Structure and functions of chromosomes. Reproduction in the organic world

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

After complete sets of chromatids move to the poles, they are called chromosomes. Chromosomes are structures in the nucleus of eukaryotic cells that spatially and functionally organize the 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 formation, formed as a result of chromatin compaction.

The structure of chromosomes. In most cases, chromosomes are clearly visible only in dividing cells, starting 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 species of plants, animals and humans. But different types the number of chromosomes varies (from two to several hundred). Smallest number The horse roundworm has chromosomes; the greatest number 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 evolutionary development living organisms. In many primitive forms it is large; for example, the nuclei of some protozoan species contain hundreds of chromosomes, while chimpanzees have only 48.

Each chromosome, formed by one DNA molecule, is elongated rod-shaped structure - chromatid, having two “arms” separated by a primary constriction, or centromere. A metaphase chromosome consists of two sister chromatids connected by a centromere, each of which contains one DNA molecule arranged in a spiral.

Centromere is a small fibrillar body that performs 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 filaments are attached during division (mitosis and meiosis) is called kinetochore(from the Greek kinetos - mobile and choros - place). It controls the movement of diverging chromosomes during cell division. A chromosome lacking a centromere is unable to move in an orderly manner and may become 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. Chromosome arms 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 thereby ensure the preservation of their integrity. For the discovery of the mechanism of chromosome protection by telomeres and the enzyme telomerase, American scientists E. Blackburn, K. Greider and D. Shostak were awarded in 2009 Nobel Prize in the field of medicine and physiology. 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 arm). 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 is nucleolar organizer, which performs the function of synthesizing the nucleolus in the nucleus.

Chromosome replication

An important property of chromosomes is their ability to duplicate (self-replicate). Chromosome duplication usually precedes cell division. Chromosome doubling is based on the process of replication (from the Latin replicatio - repetition) of DNA macromolecules, which ensures accurate copying of genetic information and its transmission from generation to generation. Chromosome doubling is difficult process, which includes not only the replication of giant DNA molecules, but also the synthesis of DNA-associated 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.

Chromosome function is:

• in storing hereditary information. Chromosomes are carriers of genetic information;
• transmission of hereditary information. Hereditary information is transmitted by replication of the DNA molecule;
• realization of hereditary information. Thanks to the reproduction of one or another type of mRNA and, accordingly, one or another type of protein, control is exercised over all life processes of the cell and the entire organism.

Thus, chromosomes with the genes contained in them determine 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 a 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 chromosomes almost identical in shape and size. Such paired, similar chromosome sets are called homologous (from the Greek homos - equal, identical, common). They come from two organisms; one set is from the mother's, and the other is from the father's. This paired set of chromosomes contains all the genetic information of the cell and organism (individual). Homologous chromosomes are identical in shape, length, structure, centromere location and carry the same genes with the same localization. They contain the same set of genes, although their alleles may differ. Thus, homologous chromosomes contain very close, but not identical, hereditary information.

The set of characteristics of chromosomes (their number, size, shape and details of microscopic structure) in the cells of the body of an organism of a particular species is called karyotype. The shape of chromosomes, their number, size, location of the centromere, and the presence of secondary constrictions are always specific to each species; using them, one can compare the relatedness of organisms and establish their belonging to a particular species.

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

Constant characteristics of the chromosome set - number and morphological features chromosomes, determined mainly by the location of centromeres, the presence of secondary constrictions, alternation of euchromatic and heterochromatic regions, etc., allow species to be identified. Therefore the karyotype is called "passport" of the type.

Chromosome is a thread-like structure containing DNA in the cell nucleus, which carries genes, units of heredity, located in 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.

Man is biological species, which reproduces sexually and has 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 experience a delay in mental development and their crying in childhood Reminds me of a cat's cry.

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. Later August Weissman 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

Heredity and variability in living nature exist thanks to chromosomes, genes, (DNA). It is stored and transmitted as a chain of nucleotides as part of DNA. What role do genes play in this phenomenon? What is a chromosome from the point of view of transmission of hereditary characteristics? Answers to questions like these provide insight into coding principles and genetic diversity on our planet. It largely depends on how many chromosomes are included in the set and on the recombination of these structures.

From the history of the discovery of “particles of heredity”

Studying plant and animal cells under a microscope, many botanists and zoologists back in the mid-19th centuries paid attention to the thinnest threads and the smallest ring-shaped structures in the core. More often than others, the German anatomist Walter Flemming is called the discoverer of chromosomes. It was he who used aniline dyes to treat nuclear structures. Flemming called the discovered substance “chromatin” for its ability to stain. The term “chromosomes” was introduced into scientific use in 1888 by Heinrich Waldeyer.

