Each species has a certain number of chromosomes. Chromosomes

Sometimes they give us amazing surprises. For example, do you know what chromosomes are and how they affect?

We propose to look into this issue in order to dot the i’s once and for all.

Looking at family photographs, you may have probably noticed that members of the same family resemble each other: children look like parents, parents look like grandparents. This similarity is passed on from generation to generation through amazing mechanisms.

All living organisms, from single-celled organisms to African elephants, contain chromosomes in the cell nucleus - thin, long threads that can only be seen with an electron microscope.

Chromosomes (ancient Greek χρῶμα - color and σῶμα - body) are nucleoprotein structures in the cell nucleus, in which most of the hereditary information (genes) is concentrated. They are designed to store this information, implement it and transmit it.

How many chromosomes does a person have

At the end of the 19th century, scientists discovered that the number of chromosomes in different species is not the same.

For example, peas have 14 chromosomes, y have 42, and in humans – 46 (that is, 23 pairs). Hence the temptation arises to conclude that the more there are, the more complex the creature that possesses them. However, in reality this is absolutely not the case.

Of the 23 pairs of human chromosomes, 22 pairs are autosomes and one pair are gonosomes (sex chromosomes). The sexes have morphological and structural (gene composition) differences.

In a female organism, a pair of gonosomes contains two X chromosomes (XX-pair), and in a male organism, one X-chromosome and one Y-chromosome (XY-pair).

The sex of the unborn child depends on the composition of the chromosomes of the twenty-third pair (XX or XY). This is determined by fertilization and the fusion of the female and male reproductive cells.

This fact may seem strange, but in terms of the number of chromosomes, humans are inferior to many animals. For example, some unfortunate goat has 60 chromosomes, and a snail has 80.

Chromosomes consist of a protein and a DNA (deoxyribonucleic acid) molecule, similar to a double helix. Each cell contains about 2 meters of DNA, and in total there are about 100 billion km of DNA in the cells of our body.

An interesting fact is that if there is an extra chromosome or if at least one of the 46 is missing, a person experiences a mutation and serious developmental abnormalities (Down's disease, etc.).

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Cell nucleus

Core(lat. nucleus, Greek karyon) is the most important component of a eukaryotic cell.

The kernel performs two main functions:

• storage and reproduction of genetic information;
• regulation of metabolic processes occurring in the cell, ensuring its normal functioning.

The nucleus contains more than 90% of the DNA of the entire cell.

Most cells have one nucleus. Some cells may contain 2 nuclei (in ciliates these are a macronucleus and a micronucleus).

In eukaryotic organisms, there are cells that do not have nuclei, but their life span is short (mature red blood cells live on average 125 days). Multinucleated cells (striated muscle fibers, fungal cells) are also known.

Multinucleated cells of striated muscle tissue

The nucleus is most often located in the center of the cell, and only in plant cells with a central vacuole - in the parietal protoplasm.

It can be of various shapes: round, ovoid, horseshoe-shaped, segmented (rarely), elongated, spindle-shaped, etc.

Round nucleus Horseshoe-shaped (bean-shaped) nucleus

The core consists of:

• nucleoplasm;
• chromatin (chromosomes);
• nucleoli;
• the nuclear membrane, which passes into part of the endoplasmic reticulum.

Nuclear envelope

The core is surrounded by a shell consisting of two membranes with a structure typical of all membranes.

The outer nuclear membrane is covered with ribosomes and passes directly into the channels of the endoplasmic reticulum (endoplasmic reticulum). The inner membrane is smooth and is in contact with the chromosomal material of the nucleus. The membranes are separated from each other perinuclear space .

The thickness of such a double-membrane nuclear envelope is 30 nm. It is permeated with many pores that provide transport of mRNA, tRNA, ATP, enzymes, ions and other substances.

Despite the active exchange between the nucleus and the cytoplasm, the nuclear envelope creates the possibility of the existence of a special internal environment in the nucleus.

Nuclear pores

The nuclear envelope is penetrated by numerous openings - pores, formed by the fusion of two nuclear membranes. These holes are filled with globular and fibrillar structures. The set of nuclear pores and these structures is called complex of nuclear pores .

Through the pores, substances are exchanged between the nucleus and the cytoplasm. RNA and ribosomal subunits leave the nucleus into the cytoplasm, and the nucleotides necessary for the assembly of RNA, enzymes and other substances that ensure the activity of nuclear structures enter the nucleus.

