The number of chromosomes in the cells of the human body. Chromosomes. Diploid set of chromosomes
Chromosomes are located in the nucleus of the cell, they are the main components of the nucleus.
The chemical composition of chromosomes is 50% DNA and 50% protein.
The function of chromosomes is the storage of hereditary information.
A chromosome can be single (from one chromatids) and double (of two chromatids). Centromere(primary constriction) is the junction of two chromatids.
- A single chromosome becomes a double chromosome in the process of DNA doubling (replication, reduplication) in interphase.
- A double chromosome turns into two single ones (chromatids become daughter chromosomes) after the separation of the centromere connecting them (in anaphase of mitosis and anaphase II of meiosis).
Chromosome sets
The set of chromosomes can be:
- single (haploid, n), in humans 23
- double (diploid, 2n), in humans 46
- triple (triploid, 3n)
- quadruple (tetraploid, 4n), etc.
The haploid set is characteristic of gametes (sex cells, spermatozoa and eggs), as well as spores. The diploid set is characteristic of somatic cells (cells of the body).
- The haploid set turns into a diploid one during fertilization (two haploid gametes merge, a diploid zygote is obtained).
- The diploid set turns into haploid in the first division (an independent divergence of homologous chromosomes occurs, the number of chromosomes is halved).
The triploid set of chromosomes is characteristic of the endosperm of the seeds of flowering plants. In double fertilization, the following merge:
- haploid sperm and egg; a diploid zygote is obtained, from which an embryo is formed;
- haploid sperm and diploid central cell of the embryo sac; resulting in triploid endosperm.
Solving problems on the number of chromosomes:
1) You need to understand where given number of chromosomes:
- if in a gamete, then the number given in the problem is n
- if in a somatic cell, then 2n
- if in the endosperm, then 3n
2) Math-time, calculate n
- if 2n=24 then n=24/2=12
- if 3n=24 then n=24/3=8
3) Math-two: if n=24, then
- there will be n=24 in the gamete
- in a somatic cell will be 2n=2x24=48
- endosperm will have 3n=3x24=72
You can also try to understand
37 related tests
Choose the one most correct option. Daughter chromatids become chromosomes after
1) separation of the centromere connecting them
2) alignment of chromosomes in the equatorial plane of the cell
3) exchange of sites between homologous chromosomes
4) pairing of homologous chromatids
Answer
Choose one, the most correct option. What set of chromosomes will the cells have after the first division of meiosis if the mother cell contained 12 chromosomes?
1) 6
2) 12
3) 3
4) 24
Answer
Choose one, the most correct option. A tetraploid organism produces gametes
1) haploid
2) diploid
3) triploid
4) tetraploid
Answer
Choose one, the most correct option. Consists of one nucleic acid molecule in combination with proteins
1) chloroplast
2) chromosome
3) gene
4) mitochondrion
Answer
Choose one, the most correct option. Storage tissue (endosperm) in flowering plants has a set of chromosomes
1) n
2) 2n
3) Zn
4) 4n
Answer
Choose one, the most correct option. Restoration of the diploid set of chromosomes in the zygote occurs as a result of
1) meiosis
2) mitosis
3) fertilization
4) conjugations
Answer
All of the features listed below, except for two, are used to describe the cellular structure depicted in the figure. Identify two signs that “fall out” from the general list, and write down the numbers under which they are indicated.
1) always have the shape of the letter "X"
2) are made up of DNA and proteins
3) when dividing, they are compact and clearly visible under a microscope
4) doubling occurs in interphase
5) when dividing, they are in the nucleus
Answer
Establish a correspondence between eukaryotic cells and sets of chromosomes in them: 1) haploid, 2) diploid. Write down the numbers 1 and 2 in the order corresponding to the letters.
A) moss spores
B) pine sperm
B) frog leukocyte
D) human neuron
D) horsetail zygote
E) bee egg
Answer
SOMATIC - ENDOSPERM
1. There are 34 chromosomes in the karyotype of an apple tree. How many chromosomes will be contained in the cells of the endosperm of her seed? Write down the correct number for your answer.
Answer
2. The diploid set of corn is 20 chromosomes. What set of chromosomes do maize endosperm cells have? Write down only the number of chromosomes in your answer.
Answer
3. There are 14 chromosomes in rye leaf cells. What set of chromosomes does a rye endosperm cell have? Write down only the number of chromosomes in your answer.
Answer
4. There are 16 chromosomes in an onion leaf cell. What set of chromosomes do the endosperm cells of an onion seed have? Write down only the number of chromosomes in your answer.
Answer
ENDOSPERM - SOMATIC
1. A cherry endosperm cell contains 24 chromosomes. What set of chromosomes does a leaf cell have? Write down only the number of chromosomes in your answer.