At the same time as Flemming, the Belgian Eduard van Beneden was looking for an answer to the question of what a chromosome is. A little earlier, German biologists Theodor Boveri and Eduard Strassburger conducted a series of experiments proving the individuality of chromosomes and the constancy of their number in different species of living organisms.

Prerequisites for the chromosomal theory of heredity

American researcher Walter Sutton found out how many chromosomes are contained in the cell nucleus. The scientist considered these structures to be carriers of units of heredity, characteristics of the organism. Sutton discovered that chromosomes consist of genes through which properties and functions are passed on to offspring from their parents. The geneticist in his publications gave descriptions of chromosome pairs and their movement during the division of the cell nucleus.

Regardless of his American colleague, work in the same direction was carried out by Theodore Boveri. Both researchers in their works studied the issues of transmission of hereditary characteristics and formulated the main provisions on the role of chromosomes (1902-1903). Further development of the Boveri-Sutton theory took place in the laboratory of Nobel laureate Thomas Morgan. The outstanding American biologist and his assistants established a number of patterns in the placement of genes on the chromosome and developed a cytological basis that explains the mechanism of the laws of Gregor Mendel, the founding father of genetics.

Chromosomes in a cell

The study of the structure of chromosomes began after their discovery and description in the 19th century. These bodies and filaments are found in prokaryotic organisms (non-nuclear) and eukaryotic cells (in nuclei). Study under a microscope made it possible to establish what a chromosome is from a morphological point of view. It is a mobile filamentous body that is visible during certain phases of the cell cycle. In interphase, the entire volume of the nucleus is occupied by chromatin. During other periods, chromosomes are distinguishable in the form of one or two chromatids.

These formations are better visible during cell division - mitosis or meiosis. In eukaryotic cells, large chromosomes with a linear structure can often be observed. In prokaryotes they are smaller, although there are exceptions. Cells often contain more than one type of chromosome, for example mitochondria and chloroplasts have their own small “particles of inheritance”.

Chromosome shapes

Each chromosome has an individual structure and differs from others in its coloring features. When studying morphology, it is important to determine the position of the centromere, the length and placement of the arms relative to the constriction. The set of chromosomes usually includes the following forms:

• metacentric, or equal arms, which are characterized by a median location of the centromere;
• submetacentric, or unequal arms (the constriction is shifted towards one of the telomeres);
• acrocentric, or rod-shaped, in which the centromere is located almost at the end of the chromosome;
• dotted with a difficult-to-define shape.

Functions of chromosomes

Chromosomes consist of genes - functional units of heredity. Telomeres are the ends of chromosome arms. These specialized elements serve to protect against damage and prevent fragments from sticking together. The centromere performs its tasks during chromosome doubling. It has a kinetochore, and it is to this that the spindle structures are attached. Each pair of chromosomes is individual in the location of the centromere. The spindle threads work in such a way that one chromosome at a time goes to the daughter cells, and not both. Uniform doubling during division is provided by the origins of replication. Duplication of each chromosome begins simultaneously at several such points, which significantly speeds up the entire division process.

Role of DNA and RNA

It was possible to find out what a chromosome is and what function this nuclear structure performs after studying its biochemical composition and properties. In eukaryotic cells, nuclear chromosomes are formed by a condensed substance - chromatin. According to the analysis, it contains high molecular weight organic matter:

Nucleic acids are directly involved in the biosynthesis of amino acids and proteins and ensure the transmission of hereditary characteristics from generation to generation. DNA is contained in the nucleus of a eukaryotic cell, RNA is concentrated in the cytoplasm.

Genes

X-ray diffraction analysis showed that DNA forms a double helix, the chains of which consist of nucleotides. They represent the carbohydrate deoxyribose, a phosphate group, and one of four nitrogenous bases:

Regions of helical deoxyribonucleoprotein strands are genes that carry encoded information about the sequence of amino acids in proteins or RNA. During reproduction, hereditary characteristics from parents are transmitted to offspring in the form of gene alleles. They determine the functioning, growth and development of a particular organism. According to a number of researchers, those sections of DNA that do not encode polypeptides perform regulatory functions. The human genome can contain up to 30 thousand genes.

Set of chromosomes

The total number of chromosomes, their features - characteristic feature kind. In the Drosophila fly their number is 8, in primates - 48, in humans - 46. This number is constant for the cells of organisms that belong to the same species. For all eukaryotes there is the concept of “diploid chromosomes”. This is a complete set, or 2n, as opposed to haploid - half the number (n).

