Chromosomes – cell structures that store and transmit hereditary information = DNA (7) + protein (6).
The structure of the chromosome is best seen in metaphase of mitosis. It is a rod-shaped structure and consists of two sister chromatid (3), held by the centromere ( kinetochore) in area primary waist (1), which divides the chromosome into 2 shoulders (2). It happens sometimes secondary constriction (4), as a result of which is formed satellite of the chromosome (5).
Individual sections of a DNA molecule - genes- responsible for each specific sign or property of the organism. Hereditary information is transmitted from cell to cell by doubling the DNA molecule (replication), transcription and translation. Main function chromosomes- storage and transmission of hereditary information, the carrier of which is the DNA molecule.
Under a microscope it can be seen that the chromosomes have cross stripes, which alternate in different chromosomes in different ways. Pairs of chromosomes are recognized, taking into account the distribution of light and dark stripes (alternating AT and GC pairs). The chromosomes of representatives are cross-striated different types. Related species, such as humans and chimpanzees, have a similar pattern of alternating bands in their chromosomes.
In all somatic cells Any plant or animal organism has the same number of chromosomes. Sex cells(gametes) always contain half as many chromosomes as somatic cells of a given type of organism.
There are 46 chromosomes in the human karyotype - 44 autosomes and 2 sex chromosomes. Males are heterogametic (XY sex chromosomes) and females are homogametic (XX sex chromosomes). The Y chromosome differs from the X chromosome in the absence of some alleles. Chromosomes of one pair are called homologous, they are wearing the same loci(locations) carry allelic genes.
All organisms belonging to the same species have the same number of chromosomes in their cells. Number of chromosomes is not a species-specific trait. However chromosome set in general, it is species-specific, that is, it is characteristic of only one type of plant or animal organism.
Karyotype - a set of external quantitative and qualitative characteristics of the chromosome set (number, shape, size of chromosomes) of a somatic cell, characteristic of a given species
Cell division - biological process, which underlies reproduction and individual development In all living organisms, the process of increasing the number of cells by dividing the original cell.
WITH cell division methods :
1.amitosis - direct (simple) division of the interphase nucleus by constriction, which occurs outside the mitotic cycle, i.e., is not accompanied by complex rearrangement of the entire cell, as well as spiralization of chromosomes. Amitosis can be accompanied by cell division, or it can be limited only to nuclear division without division of the cytoplasm, which leads to the formation of bi- and multinucleated cells. A cell that has undergone amitosis is subsequently unable to enter the normal mitotic cycle. Compared to mitosis, amitosis is quite rare. Normally, it is observed in highly specialized tissues, cells that have to divide: in the epithelium and liver of vertebrates, embryonic membranes of mammals, endosperm cells of plant seeds. Amitosis is also observed if necessary quick recovery tissues (after operations and injuries). Cells of malignant tumors also often divide by amitosis.
2 . mitosis - indirect division, in which an initially diploid cell gives rise to two daughter cells, also diploid cells; typical for somatic cells(body cells) of all eukaryotes (plants and animals); universal type of division.
3. meiosis - occurs during the formation of germ cells in animals and spores in plants.
Life cycle cells (cell cycle) - the lifetime of a cell from division to the next division, or from division to death. For different types Cells' cell cycle is different.
In the body of mammals and humans, the following three are distinguished: groups of cells, localized in different tissues and organs:
frequently dividing cells (poorly differentiated intestinal epithelial cells, basal cells of the epidermis and others);
rarely dividing cells (liver cells - hepatocytes);
non-dividing cells ( nerve cells central nervous system, melanocytes and others).
The life cycle of frequently dividing cells is the time of their existence from the beginning of division to the next division. The life cycle of such cells is often called mitotic cycle . This cell cycle is divided into two main ones period:
mitosis or period of division;
interphase is the period of cell life between two divisions.