The number of nuclear pores depends on the metabolic activity of cells: the higher the synthetic processes in cells, the more pores per unit surface of the cell nucleus.

Nuclear juice

Karyoplasm , or nucleoplasm - the fluid contained in the cell nucleus in which all processes occur.

Nuclear juice consists of:

• liquid part;
• nuclear matrix (a kind of framework that penetrates the nuclear juice - strands consisting of acidic proteins);
• various inclusions.

The liquid part is similar in composition to the corresponding component of the cytoplasm: it also contains enzymes, ribosomal and structural proteins of chromosomes, free nucleotides, amino acids and other intermediate products of cell metabolism.

Nucleolus

A dense round body, consisting of rRNA and ribosomes at different stages of formation, immersed in nuclear juice. In the nuclei of different cells and in the nucleus of the same cell, depending on its functional state, the number of nucleoli can vary from 1 to 5 - 7 or more.

Nucleoli are present only in non-dividing nuclei. During mitosis, they disappear, and then reappear around the region of the chromosome (gene) in which the rRNA structure is encoded. This gene is called the nucleolar organizer (NO). It is where rRNA synthesis occurs.

In addition to rRNA synthesis, ribosomal subunits are synthesized in the nucleolus.

Chromatin(Greek chroma- color, staining) are called DNA-protein complexes.

DNA, located in the cell nucleus, contains information about all the characteristics of an organism. The total DNA length of 46 human chromosomes is 2 meters. However, all of it is packaged in the cell nucleus thanks to special proteins - histones .

Histones- proteins in the nuclei of eukaryotic cells that are part of complexes with DNA. Histones are involved in maintaining and changing the structure of chromosomes at different stages of the cell cycle, as well as in the regulation of gene activity.

There are five types of histones: H1 (very rich in lysine), H2a and H2b (rich in lysine), H3 (rich in arginine) and H4 (rich in glycine and arginine).

The basic unit of chromatin packaging is the nucleosome. It consists of a DNA double helix that wraps 1.75 times around a complex of eight histones.

In chromatin, DNA is presented as a continuous double-stranded thread from one nucleosome to another. Nucleosomes are separated by equal sections of DNA that does not contact the histone complexes. This structure in micrographs looks like beads on a string.

Between divisions, the DNA molecules in the cell are in a despiralized state; it is almost impossible to see them with a light microscope. In a cell preparing to divide, DNA molecules double, spiral, shorten and acquire a compact shape, which makes them visible. In this state, the complex of DNA and proteins is called chromosomes . A chromosome is a continuous strand of chromatin arranged in a specific way.

Chromosomes

Thread-like bodies consisting of DNA molecules contained in the nucleus of a eukaryotic cell - chromosomes (Greek. chroma- color, soma- body). Chromosomes can consist of one or more identical DNA molecules (2, 4, 8, etc.) connected in the region of the primary constriction. The terminal sections of chromosomes (telomeres) protect their ends from sticking together.

The sizes of chromosomes in different organisms differ from each other. Thus, the length of chromosomes can vary from 0.2 to 50 microns. The smallest chromosomes are found in some protozoa and fungi. The longest ones are found in some orthopteran insects, amphibians and lilies. The length of human chromosomes ranges from 1.5 to 10 microns.

Centromere(lat. сentrum, Greek kentron- center and meros- part, lobe) - an area in the area of ​​the primary constriction to which the filaments of the spindle are attached during cell division.The centromere divides the chromosome into two arms of equal or different lengths.

A change in the position of the centromere in a particular chromosome serves as a criterion for identifying chromosomal rearrangements.

Depending on the location of the centromere, four types of chromosome structure are distinguished:

• metacentric (having arms of equal length);
• submetacentric (with arms of unequal length);
• acrocentric (with a very short second arm);
• telocentric (one shoulder is missing).

In all somatic cells of any plant or animal organism, the number of chromosomes is the same.

Sex cells always contain half as many chromosomes as somatic cells of a given type of organism.

Number of chromosomes in different types of living organisms

As can be seen from this table, in distant species the number of chromosomes can be the same, but in related species it can vary greatly.

When studying the chromosome sets of different individuals, they discovered sibling species, almost no different from each other morphologically, but having a different number of chromosomes or differences in their structure. These species do not interbreed. These are, for example, those living in the same territory crossbills spruce And pine tree, whose chromosomes differ in their structure.