Answer
2. There are 36 chromosomes in lily seed endosperm cells. What set of chromosomes does a lily leaf cell have? Write down only the number of chromosomes in your answer.
Answer
GAMETE - ENDOSPERM
1. There are 10 chromosomes in the sperm of a flowering plant. How many chromosomes do the endosperm cells of this plant contain? Write down only the number of chromosomes in your answer.
Answer
2. How many chromosomes does the endosperm cell of the seed of a flowering plant contain if there are 7 chromosomes in the sperm of this plant? Write down only the appropriate number in your answer.
Answer
ENDOSPERM - GAMETE
There are 30 chromosomes in a corn seed endosperm cell. What set of chromosomes does a corn egg have? In response, write down only the number of chromosomes.
Answer
GAMETE - SOMATIC (PLANTS)
1. The set of chromosomes in maize sperm is 10. What set of chromosomes does the somatic cells of this organism have? Write down only the number of chromosomes in your answer.
Answer
2. How many chromosomes are in cucumber leaf cells if there are 7 chromosomes in cucumber sperm? Write down the correct number for your answer.
Answer
GAMETE - SOMATIC (ANIMAL)
1. There are 19 chromosomes in a domestic cat's egg, how many chromosomes are in her brain cell? In response, write down only the number of chromosomes.
Answer
2. A fish sperm contains 28 chromosomes. What set of chromosomes does a fish somatic cell have? Write down only the number of chromosomes in your answer.
Answer
3. The set of chromosomes of a pea egg cell is 7. What set of chromosomes do the somatic cells of this organism have? Write down only the number of chromosomes in your answer.
Answer
4. The set of chromosomes of potato germ cells is 24. What set of chromosomes does the somatic cells of this organism have? Write down only the number of chromosomes in your answer.
Answer
5. There are 48 chromosomes in a hedgehog egg. What set of chromosomes does a hedgehog skin cell have? Write down only the number of chromosomes in your answer.
Answer
6. How many chromosomes does a somatic cell of an animal have if gametes contain 38 chromosomes? Write down only the appropriate number in your answer.
Answer
SOMATIC - GAMETE (PLANTS)
1. A wheat somatic cell contains 28 chromosomes. What set of chromosomes does her sperm have? Write down only the number of chromosomes in your answer.
Answer
2. There are 14 chromosomes in the somatic cells of barley. How many chromosomes are in barley sperm. In response, write down only the number of chromosomes.
Answer
3. How many chromosomes does the gooseberry sperm nucleus have if the leaf cell nucleus contains 16 chromosomes. Write down the correct number for your answer.
Answer
SOMATIC - GAMETE (ANIMAL)
1. There are 38 chromosomes in a somatic cell of a cat. What set of chromosomes does the egg of this organism have? Write down only the number of chromosomes in your answer.
Answer
2. There are 34 chromosomes in the somatic cell of the fox. What set of chromosomes does the spermatozoon of this organism have? Write down only the number of chromosomes in your answer.
Answer
3. There are 56 chromosomes in the somatic cell of the fish body. What set of chromosomes does a fish spermatozoon have? Write down only the number of chromosomes in your answer.
Answer
4. There are 78 chromosomes in the somatic cell of a wolf. What set of chromosomes does the sex cells of this organism have? Write down only the number of chromosomes in your answer.
Answer
SOMATIC - SOMATIC
Strawberry stem cells have 14 chromosomes. What set of chromosomes does a strawberry embryo cell have? Write down only the number of chromosomes in your answer.
Answer
SOMATIC - ZYGOTE
1. The diploid set of a cockroach is 48 chromosomes. What set of chromosomes does a cockroach zygote have? Write down only the number of chromosomes in your answer.
Answer
2. There are 20 chromosomes in the nuclei of the cells of the intestinal mucosa of a vertebrate animal. What number of chromosomes will the nucleus of the zygote of this animal have? Write down the correct number for your answer.
Answer
3. The nucleus of a somatic cell of the human body normally contains 46 chromosomes. How many chromosomes are in a fertilized egg? Write down the correct number for your answer.
Answer
ZYGOTE - SOMATIC
1. How many chromosomes are contained in the nucleus of a skin cell if the nucleus of a fertilized human egg contains 46 chromosomes? Write down the correct number for your answer.
Answer
The mass of all DNA molecules in 46 chromosomes of one human somatic cell is about 6x10 -9 mg. What is the mass of all DNA molecules in a spermatozoon? In your answer, write down only the corresponding number without x10 -9.
Answer
SOMATIC - AFTER MEIOSIS
How many chromosomes are in the cell nucleus after meiosis if the diploid set contains 80 chromosomes? Write down only the appropriate number in your answer.