Interphase – the period between two divisions, when the cell prepares for division: the amount of DNA in the chromosomes doubles, the number of other organelles doubles, proteins are synthesized, and cell growth occurs.
TO end of interphase Each chromosome consists of two chromatids, which during mitosis will become independent chromosomes.
Interphase periods:
1. Presynthetic period (G 1) - the period of preparation for DNA synthesis after completion of mitosis. The formation of RNA, proteins, DNA synthesis enzymes occurs, and the number of organelles increases. The content of chromosomes (n) and DNA (c) is 2n2c.
2. Synthetic period (S-phase) . Replication occurs (doubling, DNA synthesis). As a result of the work of DNA polymerases, the chromosome set for each chromosome becomes 2n4c. This is how bichromatid chromosomes are formed.
3. Postsynthetic period (G 2) - time from the end of DNA synthesis to the beginning of mitosis. The preparation of the cell for mitosis is completed, centrioles are doubled, proteins are synthesized, and cell growth is completed.
Mitosis –
This is a form of nuclear division and occurs only in eukaryotic cells. As a result of mitosis, each of the resulting daughter nuclei receives the same set of genes that the parent cell had. Both diploid and haploid nuclei can enter into mitosis. Mitosis produces nuclei of the same ploidy as the original.
Open using a light microscope in 1874 by the Russian scientist I. D. Chistyakov in plant cells.
In 1878, V. Flemming and the Russian scientist P. P. Peremezhko discovered this process in animal cells. In animal cells, mitosis lasts 30-60 minutes, in plant cells - 2-3 h.
Mitosis consists of four phases:
1. prophase- bichromatid chromosomes spiral and become visible, the nucleolus and nuclear membrane disintegrate, spindle threads are formed. The cell center is divided into two centrioles, diverging towards the poles.
2 . m etaphase - phase of chromosome accumulation at the equator of the cell: spindle threads come from the poles and join the centromeres of the chromosomes: two threads coming from the two poles approach each chromosome.
3 . A naphase - the phase of chromosome divergence, in which centromeres divide, and single-chromatid chromosomes are stretched by spindle threads to the poles of the cell; the shortest phase of mitosis.
4 . Telophase- the end of division, the movement of chromosomes ends, and they despiral (unwind into thin threads), a nucleolus is formed, the nuclear membrane is restored, a septum (in plant cells) or a constriction (in animal cells) is formed at the equator, the filaments of the fission spindle dissolve.
Cytokinesis– process of separation of the cytoplasm. Cell membrane in the central part of the cell it is drawn inward. A cleavage furrow is formed, and as it deepens, the cell bifurcates.
As a result of mitosis, two new nuclei are formed with identical sets of chromosomes, exactly copying the genetic information of the maternal nucleus.
IN tumor cells the course of mitosis is disrupted.
As a result of mitosis from one diploid cell, having two-chromatid chromosomes and a double amount of DNA (2n4c), two daughter diploid cells with single-chromatid chromosomes and a single amount of DNA (2n2c) are formed, which then enter interphase. This is how somatic cells (body cells) of a plant, animal or human body are formed.
|
Mitosis phase, set of chromosomes (n-chromosomes, c - DNA) |
Drawing |
|
|
Prophase |
Dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle filaments, “disappearance” of nucleoli, condensation of bichromatid chromosomes. |
|
|
Metaphase |
Arrangement of maximally condensed bichromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of spindle filaments at one end to the centrioles, the other to the centromeres of the chromosomes. |
|
|
Anaphase |
The division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes). |
|
|
Telophase |
Decondensation of chromosomes, formation of nuclear membranes around each group of chromosomes, disintegration of spindle threads, appearance of a nucleolus, division of the cytoplasm (cytotomy). Cytotomy in animal cells occurs due to the cleavage furrow, in plant cells – due to the cell plate. |
Thematic assignments
A1. Chromosomes are made up of
1) DNA and protein
2) RNA and protein
3) DNA and RNA
4) DNA and ATP
A2. How many chromosomes does a human liver cell contain?
