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

Let's start, perhaps, with the concept of "period of cell life". This is the period of time that begins with its very occurrence and until death. It is also customary to call this time interval the life cycle. Even within the same organism, cycle times vary by species. 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 bit of history

What is a chromosome in biology was known as early as the middle of the nineteenth century, thanks to the research of the German botanist W. Hofmeister. The scientist at that time became interested in studying cell division in a plant called tradescantia. What did he discover new? To begin with, it became clear that before cell division, nuclear division also occurs. But this is not the most interesting! 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, then we get “painted bodies”. A little later, scientists noticed that these filamentous particles are 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, to the transfer of genetic material from parents to offspring.

Thanks to the work of Walter Sutton, the number of chromosomes in cells became known. In addition, the scientist argued that these tiny particles are the carriers of units of heredity. Sutton also found that chromosomes are made up 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 main provisions of the role of chromosomes.

Cells

After the discovery and description of chromosomes in the middle of the nineteenth century, scientists began to be interested in their structure. It became clear that these little bodies are located in absolutely any cell, regardless of whether the prokaryotic or eukaryotic cell is in front of us.

Microscopes helped in the study of the structure. Scientists managed to establish several facts:

• chromosomes are threadlike bodies;
• they can be observed only in certain phases of the cycle;
• if you study in interphase, you can see 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 thing is that in the process of cell division, these little bodies are better visible);
• in eukaryotes, large chromosomes with a linear structure are most common;
• 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 possible varieties. We propose to fill this gap immediately.

So, in total it is customary to distinguish four forms:

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

Functions

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

• 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) of genes in one chromosome, it has the following features:

• the chromosome represents only one linkage group;
• arranges the arrangement of genes;
• ensures the joint inheritance of all genes.

Each individual cell has a diploid set of chromosomes. Biology is a very fascinating 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 the cells are formed with the help of chromatin. The latter includes:

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

All of the above are high molecular weight organic substances. In terms of location, DNA can be found in the nucleus in eukaryotes, while RNA can be found in the cytoplasm.

Genes and chromosomes

Biology considers the issue of genetics in some detail, starting from the school bench. 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 some trait.

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

• adenine;
• guanine;
• thymine;
• cytosine.

Chromosomal set

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

• fruit flies (eight chromosomes each);
• primates (48 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 ones.

For this reason, they reach large sizes, which is inconvenient in the process of cell division. To prevent the loss of genetic information, nature invented chromosomes.

The structure of the chromosome

These dense structures are rod-shaped. Chromosomes differ from each other in length, which ranges from 0.2 to 50 microns. The width usually has a constant value and does not differ for different pairs of dense bodies.

At the molecular level, chromosomes are a complex complex of nucleic acids and histone proteins, the ratio of which, respectively, is 40% to 60% by volume. Histones are involved in the compaction of DNA molecules.

It is worth noting that the chromosome is a non-permanent structure of the nucleus of a eukaryotic cell. Such bodies are formed only during the period of division, when it is necessary to pack all the genetic material in order to facilitate its transfer. Therefore, we consider the structure of the chromosome at the time of preparation for mitosis/meiosis.

The primary constriction is a fibrillar body that divides the chromosome into two arms. Depending on the ratio of the length of these arms, chromosomes are distinguished:

1. Metacentric, when the primary constriction is exactly in the center.
2. Submetacentric: the length of the shoulders differs slightly.
3. In acrocentric, the primary constriction is strongly displaced to one of the ends of the chromosome.
4. Telocentric, when one of the arms is completely absent (not found in humans).

Another structural feature of the chromosome of a eukaryotic cell is the presence of a secondary constriction, which is usually strongly displaced towards one of the ends. Its main function is to synthesize ribosomal RNA on the DNA template, which then form the non-membrane organelles of the ribosome cell. Secondary constrictions are also called nucleolar organizers. These formations are located at the distal part of the chromosome.

Several organizers form an integral structure - the nucleolus. The number of such formations in the nucleus can vary from 1 to several tens, and they are usually visible even with a light microscope.

During the synthetic phase of mitosis, the structure of the chromosome changes as a result of DNA duplication during replication. At the same time, a familiar shape is formed, resembling the letter X. It is in this form that one can often find chromosomes and take a high-quality picture on special microscopes.

It should be noted that the number of chromosomes in different species does not in any way indicate the degree of their evolutionary development. Here are some examples:

1. Humans have 46 chromosomes.
2. The cat has 60.
3. Carp has 100.
4. The rat has 42.
5. Luke has 16.
6. Drosophila fly has 8.
7. The mouse has 40.
8. Corn has 20.
9. Apricot has 16.
10. The crab has 254.

Functions of chromosomes

The nucleus is the central structure of any eukaryotic cell, since it contains all the genetic information. Chromosomes perform a number of important functions, namely:

1. Storage of the actual genetic information in an unchanged form.
2. Transfer of this information by replication of DNA molecules during cell division.
3. The manifestation of the characteristic features of the body due to the activation of genes responsible for the synthesis of certain proteins.
4. Assembly of rRNA in nucleolar organizers to build small and large ribosome subunits.

An important role in cell division is assigned to the primary constriction, to the proteins of which the fission spindle threads are attached in the metaphase of mitosis or meiosis. In this case, the X-structure of the chromosome is torn into two rod-shaped bodies, which are delivered to different poles and will be subsequently enclosed in the nuclei of daughter cells.