Twin species are also known in the plant kingdom. Externally practically indistinguishable Clarkia biloba And Clarkia tongue-shaped from the fireweed family, growing in California, however, the chromosome set of the second species contains an additional chromosome.

Next page "Prokaryotic cell" >

​What mutations, besides Down syndrome, threaten us? Is it possible to cross a man with a monkey? And what will happen to our genome in the future? The editor of the portal ANTHROPOGENES.RU talked about chromosomes with a geneticist, head. lab. comparative genomics SB RAS Vladimir Trifonov.

− Can you explain in simple language what a chromosome is?

− A chromosome is a fragment of the genome of any organism (DNA) in complex with proteins. If in bacteria the entire genome is usually one chromosome, then in complex organisms with a pronounced nucleus (eukaryotes) the genome is usually fragmented, and complexes of long fragments of DNA and protein are clearly visible in a light microscope during cell division. That is why chromosomes as colorable structures (“chroma” - color in Greek) were described at the end of the 19th century.

− Is there any relationship between the number of chromosomes and the complexity of an organism?

- There is no connection. The Siberian sturgeon has 240 chromosomes, the sterlet has 120, but it is sometimes quite difficult to distinguish these two species from each other based on external characteristics. Female Indian muntjac have 6 chromosomes, males have 7, and their relative, the Siberian roe deer, has more than 70 (or rather, 70 chromosomes of the main set and up to a dozen additional chromosomes). In mammals, the evolution of chromosome breaks and fusions proceeded quite intensively, and now we are seeing the results of this process, when each species often has characteristic features of its karyotype (set of chromosomes). But, undoubtedly, the general increase in genome size was a necessary step in the evolution of eukaryotes. At the same time, how this genome is distributed into individual fragments does not seem to be very important.

− What are some common misconceptions about chromosomes? People often get confused: genes, chromosomes, DNA...

− Since chromosomal rearrangements do occur frequently, people have concerns about chromosomal abnormalities. It is known that an extra copy of the smallest human chromosome (chromosome 21) leads to a rather serious syndrome (Down syndrome), which has characteristic external and behavioral features. Extra or missing sex chromosomes are also quite common and can have serious consequences. However, geneticists have also described quite a few relatively neutral mutations associated with the appearance of microchromosomes, or additional X and Y chromosomes. I think the stigmatization of this phenomenon is due to the fact that people perceive the concept of normal too narrowly.

− What chromosomal mutations occur in modern humans and what do they lead to?

− The most common chromosomal abnormalities are:

− Klinefelter syndrome (XXY men) (1 in 500) – characteristic external signs, certain health problems (anemia, osteoporosis, muscle weakness and sexual dysfunction), sterility. There may be behavioral features. However, many symptoms (except sterility) can be corrected by administering testosterone. Using modern reproductive technologies, it is possible to obtain healthy children from carriers of this syndrome;

− Down syndrome (1 in 1000) – characteristic external signs, delayed cognitive development, short life expectancy, may be fertile;

− trisomy X (XXX women) (1 in 1000) – most often there are no manifestations, fertility;

− XYY syndrome (men) (1 in 1000) – almost no manifestations, but there may be behavioral characteristics and possible reproductive problems;

− Turner syndrome (women with CP) (1 in 1500) – short stature and other developmental features, normal intelligence, sterility;

− balanced translocations (1 in 1000) – depends on the type, in some cases developmental defects and mental retardation may be observed and may affect fertility;

− small additional chromosomes (1 in 2000) – the manifestation depends on the genetic material on the chromosomes and varies from neutral to serious clinical symptoms;

Pericentric inversion of chromosome 9 occurs in 1% of the human population, but this rearrangement is considered a normal variant.

Is the difference in the number of chromosomes an obstacle to crossing? Are there any interesting examples of crossing animals with different numbers of chromosomes?

− If the crossing is intraspecific or between closely related species, then the difference in the number of chromosomes may not interfere with crossing, but the descendants may turn out to be sterile. There are a lot of hybrids known between species with different numbers of chromosomes, for example, equines: there are all kinds of hybrids between horses, zebras and donkeys, and the number of chromosomes in all equines is different and, accordingly, the hybrids are often sterile. However, this does not exclude the possibility that balanced gametes may be produced by chance.

- What unusual things have been discovered recently in the field of chromosomes?