A3. How many strands of DNA does a doubled chromosome have?
A4. If a human zygote contains 46 chromosomes, how many chromosomes are there in a human egg?
A5. What is the biological meaning of chromosome duplication in interphase of mitosis?
1) During the duplication process, hereditary information changes
2) Doubled chromosomes are better visible
3) As a result of chromosome doubling, the hereditary information of new cells remains unchanged
4) As a result of chromosome doubling, new cells contain twice as much information
A6. In which phase of mitosis does the chromatid separate to the cell poles? IN:
1) prophase
2) metaphase
3) anaphase
4) telophase
A7. Indicate the processes occurring in interphase
1) divergence of chromosomes to the poles of the cell
2) protein synthesis, DNA replication, cell growth
3) formation of new nuclei, cell organelles
4) despiralization of chromosomes, formation of a spindle
A8. Mitosis results in
1) genetic diversity of species
2) formation of gametes
3) chromosome crossing
4) germination of moss spores
A9. How many chromatids does each chromosome have before it is duplicated?
A10. As a result of mitosis, they are formed
1) zygote in sphagnum
2) sperm in a fly
3) oak buds
4) sunflower eggs
IN 1. Select the processes occurring in interphase of mitosis
1) protein synthesis
2) reduction in the amount of DNA
3) cell growth
4) chromosome doubling
5) chromosome divergence
6) nuclear fission
AT 2. Indicate the processes that are based on mitosis
1) mutations
3) fragmentation of the zygote
4) sperm formation
5) tissue regeneration
6) fertilization
VZ. Establish the correct sequence of phases of the cell life cycle
A) anaphase
B) interphase
B) telophase
D) prophase
D) metaphase
E) cytokinesis
Meiosis –
it's a division process cell nuclei, leading to a halving of the number of chromosomes and the formation of gametes, while homologous sections of paired (homologous) chromosomes, and, consequently, DNA, are exchanged before they disperse into daughter cells.
As a result of meiosis from one diploid cell (2n) four haploid cells (n) are formed.
Open in 1882 by W. Flemming in animals, in 1888 by E. Strasburger in plants.
Meiosis preceded by interphase, therefore, bichromatid chromosomes (2n4c) enter meiosis.
Meiosis passes in two stages:
1. reduction division- the most complex and important process. It is divided into phases:
A) prophase I: paired chromosomes of a diploid cell approach each other, cross, forming bridges (chiasmata), then exchange sections (crossing over), while recombination of genes occurs, after which the chromosomes diverge
B) c metaphase I these paired chromosomes are located along the equator of the cell, a spindle thread is attached to each of them: to one chromosome from one pole, to the second - from the other
B) in anaphase I bichromatid chromosomes diverge to the cell poles; one of each pair to one pole, the second to the other. In this case, the number of chromosomes at the poles becomes half as much as in the mother cell, but they remain bichromatid (n2c)
D) then passes telophase I, which immediately passes into prophase II of the second stage of meiotic division, proceeding according to the type of mitosis:
2. equational division. Interphases in in this case no, since the chromosomes are bichromatic, the DNA molecules are doubled.
A) prophase II
B) c metaphase II bichromatid chromosomes are located along the equator, with division occurring in two daughter cells at once
B) in anaphase II single-chromatid chromosomes move to the poles
D) in telophase II in four daughter cells, nuclei and partitions between cells are formed.
