologous chromosomes, or homologues. The identical chromosomes of different organisms of the same biological species are also homologous.

Genes and non-coding nucleotide sequences, enclosed in the chromosomes of cell nuclei, represent a large part of the genome of an organism.

In addition, the genome of an organism is also formed by extrachromosomal genetic elements, which during the mitotic cycle are reproduced independently of the chromosomes of the nuclei. Thus, the mitochondria of fungi and mammals contain about 1% of the total DNA, while the budding yeast Sacharomyces cerevisiae contains up to 20% of the DNA of the cell. The DNA of plant plastids (chloroplasts and mitochondria) makes up from 1 to 10% of the total amount of DNA.

The genes that make up individual chromosomes are located in one DNA molecule and form a linkage group; in the absence of recombination, they are transferred together from parent cells to daughter cells.

The physiological significance of the distribution of genes over individual chromosomes and the nature of the factors that determine the number of chromosomes in the eukaryotic genome remain not fully understood. For example, it is impossible to explain the evolutionary mechanisms of the appearance of a large number of chromosomes in specific organisms only by restrictions imposed on the maximum size of DNA molecules that make up these chromosomes. Thus, the genome of the American amphibian Amphiuma contains ~30 times more DNA than the human genome, and all DNA is contained in only 28 chromosomes, which is quite comparable with the human karyotype (46 chromosomes). However, even the smallest of these chromosomes is larger than the largest human chromosomes. The factors that limit the upper limit of the number of chromosomes in eukaryotes remain unknown. For example, in the butterfly Lysandra nivescens, the diploid set is 380-382 chromosomes, and there is no reason to believe that this value is the maximum possible.

Normally, the number of chromosomes in a person is 46. Examples: 46, XX, a healthy woman; 46, XY, healthy male.

As part of the capsid.

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✪ Chromosomes, chromatids, chromatin, etc.

✪ Genes, DNA and chromosomes

✪ The most important terms of genetics. loci and genes. homologous chromosomes. Coupling and crossing over.

✪ Chromosomal diseases. Examples and reasons. Biology video lesson Grade 10

✪ Cellular technologies. DNA. Chromosome. Genome. Program "In the first approximation"