− Recently, there have been many discoveries regarding the structure, function and evolution of chromosomes. I especially like the work that showed that sex chromosomes were formed completely independently in different groups of animals.

- Still, is it possible to cross a man with a monkey?

- Theoretically, it is possible to obtain such a hybrid. Recently, hybrids of much more evolutionarily distant mammals (white and black rhinoceros, alpaca and camel, and so on) have been obtained. The red wolf in America has long been considered a separate species, but has recently been proven to be a hybrid between a wolf and a coyote. There are a huge number of feline hybrids known.

- And a completely absurd question: is it possible to cross a hamster with a duck?

- Here, most likely, nothing will work out, because too many genetic differences have accumulated over hundreds of millions of years of evolution for the carrier of such a mixed genome to function.

- Is it possible that in the future a person will have fewer or more chromosomes?

- Yes, this is quite possible. It is possible that a pair of acrocentric chromosomes will merge and such a mutation will spread throughout the population.

− What popular science literature do you recommend on the topic of human genetics? What about popular science films?

− Books by biologist Alexander Markov, the three-volume “Human Genetics” by Vogel and Motulsky (though this is not science-pop, but there is good reference data there). Nothing comes to mind from films about human genetics... But Shubin’s “Inner Fish” is an excellent film and book of the same name about the evolution of vertebrates.

Containing genes. The name “chromosome” comes from the Greek words (chrōma - color, color and sōma - body), and is due to the fact that when cells divide, they become intensely colored in the presence of basic dyes (for example, aniline).

Many scientists, since the beginning of the 20th century, have thought about the question: “How many chromosomes does a person have?” So, until 1955, all the “minds of humanity” were convinced that the number of chromosomes in humans is 48, i.e. 24 pairs. The reason was that Theophilus Painter (Texas scientist) incorrectly counted them in preparative sections of human testes, according to a court decision (1921). Subsequently, other scientists, using different calculation methods, also came to this opinion. Even after developing a method for separating chromosomes, the researchers did not challenge Painter’s result. The error was discovered by scientists Albert Levan and Jo-Hin Thio in 1955, who accurately calculated how many pairs of chromosomes a person has, namely 23 (more modern technology was used to count them).

Somatic and germ cells contain a different chromosome set in biological species, which cannot be said about the morphological characteristics of chromosomes, which are constant. have a doubled (diploid set), which is divided into pairs of identical (homologous) chromosomes, which are similar in morphology (structure) and size. One part is always of paternal origin, the other of maternal origin. Human sex cells (gametes) are represented by a haploid (single) set of chromosomes. When an egg is fertilized, haploid sets of female and male gametes are united in one zygote nucleus. In this case, the double dialing is restored. It is possible to say with accuracy how many chromosomes a person has - there are 46 of them, with 22 pairs of them being autosomes and one pair being sex chromosomes (gonosomes). Sexes have differences - both morphological and structural (gene composition). In a female organism, a pair of gonosomes contains two X chromosomes (XX-pair), and in a male organism, one X- and a Y-chromosome (XY-pair).

Morphologically, chromosomes change during cell division, when they double (with the exception of germ cells, in which duplication does not occur). This is repeated many times, but no change in the chromosome set is observed. Chromosomes are most noticeable at one of the stages of cell division (metaphase). During this phase, the chromosomes are represented by two longitudinally split formations (sister chromatids), which narrow and unite in the area of ​​the so-called primary constriction, or centromere (an obligatory element of the chromosome). Telomeres are the ends of a chromosome. Structurally, human chromosomes are represented by DNA (deoxyribonucleic acid), which encodes the genes that make up them. Genes, in turn, carry information about a specific trait.

Individual development will depend on how many chromosomes a person has. There are such concepts as: aneuploidy (change in the number of individual chromosomes) and polyploidy (the number of haploid sets is greater than the diploid one). The latter can be of several types: loss of a homologous chromosome (monosomy), or appearance (trisomy - one extra, tetrasomy - two extra, etc.). All this is a consequence of genomic and chromosomal mutations, which can lead to pathological conditions such as Klinefelter syndrome, Shereshevsky-Turner syndrome and other diseases.

Thus, only the twentieth century gave answers to all questions, and now every educated inhabitant of planet Earth knows how many chromosomes a person has. The sex of the unborn child depends on the composition of the 23 pairs of chromosomes (XX or XY), and this is determined during fertilization and the fusion of the female and male reproductive cells.