Thus, as a result of meiosis four haploid cells with single chromatid chromosomes (nc) are obtained: these are either sex cells (gametes) of animals or plant spores.
|
Meiosis phase, set of chromosomes chromosomes, |
Drawing |
Characteristics of the phase, arrangement of chromosomes |
|
Prophase 1 |
Dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle filaments, “disappearance” of nucleoli, condensation of bichromatid chromosomes, conjugation of homologous chromosomes and crossing over. |
|
|
Metaphase 1 |
Arrangement of bivalents in the equatorial plane of the cell, attachment of spindle filaments at one end to the centrioles, the other to the centromeres of the chromosomes. |
|
|
Anaphase 1 |
Random independent divergence of bichromatid chromosomes to opposite poles of the cell (from each pair of homologous chromosomes, one chromosome goes to one pole, the other to the other), recombination of chromosomes. |
|
|
Telophase 1 |
Formation of nuclear membranes around groups of bichromatid chromosomes, division of the cytoplasm. |
|
|
Prophase 2 |
Dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle filaments. |
|
|
Metaphase 2 |
Arrangement of bichromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of spindle threads at one end to the centrioles, the other to the centromeres of the chromosomes. |
|
|
Anaphase 2 |
The division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes), recombination of chromosomes. |
|
|
Telophase 2 Total |
Decondensation of chromosomes, formation of nuclear membranes around each group of chromosomes, disintegration of the spindle threads, appearance of the nucleolus, division of the cytoplasm (cytotomy) with the formation of two, and ultimately both meiotic divisions - four haploid cells. |
Biological significance of meiosis is that a decrease in the number of chromosomes is necessary during the formation of germ cells, since during fertilization the nuclei of the gametes fuse.
If this reduction did not occur, then in the zygote (and therefore in all cells of the daughter organism) there would be twice as many chromosomes.
However, this contradicts the rule of a constant number of chromosomes.
Development of germ cells.
The process of formation of germ cells is called gametogenesis. In multicellular organisms there are spermatogenesis– formation of male reproductive cells and ovogenesis– formation of female germ cells.
Let's consider gametogenesis occurring in the gonads of animals - testes and ovaries.
Spermatogenesis- the process of transformation of diploid precursors of germ cells - spermatogonia into spermatozoa.
1. Spermatogonia are divided by mitosis into two daughter cells - first-order spermatocytes.
2. Spermatocytes of the first order are divided by meiosis (1st division) into two daughter cells - spermatocytes of the second order.
3. Spermatocytes of the second order begin the second meiotic division, as a result of which 4 haploid spermatids are formed.
4. Spermatids after differentiation turn into mature sperm.
The sperm consists of a head, neck and tail. It is mobile and thanks to this the likelihood of its meeting with gametes increases.
In mosses and ferns, sperm develop in antheridia; in angiosperms, they are formed in pollen tubes.
Oogenesis– formation of eggs in females. In animals it occurs in the ovaries. In the reproduction zone there are oogonia - primary germ cells that reproduce by mitosis.
From the oogonia, after the first meiotic division, first-order oocytes are formed.
After the second meiotic division, second-order oocytes are formed, from which one egg and three guiding bodies are formed, which then die. The eggs are immobile and have a spherical shape. They are larger than other cells and contain a reserve nutrients for the development of the embryo.
In mosses and ferns, the eggs develop in archegonia; in flowering plants, in ovules located in the ovary of the flower.
Development of germ cells and double fertilization in flowering plants.
Diagram of the life cycle of a flowering plant.
The adult is diploid. The life cycle is dominated by the sporophyte (C > G).
The adult plant here is a sporophyte, forming macro (women's) And microspores(male), which develop accordingly in embryo sac And mature pollen grain, which are gametophytes.
Female gametophyte in plants - embryo sac.
Male gametophyte in plants - pollen grain.
Calyx + corolla = perianth
Stamen and pistil - reproductive organs flower
Male reproductive cells mature into anther(pollen sac or microsporangium) located on the stamen.
It contains many diploid cells, each of which divides by meiosis and forms 4 haploid pollen grains (microspores), from all of which the male then develops gametophyte.
Each pollen grain divides by mitosis and forms 2 cells - vegetative and generative. Generative cell divides again by mitosis and forms 2 sperm.
Thus, pollen (germinated microspore, mature pollen grain) contains three cells - 1 vegetative and 2 sperm, covered with a shell.
