Functions and structural features of chromosomes. What role do chromosomes play in a cell: structure and functions How is a chromosome formed

05.08.2024

Today we propose to consider in as much detail as possible an interesting question from a school biology course - what is a chromosome? This term comes up quite often in biology, but what does it mean? Let's figure it out.

Let's start, perhaps, with the concept of “period of cell life”. This is a period of time that begins from its very origin until death. It is also customary to call this time interval the life cycle. Even within the same organism, the length of the cycle varies depending on the variety. For example, let’s take a cell of epithelial tissue and liver; the life cycle of the first is only about fifteen hours, and the second is a year. It is also important to note the fact that the entire period of cell life is divided into two intervals:

interphase;
division.

Chromosomes play an important role in the life cycle of a cell. Let's move on to the definition of what is a chromosome in biology? It is a complex of DNA molecules and proteins. We will talk about their functions in more detail later in the article.

A little history

What a chromosome is in biology was known back in the mid-nineteenth century, thanks to the research of the German botanist W. Hoffmeister. The scientist at this time became interested in studying cell division in a plant called Tradescantia. What new thing could he discover? To begin with, it became clear that before cell division, nuclear division also occurs. But that's not the most interesting thing! Even before two daughter nuclei are formed, the main one splits into very thin threads. They can only be seen under a microscope, stained with a special dye.

Then the Chamberlain gave them a name - chromosomes. What is a chromosome in biology? If we translate the term into Russian literally, we get “painted bodies.” A little later, scientists noticed that these thread-like particles are present in the nucleus of absolutely any plant or animal cell. But once again we draw your attention to the fact that their number varies depending on the type of cell and organism. If we take a person, then his cells contain only forty-six chromosomes.

Theory of heredity

We have already defined what a chromosome is in biology. Now we propose to move on to genetics, namely the transfer of genetic material from parents to descendants.

Thanks to the work of Walter Sutton, the number of chromosomes in cells became known. In addition, the scientist argued that it is these tiny particles that are carriers of units of heredity. Sutton also found out that chromosomes consist of genes.

At the same time, similar work was carried out by Theodore Boveri. It is important to note that both scientists studied this issue and came to the same conclusion. They studied and formulated the basic principles of the role of chromosomes.

Cells

After the discovery and description of chromosomes in the mid-nineteenth century, scientists began to take an interest in their structure. It became clear that these bodies are found in absolutely any cell, regardless of whether the cell in front of us is prokaryotic or eukaryotic.

Microscopes helped in studying the structure. Scientists were able to establish several facts:

chromosomes are thread-like bodies;
they can only be observed during certain phases of the cycle;
if you study in interphase, you will notice that the nucleus consists of chromatin;
during other periods, chromosomes consisting of one or two chromatids can be distinguished;
the best time to study is mitosis or meiosis (the whole point is that during the process of cell division these bodies are better visible);
in eukaryotes, large chromosomes with a linear structure are most often found;
very often cells have several types of chromosomes.

Forms

We dealt with the question - what is a chromosome in biology, but did not say anything about the possible varieties. We suggest filling this gap immediately.

So, in total there are four forms:

metacentric (if the centromere is in the middle);
submetacentric (centromere shift to one end);
acrocentric, another name is rod-shaped (if the centromere is located at any end of the chromosome);
telocentric (they are also called point ones, since it is very difficult to see the shape due to its small size).

Functions

The chromosome is the supramolecular level of organization of genetic material. The main component is DNA. It has a number of important functions:

storage of genetic material;
its transfer;
its implementation.

Genetic material is presented in the form of genes. It is important to note that there are many (from several hundred to thousands) genes on one chromosome; it has the following features:

a chromosome represents only one linkage group;
organizes the location of genes;
ensures joint inheritance of all genes.

Each individual cell has a diploid set of chromosomes. Biology is a very exciting subject that, if taught correctly, will interest many students. Now let's take a closer look at DNA and RNA.

DNA and RNA

What are chromosomes made of? If we are talking about eukaryotes, then these particles in cells are formed using chromatin. The latter includes:

deoxyribonucleic acid (abbreviated as DNA);
ribonucleic acid (abbreviated as RNA);
proteins.

Everything listed above is high molecular weight organic substances. In terms of location, DNA can be found in the nucleus in eukaryotes, and RNA in the cytoplasm.

Genes and chromosomes

Biology examines the issue of genetics in some detail, starting from school. Let's refresh our memory, what is a gene anyway? It is the smallest unit of all genetic material. A gene is a section of DNA or RNA. The second case occurs in viruses. It is he who encodes the development of a certain trait.

It is also important to note that the gene is responsible for only one trait; functionally it is indivisible. Now let's move on to X-ray diffraction analysis of DNA. So, the latter forms a double helix. Its chains consist of nucleotides. The latter are a deoxyribose carbohydrate, a phosphate group and a nitrogenous base. But here it’s a little more interesting; there can be several types of nitrogenous bases:

adenine;
guanine;
thymine;
cytosine.

Chromosome set

The species depends on the number of chromosomes and their characteristics. For example, let's take:

Drosophila flies (eight chromosomes each);
primates (forty-eight chromosomes each);
people (forty-six chromosomes each).

And this number is constant for a particular type of organism. All eukaryotic cells have a diploid set of chromosomes (2n), and haploid is half of it (that is, n). In addition, a pair of chromosomes is always homologous. What does homologous chromosomes mean in biology? These are those that are completely identical (in shape, structure, location of centromeres, and so on).