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Before diving into the mechanics of cell division, I think it would be helpful to talk about the vocabulary associated with DNA. There are many words, and some of them sound similar to each other. They can be confusing. First, I would like to talk about how DNA generates more DNA, makes copies of itself, or how it makes proteins in general. We already talked about this in the video about DNA. Let me draw a small piece of DNA. I have A, G, T, let me have two Ts and then two Cs. Such a small area. It continues like this. Of course, this is a double helix. Each letter corresponds to its own. I will paint them with this color. So, A corresponds to T, G corresponds to C, (more precisely, G forms hydrogen bonds with C), T - with A, T - with A, C - with G, C - with G. This whole spiral stretches, let's say, in this direction . So there are a couple of different processes that this DNA has to carry out. One of them has to do with your body cells - you need to produce more of your skin cells. Your DNA has to copy itself. This process is called replication. You are replicating DNA. I'll show you replication. How can this DNA copy itself? This is one of the most remarkable features of the structure of DNA. Replication. I'm making a general simplification, but the idea is that two strands of DNA are separating, and it doesn't happen on its own. This is facilitated by the mass of proteins and enzymes, but in detail I will talk about microbiology in another video. So these chains are separated from each other. I'll move the chain here. They separate from each other. I'll take another chain. This one is too big. This circuit will look something like this. They separate from each other. What can happen after that? I'll remove extra pieces here and here. So here is our double helix. They were all connected. These are base pairs. Now they are separated from each other. What can each of them do after separation? They can now become a matrix for each other. Look... If this chain is on its own, now, all of a sudden, a thymine base can come along and join here, and these nucleotides begin to line up. Thymine and cytosine, and then adenine, adenine, guanine, guanine. And so it goes. And then, in this other part, on the green chain that was previously attached to this blue one, the same thing will happen. There will be adenine, guanine, thymine, thymine, cytosine, cytosine. What just happened? By separating and bringing in complementary bases, we have created a copy of this molecule. We'll get into the microbiology of this in the future, this is just to get a general idea of ​​how DNA replicates itself. Especially when we look at mitosis and meiosis, I can say, "This is the stage where replication occurs." Now, another process that you'll hear a lot more about. I talked about him in the DNA video. This is a transcription. In the DNA video, I didn't pay much attention to how DNA doubles itself, but one of the great things about the double strand design is that it's easy to duplicate itself. You just separate 2 strips, 2 spirals, and then they become a matrix for another chain, and then a copy appears. Now transcription. This is what must happen to DNA in order to form proteins, but transcription is an intermediate step. This is the stage where you move from DNA to mRNA. Then this mRNA leaves the cell nucleus and goes to the ribosomes. I will talk about this in a few seconds. So we can do the same. These chains are again separated during transcription. One is separating out here, and the other is separating... and the other will be separating out here. Wonderful. It may make sense to use only one half of the chain - I will remove one. That's the way. We're going to transcribe the green part. There she is. I will delete all this. Wrong color. So, I'm deleting all of this. What happens if instead of deoxyribonucleic acid nucleotides that pair with this DNA strand, you have ribonucleic acid, or RNA, that pairs. I will depict RNA in magenta. RNA will pair with DNA. Thymine, found in DNA, will pair with adenine. Guanine, now when we talk about RNA, instead of thymine, we will have uracil, uracil, cytosine, cytosine. And it will continue. This is mRNA. Messenger RNA. Now she is separating. This mRNA separates and leaves the nucleus. It leaves the nucleus, and then translation takes place. Broadcast. Let's write this term. Broadcast. It comes from mRNA... In the DNA video, I had a small tRNA. The transfer RNA was like a truck transporting amino acids to the mRNA. All this happens in a part of the cell called the ribosome. Translation occurs from mRNA to protein. We've seen it happen. So, from mRNA to protein. You have this chain - I'll make a copy. I will copy the whole chain at once. This chain separates, leaves the core, and then you have these little trucks of tRNA, which, in fact, drive up, so to speak. So let's say I have tRNA. Let's see adenine, adenine, guanine and guanine. This is RNA. This is a codon. A codon has 3 base pairs and an amino acid attached to it. You have some other parts of tRNA. Let's say uracil, cytosine, adenine. And another amino acid attached to it. Then the amino acids combine and form a long chain of amino acids, which is a protein. Proteins form these strange complex shapes. To make sure you understand. We'll start with DNA. If we make copies of DNA, that's replication. You are replicating DNA. So if we make copies of DNA, that's replication. If you start with DNA and create mRNA from a DNA template, that's transcription. Let's write down. "Transcription". That is, you transcribe information from one form to another - transcription. Now, when the mRNA leaves the nucleus of the cell... I'll draw a cell to draw attention to it. We will deal with cell structure in the future. If it's a whole cell, the nucleus is the center. This is where all DNA is, all replication and transcription takes place here. The mRNA then leaves the nucleus, and then in the ribosomes, which we will discuss in more detail in the future, translation occurs and protein is formed. So from mRNA to protein is translation. You are translating from the genetic code into the so-called protein code. So this is the broadcast. These are exactly the words that are commonly used to describe these processes. Make sure you use them correctly by naming the various processes. Now another part of DNA terminology. When I first met her, I thought she was extremely confusing. The word is "chromosome". I'll write down the words here - you can appreciate how confusing they are: chromosome, chromatin and chromatid. Chromatid. So, the chromosome, we've already talked about it. You may have a DNA strand. This is a double helix. This chain, if I enlarge it, is actually two different chains. They have connected base pairs. I just drew base pairs connected together. I want to be clear: I drew this little green line here. This is a double helix. It