Female reproductive cells develop into ovule(ovule or megasporangium), located in the ovary of the pistil.
One of its diploid cells divides by meiosis to form 4 haploid cells. Of these, only one haploid cell (megaspore) divides three times by mitosis and grows into the embryo sac ( female gametophyte),
the other three haploid cells die.
As a result of division megaspores form 8 haploid nuclei of the embryo sac, in which 4 nuclei are located at one pole, and 4 at the opposite pole.
Then, one nucleus migrates from each pole to the center of the embryo sac, merging, they form the central diploid nucleus of the embryo sac.
One of the three haploid cells located at the pollen entrance is a large egg cell, the other 2 are auxiliary synergid cells.
Pollination- transfer of pollen from the anthers to the stigma.
Fertilization is the process of fusion of an egg and a sperm, resulting in the formation zygote– germ cell or first cell of a new organism
At fertilization The pollen grain, once on the stigma, germinates towards the ovules located in the ovary due to its vegetative cell, which forms a pollen tube. At the anterior end of the pollen tube there are 2 sperm cells (the sperm cells themselves cannot move, so they move forward due to the growth of the pollen tube). Penetrating into the embryo sac through a canal in the integument - the pollen passage (micropyle), one sperm fertilizes the egg, and the second merges with 2n central cell (diploid nucleus of the embryo sac) with the formation 3n triploid nucleus. This process is called double fertilization , was discovered by S.G. Navashin in 1898 in Liliaceae. Subsequently from fertilized egg - zygotes develops embryo seed, and from triploid nucleus- nutritional tissue - endosperm. Thus, a seed is formed from the ovule, and the seed coat is formed from its integument. Around the seed from ovary and other parts of the flower is being formed fetus.
Thematic assignments
A1. Meiosis is the process
1) changes in the number of chromosomes in a cell
2) doubling the number of chromosomes in the cell
3) formation of gametes
4) chromosome conjugation
A2. The basis of changes in the hereditary information of children
Processes lie compared to parent information
1) doubling the number of chromosomes
2) reducing the number of chromosomes by half
3) doubling the amount of DNA in cells
4) conjugation and crossing over
A3. The first division of meiosis ends with the formation of:
2) cells with a haploid set of chromosomes
3) diploid cells
4) cells of different ploidy
A4. As a result of meiosis, the following are formed:
1) fern spores
2) cells of the fern antheridium walls
3) cells of the fern archegonium walls
4) somatic cells of bee drones
A5. Metaphase of meiosis from metaphase of mitosis can be distinguished by
1) the location of bivalents in the equatorial plane
2) doubling of chromosomes and their twisting
3) formation of haploid cells
4) divergence of chromatids to the poles
A6. Telophase of the second division of meiosis can be recognized by
1) the formation of two diploid nuclei
2) divergence of chromosomes to the poles of the cell
3) the formation of four haploid nuclei
4) doubling the number of chromatids in the cell
A7. How many chromatids will be contained in the nucleus of rat sperm, if it is known that the nuclei of its somatic cells contain 42 chromosomes
A8. The gametes formed as a result of meiosis contain
1) copies of the complete set of parental chromosomes
2) copies of half the set of parental chromosomes
3) a complete set of recombined parental chromosomes
4) half of the recombined set of parental chromosomes
IN 1. Establish the correct sequence of processes occurring in meiosis
A) Location of bivalents in the equatorial plane
B) Formation of bivalents and crossing over
B) Divergence of homologous chromosomes to the cell poles
D) formation of four haploid nuclei
D) the formation of two haploid nuclei containing two chromatids
Cell cycle
Cytokinesis of plant and animal cell
Phases of mitosis. Biological significance of mitosis
Phases of meiosis.
Cell cycle
Cell cycle is a set of processes occurring in a cell during its preparation for division and during division itself, as a result of which the miterian cell is divided into two daughter cells. There are two phases in the cycle: autosynthetic, or interphase(preparing the cell for division), including presynthetic (G, from the English gap - gap), synthetic (S) and postsynthetic (G 2) periods, and cell division - mitosis.