Chromosomes in one pair are homologous, identical in shape, structure, location of centromeres and other elements. Homologs have their own characteristics, which distinguish them from other chromosomes in the set. Staining with basic dyes allows you to examine and study distinctive features each pair. is present in the somatic ones - in the reproductive ones (the so-called gametes). In mammals and other living organisms with a heterogametic male sex, two types of sex chromosomes are formed: the X chromosome and the Y. Males have a set of XY, females have a set of XX.

Human chromosome set

The cells of the human body contain 46 chromosomes. All of them are combined into 23 pairs that make up the set. There are two types of chromosomes: autosomes and sex chromosomes. The first form 22 pairs - common for women and men. What differs from them is the 23rd pair - sex chromosomes, which are non-homologous in the cells of the male body.

Genetic traits are associated with gender. They are transmitted by a Y and an X chromosome in men and two X chromosomes in women. Autosomes contain the rest of the information about hereditary traits. There are techniques that allow you to individualize all 23 pairs. They are clearly distinguishable in the drawings when painted in a certain color. It is noticeable that the 22nd chromosome in the human genome is the smallest. Its DNA, when stretched, is 1.5 cm long and has 48 million nitrogen base pairs. Special histone proteins from chromatin perform compression, after which the thread takes up thousands of times less space in the cell nucleus. Under electron microscope the histones in the interphase core resemble beads strung on a strand of DNA.

Genetic diseases

There are more than 3 thousand hereditary diseases of various types caused by damage and abnormalities in chromosomes. These include Down syndrome. A child with such a genetic disease is characterized by mental and mental retardation. physical development. With cystic fibrosis, a malfunction occurs in the functions of the exocrine glands. Violation leads to problems with sweating, secretion and accumulation of mucus in the body. It makes it difficult for the lungs to function and can lead to suffocation and death.

Color vision impairment - color blindness - insensitivity to certain parts of the color spectrum. Hemophilia leads to weakened blood clotting. Lactose intolerance prevents the human body from digesting milk sugar. In family planning offices you can find out about your predisposition to a particular genetic disease. In large medical centers there is an opportunity to undergo appropriate examination and treatment.

Gene therapy is a direction of modern medicine, identifying the genetic cause of hereditary diseases and eliminating it. By using the latest methods Normal genes are introduced into pathological cells instead of damaged ones. In this case, doctors relieve the patient not from the symptoms, but from the causes that caused the disease. Only correction of somatic cells is carried out; gene therapy methods are not yet applied en masse to germ cells.

A chromosome is an elongated, structured collection of genes that carries information about heredity and is formed from condensed. Chromatin is made up of DNA and proteins that are tightly packed together to form chromatin fibers. Condensed chromatin fibers form chromosomes. Chromosomes are located in our. The sets of chromosomes join together (one from the mother and one from the father) and are known as .

Scheme of chromosome structure at the metaphase stage

Unduplicated chromosomes are single-stranded and consist of a region that connects the chromosome arms. The short arm is indicated by the letter p, and the long one is a letter q. The terminal regions of chromosomes are called telomeres, which consist of repeated non-coding DNA sequences that are shortened during cell division.

Chromosome duplication

Chromosomal duplication occurs before division processes through or. DNA replication processes make it possible to preserve correct number chromosomes after parent cell division. A duplicated chromosome consists of two identical chromosomes, called chromosomes, which are linked at the centromere. The sisters remain together until the end of the division process, where they are separated by spindle fibers and enclosed in. Once paired chromatids are separated from each other, each of them becomes .

Chromosomes and cell division

One of the most important elements successful cell division is the correct distribution of chromosomes. In mitosis, this means that the chromosomes must be distributed between the two daughter cells. In meiosis, chromosomes are distributed among four daughter cells. The spindle is responsible for moving chromosomes during cell division.

This type of cell movement involves interactions between spindle microtubules and motor proteins working together to separate chromosomes. It is vitally important that the daughter cells maintain correct amount chromosomes. Errors that occur during cell division can result in unbalanced chromosome numbers, having too many or not enough chromosomes. This abnormality is known as aneuploidy and can occur on autosomal chromosomes during mitosis or on sex chromosomes during meiosis. Abnormalities in chromosomal numbers can lead to birth defects, developmental disorders and death.

Chromosomes and protein production

Protein production is a vital cellular process that depends on DNA and chromosomes. DNA contains segments called genes that code for proteins. During protein production, the DNA is unwound and its coding segments are transcribed into an RNA transcript. The RNA transcript is then translated to form a protein.

Chromosome mutation

Chromosome mutations are changes that occur in chromosomes and are usually the result of errors that occur during meiosis or when exposed to mutagens such as chemicals or radiation.

Chromosome breakage and duplication can lead to several types of chromosome structural changes that are usually harmful to humans. These types of mutations result in chromosomes with extra genes that are in the wrong sequence. Mutations can also produce cells with the wrong number of chromosomes. Abnormal chromosome numbers usually result from nondisjunction or disruption of homologous chromosomes during meiosis.