Chromosomes are the main structural elements cell nucleus, which are carriers of genes in which hereditary information is encoded. Possessing the ability to self-reproduce, chromosomes provide a genetic link between generations.
The morphology of chromosomes is related to the degree of their spiralization. For example, if at the stage of interphase (see Mitosis, Meiosis) the chromosomes are maximally deployed, i.e., despiralized, then with the onset of division, the chromosomes intensively spiralize and shorten. The maximum spiralization and shortening of the chromosome is reached at the metaphase stage, when relatively short, dense, intensely stained with basic dye structures are formed. This stage is most convenient for studying the morphological characteristics of chromosomes.
The metaphase chromosome consists of two longitudinal subunits - chromatids [reveals in the structure of chromosomes elementary filaments (the so-called chromonema, or chromofibrils) 200 Å thick, each of which consists of two subunits].
The sizes of chromosomes of plants and animals fluctuate considerably: from fractions of a micron to tens of microns. The average lengths of human metaphase chromosomes lie in the range of 1.5-10 microns.
The chemical basis of the structure of chromosomes are nucleoproteins - complexes (see) with the main proteins - histones and protamines.
Rice. 1. The structure of a normal chromosome.
A - appearance; B - internal structure: 1-primary constriction; 2 - secondary constriction; 3 - satellite; 4 - centromere.
Individual chromosomes (Fig. 1) are distinguished by the localization of the primary constriction, i.e., the location of the centromere (during mitosis and meiosis, spindle threads are attached to this place, pulling it towards the pole). With the loss of the centromere, fragments of chromosomes lose their ability to disperse during division. The primary constriction divides the chromosomes into 2 arms. Depending on the location of the primary constriction, chromosomes are divided into metacentric (both arms of equal or almost equal length), submetacentric (arms of unequal length) and acrocentric (the centromere is shifted to the end of the chromosome). In addition to the primary, less pronounced secondary constrictions can occur in the chromosomes. A small terminal section of chromosomes, separated by a secondary constriction, is called a satellite.
Each type of organism is characterized by its specific (in terms of the number, size and shape of chromosomes) so-called chromosome set. The set of a double, or diploid, set of chromosomes is designated as a karyotype.
Rice. 2. Normal female chromosome set (two X-chromosomes in the lower right corner).
Rice. 3. Normal chromosomal set of a man (in the lower right corner - sequentially X- and Y-chromosomes).
Mature eggs contain a single, or haploid, set of chromosomes (n), which is half of the diploid set (2n) inherent in the chromosomes of all other cells of the body. In a diploid set, each chromosome is represented by a pair of homologues, one of which is maternal and the other paternal. In most cases, the chromosomes of each pair are identical in size, shape, and genetic composition. The exception is the sex chromosomes, the presence of which determines the development of the organism in the male or female direction. The normal human chromosome set consists of 22 pairs of autosomes and one pair of sex chromosomes. In humans and other mammals, the female is determined by the presence of two X chromosomes, and the male is determined by the presence of one X and one Y chromosome (Fig. 2 and 3). In female cells, one of the X chromosomes is genetically inactive and is found in the interphase nucleus in the form (see). The study of human chromosomes in normal and pathological conditions is the subject of medical cytogenetics. It has been established that deviations in the number or structure of chromosomes from the norm that occur in the sex! cells or in the early stages of cleavage of a fertilized egg, cause disturbances in the normal development of the body, causing in some cases the occurrence of spontaneous abortions, stillbirths, congenital deformities and developmental anomalies after birth (chromosomal diseases). Examples of chromosomal diseases are Down's disease (an extra G chromosome), Klinefelter's syndrome (an extra X chromosome in men) and (absence of a Y or one of the X chromosomes in the karyotype). In medical practice, chromosomal analysis is carried out either by a direct method (on bone marrow cells) or after a short-term cultivation of cells outside the body (peripheral blood, skin, embryonic tissues).
Chromosomes (from the Greek chroma - color and soma - body) are thread-like, self-reproducing structural elements of the cell nucleus, containing heredity factors in a linear order - genes. Chromosomes are clearly visible in the nucleus during the division of somatic cells (mitosis) and during the division (maturation) of germ cells - meiosis (Fig. 1). In both cases, the chromosomes are intensely stained with basic dyes, and are also visible on unstained cytological preparations in phase contrast. In the interphase nucleus, the chromosomes are despiralized and are not visible in a light microscope, since their transverse dimensions are beyond the resolution light microscope. At this time, separate sections of chromosomes in the form of thin threads with a diameter of 100-500 Å can be distinguished using an electron microscope. Separate non-despiralized sections of chromosomes in the interphase nucleus are visible through a light microscope as intensely stained (heteropyknotic) sections (chromocenters).