wraps around proteins called histones. Histones. Let her turn around like this and something like this, and then something like this. Here you have substances called histones, which are proteins. Let's draw them like this. Like this. It is a structure, that is, DNA in combination with proteins that structure it, causing it to wrap around further and further. Ultimately, depending on the life stage of the cell, different structures will form. And when you talk about nucleic acid , which is DNA, and combine it with proteins, then you are talking about chromatin. So chromatin is DNA plus the structural proteins that give DNA its shape. structural proteins. The idea of ​​chromatin was first used because of what people saw when they looked at a cell... Remember? Each time I drew the cell nucleus in a certain way. So to speak. This is the nucleus of the cell. I drew very distinct structures. This is one, this is another. Maybe she's shorter, and she has a homologous chromosome. I drew the chromosomes, right? And each of these chromosomes, as I showed in the last video, are essentially long structures of DNA, long strands of DNA wrapped tightly around each other. I drew it like this. If we zoom in, we'll see one chain, and it's really wrapped around itself like this. This is her homologous chromosome. Remember, in the video on variability, I talked about a homologous chromosome that codes for the same genes, but a different version of them. Blue is from dad and red is from mom, but they essentially code for the same genes. So this is one strand that I got from my dad with the DNA of this structure, we call it a chromosome. So chromosome. I want to make it clear, DNA only takes this form at certain life stages when it reproduces itself, ie. is replicated. More precisely, not so ... When the cell divides. Before a cell becomes capable of dividing, the DNA assumes this well-defined shape. For most of a cell's life, when the DNA is doing its job, when it's making proteins, meaning the proteins are being transcribed and translated from the DNA, it doesn't fold in that way. If it were folded, it would be difficult for the replication and transcription system to get to the DNA, make proteins, and do anything else. Usually DNA... Let me draw the nucleus again. Most of the time, you can't even see it with a regular light microscope. It is so thin that the entire helix of DNA is completely distributed in the nucleus. I draw it here, another one might be here. And then you have a shorter chain like this one. You can't even see her. It is not in this well-defined structure. It usually looks like this. Let there be such a short chain. You can only see a similar mess, consisting of a jumble of combinations of DNA and proteins. This is what people generally call chromatin. This needs to be written down. "Chromatin" So the words can be very ambiguous and very confusing, but the common usage when you talk about a well-defined single strand of DNA, well-defined structure like this, is chromosome. The concept of "chromatin" can refer either to a structure such as a chromosome, a combination of DNA and proteins that structure it, or to a disorder of many chromosomes that contain DNA. That is, from many chromosomes and proteins mixed together. I want this to be clear. Now the next word. What is a chromatid? Just in case I haven't done it already... I don't remember if I flagged it. These proteins that provide structure to chromatin or make up chromatin and also provide structure are called "histones". There are different types that provide structure at different levels, we'll look at them in more detail later. So what is a chromatid? When the DNA replicates... Let's say it was my DNA, it's in a normal state. One version is from dad, one version is from mom. Now it is replicated. The version from dad first looks like this. It's a big strand of DNA. It creates another version of itself, identical if the system is working properly, and that identical part looks like this. They are initially attached to each other. They are attached to each other at a place called the centromere. Now, despite the fact that I have 2 chains here, fastened together. Two identical chains. One chain here, one here ... Although let me put it differently. In principle, this can be represented in many different ways. This is one chain here, and here is another chain here. So we have 2 copies. They code for exactly the same DNA. So. They are identical, which is why I still call it a chromosome. Let's write it down too. All this together is called a chromosome, but now each individual copy is called a chromatid. So this is one chromatid and this is the other. They are sometimes called sister chromatids. They can also be called twin chromatids because they share the same genetic information. So this chromosome has 2 chromatids. Now, before replication, or before DNA duplication, you can say that this chromosome right here has one chromatid. You can call it a chromatid, but it doesn't have to be. People start talking about chromatids when two of them are present on a chromosome. We learn that in mitosis and meiosis these 2 chromatids separate. When they separate, there is a strand of DNA that you once called a chromatid, now you will call a single chromosome. So this is one of them, and here's another one that could have branched off in that direction. I'll circle this one in green. So this one can go to this side, and this one that I circled in orange, for example, to this ... Now that they are separated and no longer connected by a centromere, what we originally called one chromosome with two chromatids, now you call two separate chromosomes. Or you could say that you now have two separate chromosomes, each consisting of one chromatid. I hope this clears things up a bit meaning of terms associated with DNA. I have always found them rather confusing, but they will be a useful tool when we start mitosis and meiosis and I will talk about how a chromosome becomes a chromatid. You will ask how one chromosome became two chromosomes, and how a chromatid became a chromosome. It all revolves around vocabulary. I would choose another instead of calling it a chromosome and each of these individual chromosomes, but that's what they decided to call for us. You might be wondering where the word "chromo" comes from. Maybe you know an old Kodak film called "chrome color". Basically "chromo" means "color". I think it comes from the Greek word for color. When people first looked at the nucleus of a cell, they used a dye, and what we call chromosomes was stained with the dye. And we could see it with a light microscope. The part "soma" comes from the word "soma" meaning "body", that is, we get a colored body. Thus the word "chromosome" was born. Chromatin also stains... I hope this clarifies a little the concepts of "chromatid", "chromosome", "chromatin", and now we are prepared for the study of mitosis and meiosis.