The most important organelles of the cell are microscopic structures located in the core. They were discovered simultaneously by several scientists, including Russian biologist Ivan Chistyakov.

The name of the new cellular component was not immediately invented. He gave it German scientist W. Waldeyer, who, while staining histological preparations, discovered certain bodies that stained well with fuchsin. At that time it was not yet known exactly what role chromosomes play.

In contact with

Meaning

Structure

Let's consider what structure and functions these unique cellular formations have. In the interphase state they are practically invisible. At this stage, the molecule doubles and forms two sister chromatids.

The structure of a chromosome can be examined at the time of its preparation for mitosis or meiosis (division). Such chromosomes are called metaphase, because they are formed at the stage of metaphase, preparation for division. Until this moment, the bodies are inconspicuous thin dark threads which are called chromatin.

During the transition to the metaphase stage, the structure of the chromosome changes: it is formed by two chromatids connected by a centromere - this is called primary constriction. During cell division the amount of DNA also doubles. Schematic drawing resembles the letter X. They contain, in addition to DNA, proteins (histone, non-histone) and ribonucleic acid - RNA.

The primary constriction divides the cell body (nucleoprotein structure) into two arms, slightly bending them. Based on the location of the constriction and the length of the arms, a next classification types:

• metacentric, they are also equal-armed, the centromere divides the cell exactly in half;
• submetacentric. Shoulders are not the same, the centromere is shifted closer to one end;
• acrocentric. The centromere is strongly shifted and is located almost at the edge;
• telocentric. One shoulder is completely missing does not occur in humans.

Some species have secondary constriction, which can be located at different points. It separates a part called the satellite. It differs from the primary one in that doesn't have visible angle between segments. Its function is to synthesize RNA on a DNA template. It occurs in people in 13, 14, 21 and 15, 21 and 22 pairs of chromosomes. Appearing in another couple carries the risk of serious illness.

Now let's look at what function the chromosomes perform. Thanks to reproduction different types mRNA and proteins they carry out clear control over all processes of cell life and the body as a whole. Chromosomes in the nucleus of eukaryotes perform the functions of synthesizing proteins from amino acids, carbohydrates from inorganic compounds, breaking down organic substances into inorganic ones, store and transmit hereditary information.

Diploid and haploid sets

The specific structure of chromosomes may differ depending on where they are formed. What is the name of the set of chromosomes in somatic cell structures? It is called diploid or double. Somatic cells reproduce by simple division into two daughters. In ordinary cellular formations, each cell has its own homologous pair. This happens because each of the daughter cells must have the same volume of hereditary information, as the mother's.

How do the number of chromosomes in somatic and germ cells compare? Here the numerical ratio is two to one. During the formation of germ cells, special type of division, as a result, the set in mature eggs and sperm becomes single. What function chromosomes perform can be explained by studying the features of their structure.

Male and female reproductive cells have half set called haploid, that is, there are 23 of them in total. The sperm merges with the egg, resulting in a new organism with a complete set. The genetic information of a man and a woman is thus combined. If germ cells carried a diploid set (46), then when united, the result would be non-viable organism.

Genome diversity

The number of carriers of genetic information differs among different classes and species of living beings.

They have the ability to be painted with specially selected dyes; they alternate in their structure light and dark transverse sections - nucleotides. Their sequence and location are specific. Thanks to this, scientists have learned to distinguish cells and, if necessary, clearly indicate the “broken” one.

Currently, geneticists deciphered the person and compiled genetic maps, which allows the analysis method to suggest some serious hereditary diseases even before they appear.

There is now an opportunity to confirm paternity, determine ethnicity, to identify whether a person is a carrier of any pathology that has not yet manifested itself or is dormant inside the body, to determine the characteristics negative reaction to medications and much more.

A little about pathology

During the transfer of the gene set, there may be failures and mutations, leading to serious consequences, among them are

• deletions - loss of one section of the shoulder, causing underdevelopment of organs and brain cells;
• inversions - processes in which a fragment is flipped 180 degrees, the result is incorrect gene sequence;
• duplications – bifurcation of a section of the shoulder.

Mutations can also occur between adjacent bodies - this phenomenon was called translocation. The well-known Down, Patau, and Edwards syndromes are also a consequence disruption of the gene apparatus.

Chromosomal diseases. Examples and reasons

Classification of cells and chromosomes

Conclusion

The importance of chromosomes is great. Without these tiny ultrastructures transfer of genetic information is impossible, therefore, the organisms will not be able to reproduce. Modern technologies can read the code embedded in them and successfully prevent possible diseases which were previously considered incurable.