Chromosomes continuously exist in the cell nucleus, undergoing a cycle of reversible spiralization: mitosis-interphase-mitosis. The main regularities of the structure and behavior of chromosomes in mitosis, meiosis and during fertilization are the same in all organisms.
Chromosomal theory of heredity. For the first time chromosomes were described by I. D. Chistyakov in 1874 and Strasburger (E. Strasburger) in 1879. In 1901, E. V. Wilson, and in 1902 W. S. Sutton paid attention to parallelism in the behavior of chromosomes and Mendelian factors of heredity - genes - in meiosis and during fertilization and came to the conclusion that genes are located in chromosomes. In 1915-1920. Morgan (T. N. Morgan) and his collaborators proved this position, localized several hundred genes in Drosophila chromosomes and created genetic maps of chromosomes. Data on chromosomes, obtained in the first quarter of the 20th century, formed the basis of the chromosome theory of heredity, according to which the continuity of the characteristics of cells and organisms in a number of their generations is ensured by the continuity of their chromosomes.
Chemical composition and autoreproduction of chromosomes. As a result of cytochemical and biochemical studies of chromosomes in the 30s and 50s of the 20th century, it was established that they consist of permanent components [DNA (see Nucleic acids), basic proteins (histones or protamines), non-histone proteins] and variable components (RNA and associated acidic protein). Chromosomes are based on deoxyribonucleoprotein filaments with a diameter of about 200 Å (Fig. 2), which can be connected into bundles with a diameter of 500 Å.
The discovery by Watson and Crick (J. D. Watson, F. N. Crick) in 1953 of the structure of the DNA molecule, the mechanism of its autoreproduction (reduplication) and the DNA nucleic code and the development of molecular genetics that arose after that led to the idea of genes as sections of the DNA molecule. (see Genetics). The regularities of autoreproduction of chromosomes [Taylor (J. N. Taylor) et al., 1957], which turned out to be similar to the regularities of autoreproduction of DNA molecules (semiconservative reduplication), were revealed.
Chromosomal set is the totality of all chromosomes in a cell. Each biological species has a characteristic and constant set of chromosomes, fixed in the evolution of this species. There are two main types of chromosome sets: single, or haploid (in animal germ cells), denoted n, and double, or diploid (in somatic cells, containing pairs of similar, homologous chromosomes from mother and father), denoted 2n.
Individual sets of chromosomes species differ significantly in the number of chromosomes: from 2 (horse roundworm) to hundreds and thousands (some spore plants and protozoa). The diploid numbers of chromosomes of some organisms are as follows: humans - 46, gorillas - 48, cats - 60, rats - 42, Drosophila - 8.
The size of the chromosomes different types are also different. The length of chromosomes (in the metaphase of mitosis) varies from 0.2 microns in some species to 50 microns in others, and the diameter is from 0.2 to 3 microns.
Chromosome morphology is well expressed in the metaphase of mitosis. Metaphase chromosomes are used to identify chromosomes. In such chromosomes, both chromatids are clearly visible, into which each chromosome is split longitudinally and the centromere (kinetochore, primary constriction) connecting the chromatids (Fig. 3). The centromere is visible as the narrowed site which is not containing chromatin (see); threads of the achromatin spindle are attached to it, due to which the centromere determines the movement of chromosomes to the poles in mitosis and meiosis (Fig. 4).
Loss of the centromere, for example, when a chromosome is broken by ionizing radiation or other mutagens, leads to the loss of the ability of a piece of chromosome devoid of a centromere (acentric fragment) to participate in mitosis and meiosis and to its loss from the nucleus. This can lead to severe cell damage.
The centromere divides the body of the chromosome into two arms. The location of the centromere is strictly constant for each chromosome and determines three types of chromosomes: 1) acrocentric, or rod-shaped, chromosomes with one long and the second very short arm resembling a head; 2) submetacentric chromosomes with long arms of unequal length; 3) metacentric chromosomes with arms of the same or almost the same length (Fig. 3, 4, 5 and 7).
Rice. Fig. 4. Scheme of the structure of chromosomes in the metaphase of mitosis after longitudinal splitting of the centromere: A and A1 - sister chromatids; 1 - long shoulder; 2 - short shoulder; 3 - secondary constriction; 4-centromere; 5 - spindle fibers.
Characteristic features of the morphology of certain chromosomes are secondary constrictions (not having the function of a centromere), as well as satellites - small sections of chromosomes connected to the rest of her body fine thread(Fig. 5). Satellite filaments have the ability to form nucleoli. A characteristic structure in the chromosome (chromomeres) is thickening or more densely spiralized sections of the chromosome thread (chromonema). The chromomere pattern is specific for each pair of chromosomes.