Compactization levels

The first level is called the nucleosome. At the same time, DNA wraps around histone proteins, forming “beads on a string”.

The second level is nucleomeric. Here, the “beads” approach each other and form threads up to 30 nm thick.

The third level is called the chromomeric level. In this case, the strands begin to form loops of several orders, thereby shortening the initial length of DNA many times over.

The fourth level is chromonemic. Compaction reaches its maximum, and the resulting rod-shaped formations are already visible in a light microscope.

Features of the genetic material of prokaryotes

A distinctive feature of bacteria is the absence of a nucleus. Genetic information is also stored using DNA, which is scattered throughout the cell as part of the cytoplasm. Among nucleic acid molecules, one ring will stand out. It is usually located in the center and is responsible for all the functions of the prokaryotic cell.

Sometimes this DNA is called the chromosome of a bacterium, the structure of which, of course, does not coincide with that of a eukaryote. Therefore, such a comparison is relative and simply simplifies the understanding of some biochemical mechanisms.

Chromosomes are the most important element of the cell. They are responsible for the transmission and implementation of hereditary information and in the eukaryotic cell are localized in the nucleus.

According to the chemical structure, chromosomes are complexes of deoxyribonucleic acids (DNA) and associated proteins, as well as a small number of other substances and ions. Thus, chromosomes are deoxyribonucleoproteins (DNP).

Each chromosome in interphase contains one long double-stranded DNA molecule. A gene is a sequence of a certain number of consecutive nucleotides that make up DNA. The genes that make up the DNA of one chromosome follow each other. In the interphase, many processes take place in the cell, many parts of the chromosome are despiralized to varying degrees. Many regions of DNA are involved in RNA synthesis.

During cell division (both during mitosis and meiosis), the chromosomes spiralize (their compaction occurs). At the same time, their length is reduced, and RNA synthesis on them becomes impossible. Prior to spiralization, each chromosome doubles. The chromosome is said to be made up of two chromatids. That is, during interphase, the chromosome consisted of one chromatid.

Proteins that make up the chromosome play an important role in the compaction of chromatids.

Thus, depending on the phase of the cell cycle, according to the external structure of the chromosome, they can be represented 1) as invisible in a light microscope chromatin(in interphase) and consist of one chromatid or 2) in the form of two spiralized chromatids visible in a light microscope (in the phases of cell division, starting from metaphase).

There is another important element in the structure of chromosomes - centromere(primary stretch). It is of a protein nature and is responsible for the movement of the chromosome, and the fission spindle threads are also attached to it. Depending on the location of the centromere, equal-arm (metacentric), unequal-arm (submetacentric) and rod-shaped (acrocentric) chromosomes are distinguished. In the first, the centromere is located in the middle, dividing each chromatid into two equal arms, in the second, the arms are of unequal length, and in the third, the centromere is located at one of the ends of the chromatid.

In doubled chromosomes, the chromatids are interconnected at the centromere.

1 - chromatid; 2 - centromere; 3 - short shoulder; 4 - long shoulder.

The presence of a primary constriction in the structure of chromosomes is mandatory. However, in addition to them, there are secondary constrictions ( nucleolar organizers), they are not observed in all chromosomes. In the nucleus, on the secondary constrictions of chromosomes, the synthesis of nucleoli occurs.

At the ends of the chromatids are the so-called telomeres. They prevent chromosomes from sticking together.

In a haploid set, each chromosome is unique in its structure. The position of the centromere (and the lengths of the chromosome arms resulting from this) makes it possible to distinguish each from the rest.

In the diploid set, each chromosome has a homologous one, having the same structure and the same set of genes (but possibly their other alleles) and inherited from another parent.

Each type of living organism is characterized by its own karyotype, i.e. its own number of chromosomes and their features (length, position of centromeres, chemical structure features). Biological species can be determined by karyotype.

Chromosomes - self-reproducing structures of the cell nucleus. In both prokaryotic and eukaryotic organisms, genes are arranged in groups on individual 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 of 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 cell structures that store and transmit hereditary information. A chromosome is made up of DNA and protein. The complex of proteins associated with DNA forms chromatin. Proteins play an important role in the packaging of DNA molecules in the nucleus.

DNA in chromosomes is packed in such a way that it fits in the nucleus, which usually does not exceed 5 microns (5-10-4 cm) in diameter. The packaging of DNA takes the form of a looped structure, similar to amphibian lampbrush chromosomes or insect polytene chromosomes. The loops are maintained by proteins that recognize specific nucleotide sequences and bring them closer together. The structure of the chromosome is best seen in the metaphase of mitosis.

The chromosome is a rod-shaped structure and consists of two sister chromatids, which are held by the centromere in the region of the primary constriction. Each chromatid a is built from chromatin loops. Chromatin does not replicate. Only DNA is replicated.

When DNA replication starts, RNA synthesis stops. Chromosomes can be in two states: condensed (inactive) and decondensed (active).

The diploid set of chromosomes in an organism is called a karyotype. Modern research methods make it possible to determine each chromosome in the karyotype. For this, the distribution of light and dark bands visible under a microscope (alternation of AT and GC pairs) in chromosomes treated with special dyes is taken into account. Transverse striation is possessed by chromosomes of representatives of different species. In related species, for example, in humans and chimpanzees, the pattern of alternation of bands in the chromosomes is very similar.