The history of the discovery of chromosomes

The first descriptions of chromosomes appeared in articles and books by various authors in the 70s of the 19th century, and the priority of discovering chromosomes is given to different people. Among them are such names as I. D. Chistyakov (1873), A. Schneider (1873), E. Strasburger (1875), O. Büchli (1876) and others. Most often, the year of discovery of chromosomes is called 1882, and their discoverer is the German anatomist W. Fleming, who in his fundamental book "Zellsubstanz, Kern und Zelltheilung" collected and streamlined information about them, supplementing the results of his own research. The term "chromosome" was proposed by the German histologist G. Waldeyer in 1888. "Chromosome" literally means "colored body", since the basic dyes are well linked by chromosomes.

After the rediscovery of Mendel's laws in 1900, it took only one or two years for it to become clear that chromosomes during meiosis and fertilization behave exactly as expected from "heredity particles". In 1902 T. Boveri and in 1902-1903 W. Setton ( Walter Sutton) independently put forward a hypothesis about the genetic role of chromosomes.

In 1933, T. Morgan received the Nobel Prize in Physiology or Medicine for the discovery of the role of chromosomes in heredity.

Morphology of metaphase chromosomes

In the metaphase stage of mitosis, chromosomes consist of two longitudinal copies called sister chromatids, which are formed during replication. In metaphase chromosomes, sister chromatids are connected in the region primary constriction called the centromere. The centromere is responsible for separating sister chromatids into daughter cells during division. At the centromere, the kinetochore is assembled - a complex protein structure that determines the attachment of the chromosome to the microtubules of the spindle division - the movers of the chromosome in mitosis. The centromere divides chromosomes into two parts called shoulders. In most species, the short arm of the chromosome is denoted by the letter p, long shoulder - letter q. Chromosome length and centromere position are the main morphological features of metaphase chromosomes.

Three types of chromosome structure are distinguished depending on the location of the centromere:

This classification of chromosomes based on the ratio of arm lengths was proposed in 1912 by the Russian botanist and cytologist S. G. Navashin. In addition to the above three types, S. G. Navashin also singled out telocentric chromosomes, that is, chromosomes with only one arm. However, according to modern concepts, truly telocentric chromosomes do not exist. The second arm, even if very short and invisible in a conventional microscope, is always present.

An additional morphological feature of some chromosomes is the so-called secondary constriction, which outwardly differs from the primary one by the absence of a noticeable angle between the segments of the chromosome. Secondary constrictions are of various lengths and can be located at various points along the length of the chromosome. In the secondary constrictions, as a rule, there are nucleolar organizers containing multiple repeats of genes encoding ribosomal RNA. In humans, secondary constrictions containing ribosomal genes are located in the short arms of acrocentric chromosomes; they separate small chromosome segments from the main body of the chromosome, called satellites. Chromosomes that have a satellite are called SAT chromosomes (lat. SAT (Sine Acid Thymonucleinico)- without DNA).