It is also very important to note that the diploid set is inherent in somatic cells, and the haploid set is inherent in sexual cells.

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

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

History of the discovery of chromosomes

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

Chromosomal theory of heredity

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

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

Chromosome structure

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

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

Shapes and types of chromosomes

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

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

Functions of chromosomes

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

Set of chromosomes

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

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

Human chromosome set

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

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

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

Genetic diseases associated with chromosomes

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

Chromosomes, video

And finally, an interesting educational video about chromosomes.

This article is available in English - .

Eukaryotic chromosomes

Centromere

Primary constriction

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

Secondary constrictions

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

Types of chromosome structure

There are four types of chromosome structure:

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

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

Satellites

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

Nucleolar zone

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

Chromonema

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

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

Chromosomal rearrangements

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

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

Giant chromosomes

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

Polytene chromosomes

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

Lamp brush chromosomes

Bacterial chromosomes

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

Literature

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

History of the discovery of chromosomes

Drawing from W. Flemming’s book depicting different stages of salamander epithelial cell division (W. Flemming. Zellsubstanz, Kern und Zelltheilung. 1882)

In different articles and books, priority for the discovery of chromosomes is given to different people, but most often the year of discovery of chromosomes is called 1882, and their discoverer is the German anatomist W. Fleming. However, it would be fairer to say that he did not discover chromosomes, but in his fundamental book “Zellsubstanz, Kern und Zelltheilung” (German) he collected and organized information about them, supplementing them with the results of his own research. The term "chromosome" was proposed by the German histologist Heinrich Waldeyer in 1888; "chromosome" literally means "colored body", since the basic dyes are well bound by chromosomes.

Now it is difficult to say who made the first description and drawing of chromosomes. In 1872, the Swiss botanist Carl von Negili published a work in which he depicted certain bodies that appear in place of the nucleus during cell division during the formation of pollen in a lily ( Lilium tigrinum) and Tradescantia ( Tradescantia). However, his drawings do not allow us to unequivocally state that K. Negili saw exactly chromosomes. In the same 1872, the botanist E. Russov presented his images of cell division during the formation of spores in a fern of the genus Zhovnik ( Ophioglossum) and lily pollen ( Lilium bulbiferum). In his illustrations it is easy to recognize individual chromosomes and stages of division. Some researchers believe that the German botanist Wilhelm Hofmeister was the first to see chromosomes long before K. Negili and E. Russow, back in 1848-1849. At the same time, neither K. Negili, nor E. Russov, nor even more so V. Hofmeister realized the significance of what they saw.

After the rediscovery of Mendel's laws in 1900, it took only one or two years for it to become clear that chromosomes behaved exactly as expected from “particles of heredity.” In 1902 T. Boveri and in 1902-1903 W. Setton ( Walter Sutton) independently of each other were the first to put forward a hypothesis about the genetic role of chromosomes. T. Boveri discovered that the sea urchin embryo Paracentrotus lividus can develop normally only if there is at least one, but complete set of chromosomes. He also found that different chromosomes are not identical in composition. W. Setton studied gametogenesis in the locust Brachystola magna and realized that the behavior of chromosomes in meiosis and during fertilization fully explains the patterns of divergence of Mendelian factors and the formation of their new combinations.

Experimental confirmation of these ideas and the final formulation of the chromosome theory was made in the first quarter of the 20th century by the founders of classical genetics, who worked in the USA with the fruit fly ( D. melanogaster): T. Morgan, K. Bridges ( C.B.Bridges), A. Sturtevant ( A.H. Sturtevant) and G. Möller. Based on their data, they formulated the “chromosomal theory of heredity,” according to which the transmission of hereditary information is associated with chromosomes in which genes are localized linearly, in a certain sequence. These findings were published in 1915 in the book The Mechanisms of Mendelian Heredity.

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

Eukaryotic chromosomes

The basis of the chromosome is a linear (not closed in a ring) macromolecule of deoxyribonucleic acid (DNA) of considerable length (for example, in the DNA molecules of human chromosomes there are from 50 to 245 million pairs of nitrogenous bases). When stretched, the length of a human chromosome can reach 5 cm. In addition to it, the chromosome includes five specialized proteins - H1, H2A, H2B, H3 and H4 (the so-called histones) and a number of non-histone proteins. The amino acid sequence of histones is highly conserved and practically does not differ in the most diverse groups of organisms.

Primary constriction

Chromosome constriction (X. n.), in which the centromere is localized and which divides the chromosome into arms.

Secondary constrictions

Types of chromosome structure

There are four types of chromosome structure:

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

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

Satellites

Nucleolar zone

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

Chromonema

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

Chromosomal rearrangements

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

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

Giant chromosomes

Polytene chromosomes

Lamp brush chromosomes

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

Human chromosomes

Each nucleated human somatic cell contains 23 pairs of linear chromosomes, as well as numerous copies of mitochondrial DNA. The table below shows the number of genes and bases in human chromosomes.

Number of genes	Total bases	Sequenced bases
4 234	247 199 719	224 999 719
1 491	242 751 149	237 712 649
1 550	199 446 827	194 704 827
446	191 263 063	187 297 063
609	180 837 866	177 702 766
2 281	170 896 993	167 273 993

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.

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. The 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, the following classification of types was developed:

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 has no 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 the reproduction of different types of mRNA and proteins, they carry out a 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 there 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.

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