Rice. 5. Scheme of chromosome morphology in the anaphase of mitosis (chromatid moving towards the pole). A - the appearance of the chromosome; B - the internal structure of the same chromosome with two chromonemes (semichromatids) that make it up: 1 - primary constriction with chromomeres that make up the centromere; 2 - secondary constriction; 3 - satellite; 4 - satellite thread.
The number of chromosomes, their size and shape at the metaphase stage are characteristic of each type of organism. The totality of these features of a set of chromosomes is called a karyotype. A karyotype can be represented as a diagram called an idiogram (see human chromosomes below).
sex chromosomes. Sex-determining genes are localized in a special pair of chromosomes - the sex chromosomes (mammals, humans); in other cases, iol is determined by the ratio of the number of sex chromosomes and all the rest, called autosomes (drosophila). In humans, as in other mammals, the female sex is determined by two identical chromosomes, designated as X chromosomes, the male sex is determined by a pair of heteromorphic chromosomes: X and Y. As a result of reduction division (meiosis) during the maturation of oocytes (see Ovogenesis) in women All eggs contain one X chromosome. In men, as a result of the reduction division (maturation) of spermatocytes, half of the sperm contains the X chromosome, and the other half the Y chromosome. The sex of a child is determined by the random fertilization of an egg by a sperm that carries an X or Y chromosome. The result is a female (XX) or male (XY) fetus. In the interphase nucleus in females, one of the X chromosomes is visible as a lump of compact sex chromatin.
Chromosome Function and Nuclear Metabolism. Chromosomal DNA is a template for the synthesis of specific messenger RNA molecules. This synthesis occurs when a given region of the chromosome is despiralized. Examples of local activation of chromosomes are: the formation of despiralized loops of chromosomes in the oocytes of birds, amphibians, fish (the so-called X-lamp brushes) and swellings (puffs) of certain chromosome loci in multifilamentous (polytene) chromosomes of the salivary glands and other secretory organs of dipteran insects (Fig. 6). An example of the inactivation of an entire chromosome, i.e., its exclusion from the metabolism of a given cell, is the formation of one of the X chromosomes of a compact body of sex chromatin.
Rice. Fig. 6. Polytene chromosomes of the dipteran insect Acriscotopus lucidus: A and B - the area bounded by dotted lines, in a state of intensive functioning (puff); B - the same site in a non-functioning state. Numbers indicate individual loci of chromosomes (chromomeres).
Rice. 7. Chromosomal set in the culture of male peripheral blood leukocytes (2n=46).
The discovery of the mechanisms of functioning of polytene chromosomes such as lampbrushes and other types of spiralization and despiralization of chromosomes is of decisive importance for understanding the reversible differential activation of genes.
human chromosomes. In 1922, T. S. Painter established the diploid number of human chromosomes (in spermatogonia) equal to 48. In 1956, Tio and Levan (N. J. Tjio, A. Levan) used a set of new methods for studying human chromosomes : cell culture; the study of chromosomes without histological sections on total cell preparations; colchicine, which leads to the arrest of mitosis at the metaphase stage and the accumulation of such metaphases; phytohemagglutinin, which stimulates the entry of cells into mitosis; treatment of metaphase cells with hypotonic saline solution. All this made it possible to clarify the diploid number of chromosomes in humans (it turned out to be 46) and to give a description of the human karyotype. In 1960, in Denver (USA), an international commission developed a nomenclature of human chromosomes. According to the proposals of the commission, the term "karyotype" should be applied to a systematized set of chromosomes of a single cell (Fig. 7 and 8). The term "idiotram" is retained to represent a set of chromosomes in the form of a diagram built on the basis of measurements and a description of the morphology of the chromosomes of several cells.
Human chromosomes are numbered (somewhat serially) from 1 to 22 in accordance with morphological features that allow their identification. Sex chromosomes do not have numbers and are designated as X and Y (Fig. 8).
A connection has been found between a number of diseases and birth defects in human development and changes in the number and structure of its chromosomes. (see. Heredity).
See also Cytogenetic studies.
All these achievements have created a solid basis for the development of human cytogenetics.
Rice. 1. Chromosomes: A - at the stage of anaphase of mitosis in shamrock microsporocytes; B - at the metaphase stage of the first division of meiosis in pollen mother cells in Tradescantia. In both cases, the helical structure of the chromosomes is visible.
Rice. Fig. 2. Elementary chromosome filaments with a diameter of 100 Å (DNA + histone) from the interphase nuclei of the calf thymus gland (electron microscopy): A - filaments isolated from the nuclei; B - thin section through the film of the same preparation.
Rice. 3. Chromosomal set of Vicia faba (horse beans) in the metaphase stage.