Differential staining of metaphase chromosomes

With monochrome staining of chromosomes (aceto-carmine, aceto-orcein, Fölgen or Romanovsky-Giemsa staining), the number and size of chromosomes can be identified; their shape, determined primarily by the position of the centromere, the presence of secondary constrictions, satellites. In the vast majority of cases, these signs are not enough to identify individual chromosomes in the chromosome set. In addition, monochrome-stained chromosomes are often very similar across species. Differential staining of chromosomes, various methods of which were developed in the early 1970s, provided cytogenetics with a powerful tool for identifying both individual chromosomes as a whole and their parts, thereby facilitating the analysis of the genome.

Differential staining methods fall into two main groups:

Levels of compaction of chromosomal DNA

The basis of the chromosome is a linear DNA macromolecule of considerable length. In the DNA molecules of human chromosomes, there are from 50 to 245 million pairs of nitrogenous bases. The total length of DNA from one human cell is about two meters. At the same time, a typical human cell nucleus, which can only be seen with a microscope, occupies a volume of about 110 microns, and the average human mitotic chromosome does not exceed 5-6 microns. Such compaction of the genetic material is possible due to the presence in eukaryotes of a highly organized system of packing DNA molecules both in the interphase nucleus and in the mitotic chromosome. It should be noted that in proliferating cells in eukaryotes there is a constant regular change in the degree of compaction of chromosomes. Before mitosis, chromosomal DNA is compacted 105 times compared to the linear length of DNA, which is necessary for successful segregation of chromosomes into daughter cells, while in the interphase nucleus, for successful transcription and replication processes, the chromosome must be decompacted. At the same time, DNA in the nucleus is never completely elongated and is always packed to some extent. Thus, the estimated size reduction between a chromosome in interphase and a chromosome in mitosis is only about 2 times in yeast and 4-50 times in humans.

One of the latest levels of packaging in the mitotic chromosome, some researchers consider the level of the so-called chromonemes, the thickness of which is about 0.1-0.3 microns. As a result of further compaction, the chromatid diameter reaches 700 nm by the time of metaphase. The significant thickness of the chromosome (diameter 1400 nm) at the metaphase stage allows, finally, to see it in a light microscope. The condensed chromosome looks like the letter X (often with unequal arms), since the two chromatids resulting from replication are connected to each other in the centromere region (more on the fate of chromosomes during cell division see articles mitosis and meiosis).

Chromosomal abnormalities

Aneuploidy

With aneuploidy, a change in the number of chromosomes in the karyotype occurs, in which the total number of chromosomes is not a multiple of the haploid chromosome set n. In the case of the loss of one chromosome from a pair of homologous chromosomes, mutants are called monosomics, in the case of one extra chromosome, mutants with three homologous chromosomes are called trisomics, in case of loss of one pair of homologues - nullisomics. Autosomal aneuploidy always causes significant developmental disorders, being the main cause of spontaneous abortions in humans. One of the most famous aneuploidies in humans is trisomy 21, which leads to the development of Down syndrome. Aneuploidy is characteristic of tumor cells, especially of solid tumor cells.

Polyploidy

Change in the number of chromosomes, a multiple of the haploid set of chromosomes ( n) is called polyploidy. Polyploidy is widely and unevenly distributed in nature. Polyploid eukaryotic microorganisms are known - fungi and algae, polyploids are often found among flowering plants, but not among gymnosperms. Whole-body polyploidy is rare in metazoans, although they often have endopolyploidy some differentiated tissues, for example, the liver in mammals, as well as intestinal tissues, salivary glands, Malpighian vessels of a number of insects.

Chromosomal rearrangements

Chromosomal rearrangements (chromosomal aberrations) are mutations that disrupt the structure of chromosomes. They can arise in somatic and germ cells spontaneously or as a result of external influences (ionizing radiation, chemical mutagens, viral infection, etc.). As a result of chromosomal rearrangement, a fragment of a chromosome can be lost or, conversely, doubled (deletion and duplication, respectively); a segment of a chromosome can be transferred to another chromosome (translocation) or it can change its orientation within the chromosome by 180° (inversion). There are other chromosomal rearrangements.