Rice. 8. Chromosomes of the same as in fig. 7, sets classified according to Denver nomenclature into pairs of homologues (karyotype).
The set of chromosomes contained in the nucleus is called chromosome set . The number of chromosomes in a cell and their shape are constant for each type of living organism.
Number (diploid set) of chromosomes in some plant and animal species
Somatic cells usually diploid
(contain a double set of chromosomes - 2n). In these cells, the chromosomes are presented in pairs. The diploid set of chromosomes in cells of a particular type of living organisms, characterized by the number, size and shape of chromosomes, is called karyotype
. Chromosomes belonging to the same pair are called homologous. One of them is inherited from the paternal organism, the other - from the maternal. Chromosomes of different pairs are called non-homologous
. They differ from each other in size, shape, location of primary and secondary constrictions. Chromosomes that are the same in both sexes are called autosomes. The chromosomes on which the male and female sexes differ from each other are called sex chromosomes, or heterochromosomes
. A human cell contains 46 chromosomes or 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes. Sex chromosomes are referred to as X- and Y-chromosomes. Women have two X chromosomes, while men have one X and one Y chromosome.
sex cells haploid
(contain a single set of chromosomes - n). In these cells, the chromosomes are in singular and do not have a pair in the form of a homologous chromosome.
cell division
Chromosomal set
Chromosomal set - a set of chromosomes contained in the nucleus. Depending on the chromosome set, cells are somatic and sexual.
Somatic and germ cells
cell cycle
cell cycle
(life cycle cell) - the existence of a cell from the moment it arises as a result of the division of the mother cell to its own division or death. The duration of the cell cycle depends on the type of cell, its functional state and environmental conditions. The cell cycle includes a mitotic cycle and a dormant period.
IN rest period
(G 0) the cell performs its inherent functions and chooses a further fate - it dies or returns to the mitotic cycle. In continuously reproducing cells, the cell cycle coincides with the mitotic cycle, and there is no rest period.
Mitotic cycle
consists of four periods: presynthetic (postmitotic) - G 1, synthetic - S, postsynthetic (premitotic) - G 2, mitosis - M. The first three periods are the preparation of the cell for division ( interphase), the fourth period is the division itself (mitosis).
Interphase - preparation of a cell for division - consists of three periods.
Interphase periods
division of eukaryotic cells
basis for reproduction and individual development organisms is cell division.
Eukaryotic cells have three ways of dividing:
- amitosis (direct division),
- mitosis (indirect division),
- meiosis (reduction division).
Amitosis- a rare way of cell division, characteristic of aging or tumor cells. During amitosis, the nucleus is divided by constriction and the uniform distribution of hereditary material is not ensured. After amitosis, the cell is not able to enter into mitotic division.
Mitosis
Mitosis- a type of cell division, as a result of which daughter cells receive genetic material identical to that contained in the mother cell. As a result of mitosis, two diploid cells, genetically identical to the parent, are formed from one diploid cell.
Mitosis consists of four phases.
Phases of mitosis
| Phases | Number of chromosomes and chromatids | Processes |
| Prophase | 2n4c | Chromosomes spiralize, centrioles (in animal cells) diverge to the poles of the cell, the nuclear membrane disintegrates, the nucleoli disappear, and the division spindle begins to form. |
| metaphase | 2n4c | Chromosomes, consisting of two chromatids, are attached by their own centromeres(primary constrictions) to the fission spindle threads. Moreover, all of them are located in the equatorial plane. This structure is called metaphase plate. |
| Anaphase | 2n2c | The centromeres divide and the spindle fibers stretch the separated chromatids to opposite poles. The separated chromatids are now called daughter chromosomes. |
| Telophase | 2n2c | Daughter chromosomes reach the poles of the cell, despiralize, the spindle fibers are destroyed, a nuclear envelope is formed around the chromosomes, and the nucleoli are restored. The two nuclei formed are genetically identical. After that follows cytokinesis(division of the cytoplasm), which results in the formation of two daughter cells. Organelles are distributed between them more or less evenly. |
The biological significance of mitosis:
- genetic stability is achieved;
- the number of cells in the body increases;
- body grows;
- possible phenomena of regeneration and asexual reproduction in some organisms.
Meiosis
Meiosis- a type of cell division, accompanied by a reduction in the number of chromosomes. As a result of meiosis, four haploid cells are formed from one diploid cell, genetically different from the mother. During meiosis, two cell divisions occur (the first and second meiotic divisions), and the doubling of the number of chromosomes occurs only before the first division.
Like mitosis, each of the meiotic divisions consists of four phases.
Phases of meiosis
| Phases | Number of chromosomes and chromatids | Processes |
| Prophase I | 2n4c | There are processes similar to the processes of prophase of mitosis. In addition, homologous chromosomes, represented by two chromatids, approach and "stick" to each other. This process is called conjugation. In this case, there is an exchange of sections of homologous chromosomes - crossing over(cross chromosomes), that is, the exchange of hereditary information. After conjugation, homologous chromosomes separate from each other. |
| Metaphase I | 2n4c | There are processes similar to those of the metaphase of mitosis. |
| Anaphase I | 1n2c | Unlike the anaphase of mitosis, the centromeres do not divide, and not one chromatid from each chromosome departs to the poles of the cell, but one chromosome, consisting of two chromatids and held together by a common centromere. |
| Telophase I | 1n2c | Two cells with a haploid set are formed. |
| Interphase | 1n2c | Short. DNA replication (doubling) does not occur and, therefore, diploidy is not restored. |
| Prophase II | 1n2c | |
| Metaphase II | 1n2c | Similar to the processes during mitosis. |
| Anaphase II | 1n1c | Similar to the processes during mitosis. |
| Telophase II | 1n1c | Similar to the processes during mitosis. |
The biological significance of meiosis:
- the basis of sexual reproduction;
- basis of combinative variability.
division of prokaryotic cells
Prokaryotes do not have mitosis or meiosis. Bacteria reproduce asexually - cell division with the help of constrictions or partitions, less often budding. These processes are preceded by the duplication of the circular DNA molecule.
In addition, bacteria are characterized by a sexual process - conjugation. When conjugated through a special channel formed between two cells, a DNA fragment of one cell is transferred to another cell, that is, the hereditary information contained in the DNA of both cells changes. Since the number of bacteria does not increase, for correctness, the concept of "sexual process" is used, but not "sexual reproduction".
Answer: 1) in the cells of the seed embryo, the diploid set of chromosomes is 2n, since the embryo develops from a zygote - a fertilized egg; 2) in the cells of the endosperm of the seed, the triploid set of chromosomes is 3n, since it is formed by the fusion of two nuclei of the central cell of the ovule (2n) and one sperm (n); 3) cells of the leaves of a flowering plant have a diploid set of chromosomes - 2n, since mature plant develops from the embryo.
What chromosome set is typical for the pulp cells of pine needles and sperm? Explain from what initial cells and as a result of what division these cells are formed?
Answer: 1) Needle pulp cells have a diploid set of chromosomes. 2) Pine sperm have a haploid set of chromosomes. 3) Needle pulp cells are somatic and are formed as a result of mitosis from a diploid zygote, and sperm cells are formed from haploid pollen grain cells as a result of mitotic division.
What chromosome set is typical for the microspore, which is formed in the anther, and the sperm of a flowering plant? Explain from what initial cells and as a result of what division these cells are formed?
Answer: 1) Microspores have a haploid set of chromosomes. 2) Sperm have a haploid set of chromosomes. 3) Microspores in the anther are formed from diploid mother cells as a result of meiosis, and spermatozoa are formed from haploid microspores by division according to the type of mitosis.
Cattle have 60 chromosomes in their somatic cells. Determine the number of chromosomes and DNA molecules in ovarian cells in the interphase before the beginning of division and after the division of meiosis I. Explain how such a number of chromosomes and DNA molecules are formed.
Answer: 1) In somatic cells, a double set of chromosomes (2n), equal to 60. 2) In ovarian cells in interphase before meiosis, the number of chromosomes is 60 (2n), but the number of DNA molecules is 120, since DNA doubling occurs in interphase. 3) After the first division of meiosis, the set of chromosomes in the cells is haploid and the number of chromosomes is 30, and the number of DNA molecules is 60, since the chromosomes are two-chromatid.
What chromosome set is typical for sperm and endosperm cells of the seed of a flowering plant? Explain from what initial cells and as a result of what division these cells are formed.
Answer: 1) Sperm cells (male gametes) contain a haploid (n) set of chromosomes. 2) Endosperm cells contain a triploid (3p) set of chromosomes, since the endosperm, the nutrient tissue for the seed embryo, is formed as a result of the fusion of a diploid central cell with a second sperm during double fertilization of a flowering plant. 3) Sperms are formed from haploid pollen cells by division by mitosis
The ability to transfer genetic information is very important for procreation. Features of the chromosome set of the male germ cell in the future after conception determine the inheritance of certain traits. This article will tell you how many chromosomes the sperm nucleus contains.
Features of the structure of the male germ cell
The genetic information that is inherited by genus is encrypted in individual genes located on the chromosomes.
The very first ideas of scientists about the chromosomes that are inside human cells appeared in the 70s of the XIX century. To date scientific world never came to a consensus about which of the researchers discovered chromosomes. At various times, this discovery was "assigned" to ID Chistyakov, A. Schneider and many other scientists. However, the term “chromosome” itself was first proposed by the German histologist G. Waldeyer in 1888. The literal translation means "painted body", since these elements are quite well stained with basic dyes during research.