Unusual types of chromosomes

microchromosomes

B chromosomes

B chromosomes are extra chromosomes that are found in the karyotype only in certain individuals in a population. They are often found in plants and have been described in fungi, insects, and animals. Some B chromosomes contain genes, often rRNA genes, but it is not clear how functional these genes are. The presence of B chromosomes can affect the biological characteristics of organisms, especially in plants, where their presence is associated with reduced viability. It is assumed that B chromosomes are gradually lost in somatic cells as a result of their irregular inheritance.

Holocentric chromosomes

Holocentric chromosomes do not have a primary constriction, they have a so-called diffuse kinetochore, therefore, during mitosis, spindle microtubules are attached along the entire length of the chromosome. During chromatid divergence to the poles of division in holocentric chromosomes, they go to the poles parallel to each other, while in a monocentric chromosome, the kinetochore is ahead of the rest of the chromosome, which leads to a characteristic V-shaped diverging chromatids at the anaphase stage. During fragmentation of chromosomes, for example, as a result of exposure to ionizing radiation, fragments of holocentric chromosomes diverge towards the poles in an orderly manner, and fragments of monocentric chromosomes that do not contain centromeres are randomly distributed between daughter cells and may be lost.

Holocentric chromosomes are found in protists, plants, and animals. Nematodes have holocentric chromosomes C. elegans .

Giant forms of chromosomes

Polytene chromosomes

Polytene chromosomes are giant agglomerations of chromatids that occur in certain types of specialized cells. First described by E. Balbiani ( Edouard-Gerard Balbiani) in 1881 in the cells of the salivary glands of the bloodworm ( Chironomus), their study was continued already in the 30s of the XX century by Kostov, T. Paynter, E. Heitz and G. Bauer ( Hans Bauer). Polytene chromosomes have also been found in the cells of the salivary glands, intestines, trachea, fat body, and Malpighian vessels of Diptera larvae.

Lampbrush chromosomes

The lampbrush chromosome is a giant form of chromosome that occurs in meiotic female cells during the diplotene stage of prophase I in some animals, notably some amphibians and birds. These chromosomes are extremely transcriptionally active and are observed in growing oocytes when the processes of RNA synthesis leading to the formation of the yolk are most intense. At present, 45 animal species are known in whose developing oocytes such chromosomes can be observed. Lampbrush chromosomes are not produced in mammalian oocytes.

Lampbrush-type chromosomes were first described by W. Flemming in 1882. The name "lampbrush chromosomes" was proposed by the German embryologist I. Rückert ( J. Rϋckert) in 1892.

Lampbrush-type chromosomes are longer than polytene chromosomes. For example, the total length of the chromosome set in the oocytes of some caudate amphibians reaches 5900 µm.

Bacterial chromosomes

There is evidence of the presence of proteins associated with nucleoid DNA in bacteria, but no histones have been found in them.

human chromosomes

The normal human karyotype is represented by 46 chromosomes. These are 22 pairs of autosomes and one pair of sex chromosomes (XY in the male karyotype and XX in the female). The table below shows the number of genes and bases in human chromosomes.

Chromosome	Total bases	Number of genes	Number of protein-coding genes
	249250621	3511	2076
	243199373	2368	1329
	198022430	1926	1077
	191154276	1444	767
	180915260	1633	896
	171115067	2057	1051
	159138663	1882	979
	146364022	1315	702
	141213431	1534	823
	135534747	1391	774
	135006516	2168	1914
	133851895	1714	1068
	115169878	720	331
	107349540	1532	862
	102531392	1249	615
	90354753	1326	883
	81195210	1773	1209
	78077248	557	289
	59128983	2066	1492
	63025520	891	561
	48129895	450	246
	51304566	855	507
X chromosome	155270560	1672	837
Y chromosome	59373566	429	76
Total	3 079 843 747	36463

see also

Notes

1. Tarantula V.Z. Explanatory biotechnological dictionary. - M.: Languages ​​of Slavic cultures, 2009. - 936 p. - 400 copies. - ISBN 978-5-9551-0342-6.