Most of the scientific experiments that brought clarity to the definition of the structure of chromosomes were carried out mainly in the 20th century. Modern researchers continue scientific experiments aimed at accurately deciphering the genetic information contained in chromosomes.
For a better and simpler understanding of how the chromosome set of the male germ cell is formed, let's touch on biology a bit. Each spermatozoon consists of a head, a middle part (body) and a tail. Average length male cell to the tail is 55 microns.
The head of the spermatozoon is elliptical in shape. Almost all of its internal space is filled with a special anatomical formation, which is called the nucleus. It contains chromosomes - the main structures of the cell that carry genetic information.
Each of them contains a different number of genes. So, there are areas more and less rich in genes. Currently, scientists are conducting experiments aimed at studying this interesting feature.
The main component of each chromosome is DNA. It is in it that the main genetic information inherited from parents by their children is stored. Each of these molecules contains a certain sequence of genes that determine the development of various traits.
The DNA chain is quite long. In order for chromosomes to have a microscopic size, DNA strands are strongly twisted. Recent genetic studies have determined that for the twisting of DNA molecules, special proteins are also needed - histones, which are also located in the nucleus of the germ cell.
A more detailed study of the structure of chromosomes showed that, in addition to DNA molecules, they also consist of protein. This combination is called chromatin.
In the middle of each chromosome there is a centromere - this is a small section that divides it into two sections. This division determines the presence of a long and short arm in each chromosome. Thus, when studied under a microscope, it has a striated appearance. Each chromosome also has its own serial number.
The total chromosome set of a living organism is called a karyotype. In humans, it is 46 chromosomes, and, for example, in the fruit fly Drosophila, only 8. Features of the structure of the karyotype determine the inheritance of a certain set of various traits.
Interestingly, the formation of sex chromosomes occurs during the period of intrauterine development. The fetus, which is still in the mother's womb, is already forming germ cells, which it will need in the future.
Spermatozoa acquire their activity much later - during puberty (puberty). At this time, they are already quite mobile and capable of fertilizing eggs.
Haploid set - what is it?
To begin with, you should understand what experts mean by “ploidy”. More in simple words, this term means multiplicity. Under the ploidy of a chromosome set, scientists mean the total number of such sets in a particular cell.
Speaking of this concept, experts use the term "haploid" or "single". That is, the sperm nucleus contains 22 single chromosomes and 1 sex chromosome. Each chromosome is not paired.
The haploid set is distinctive feature namely germ cells. It is conceived by nature not by chance. During fertilization, part of the inherited genetic information is transferred from the paternal chromosomes, and part from the maternal. Thus, the zygote, resulting from the fusion of germ cells, has a complete (diploid) set of chromosomes, in the amount of 46 pieces.
One more interesting feature The haploid set of sperm is the presence of a sex chromosome in it. It can be of two types: X or Y. Each of them determines the gender of the unborn child in the future.
Each sperm contains only one sex chromosome. It can be either X or Y. The egg has only one X chromosome. With the fusion of germ cells and the unification of the chromosome set, various combinations are possible.
- XY. In this case, the Y chromosome is inherited from the father, and the X chromosome is inherited from the mother. With such a combination of germ cells, a male body is formed, that is, a couple in love will soon have an heir.
- XX. In this case, the child "receives" the X chromosome from the father and a similar one from the mother. This combination ensures the formation of the female body, that is, the birth of a little girl in the future.
Unfortunately, the process of inheritance of genetic information does not always occur physiologically. Quite rare, but there are certain pathologies. This occurs when only one X chromosome (monosomy) is present in the zygote formed after fertilization, or, conversely, their number increases (trisomy). In such cases, children develop quite severe pathologies, which further significantly worsen their quality of life.
Down's disease is one of the clinical examples of pathologies associated with a violation of the inheritance of the chromosome set. In this case, a certain "failure" occurs in the 21st pair of chromosomes, when the same third pair is added to them.
A change in the chromosome set in this situation also contributes to a change in inherited traits. In this case, the baby has certain developmental defects, and the appearance changes.
human genome
For the implementation of normal life, each somatic cell of our body needs 23 pairs of chromosomes, received by it after the fusion of the genetic material of the maternal and paternal cells. The totality of such acquired genetic material is called the human genome by geneticists.
The study of the genome allowed specialists to determine that the human chromosome set includes a sequence of more than 30,000 different genes. Each of the genes is responsible for the development of a particular trait in a person.
A certain sequence of genes can thus determine the shape of the eyes or nose, the color of the hair, the length of the fingers, and many other traits.