Highflick expressed the point of view according to which cells from the beginning of their appearance after the first division can go through several dozen cell cycles. After this they die. It was believed that the loss of the ability of cells to enter into new cycles and divide is one of the reasons for the aging of the body.
Interphase- the sequence of events that prepares mitosis. Very important in interphase is template DNA synthesis And chromosome doubling- S-phase. The interval between division and the onset of the S phase is called phaseG1 (postmitotic, or presynthetic, phase), and between the S-phase and mitosis - phaseG 2 (postsynthetic, or premitotic, phase).
During the G 1 phase the cell is diploid, during the S phase the ploidy increases to four, and in the G 2 phase the cell is tetraploid.
In interphase, the rate of biosynthetic processes increases in the direction G t -> S -> G 2.
At this time, the mass of the cell and all its components doubles, and centrioles also double. G 1 During the presynthetic phase
The general essence of the S-phase has already been revealed in the previous paragraph. Self-duplication (replication) of chromosomes is very complex and occurs gradually. The essence of doubling is that exactly the same parallel chain is synthesized on a DNA chain. . Replication
(from Latin replicatio - repetition) is the process of transferring genetic information stored in parental DNA by accurately reproducing it in a daughter cell In this case, each parent DNA strand is a template for the synthesis of a daughter strand (template DNA synthesis). The chromosome has a structure that enables this process. Located on the chromosome small area - , which does not participate in matrix synthesis centromere(or centromere). It divides the chromosome into two shoulders.
At the ends of the chromosome there are also regions that are not involved in synthesis -
telomeres.
In the S period, RNA and proteins associated with DNA are synthesized most intensively, and centrioles double.
In the cytoplasm, during the S phase, not only the DNA chains are doubled, but also each of the centrioles of the cell center. The mother centriole builds its new daughter centriole. On the ER membranes, proteins (including histones) necessary for inclusion in the new chromatid are simultaneously synthesized.
Mitosis During the premitotic phase G2, the syntheses necessary to directly support the division process take place. During this period, the formation of lysosomes intensifies, mitochondria divide and new proteins are synthesized, which are absolutely necessary for mitosis. By the end of interphase, the chromatin is condensed, the nucleolus is clearly visible, the nuclear envelope is not damaged, and the organelles are not changed.
Phase G 2 lasts up to 6 hours.
During each of these phases there are so-called critical points Prophase(regulatory points).
IN - a method of indirect division of somatic cells. The chromosomes are lined up along the equator of the cell with their centromeres, to which the microtubules of the fully formed spindle are attached. At this stage of division, the chromosomes are most compacted and have a characteristic shape, which makes it possible to study the karyotype.
IN anaphase Rapid DNA replication occurs at centromeres, as a result of which chromosomes are split and chromatids diverge to the poles of the cell, stretched by microtubules. The distribution of chromatids must be absolutely equal, since it is this process that ensures the maintenance of a constant number of chromosomes in the cells of the body.
On the stage telophases daughter chromosomes gather at the poles, despiral, nuclear membranes form around them from vesicles, and nucleoli appear in the newly formed nuclei.
After nuclear division, cytoplasmic division occurs - cytokinesis, during which more or less happens uniform distribution all organelles of the mother cell.
Thus, as a result of mitosis, two daughter cells are formed from one mother cell, each of which is a genetic copy of the mother cell (2n2c).
In sick, damaged, aging cells and specialized tissues of the body, a slightly different division process can occur - amitosis. Amitosis called direct division of eukaryotic cells, in which the formation of genetically equivalent cells does not occur, since the cellular components are distributed unevenly. It is found in plants in the endosperm, and in animals - in the liver, cartilage and cornea of the eye.
Meiosis. Phases of meiosis
Meiosis is a method of indirect division of primary germ cells (2n2c), which results in the formation of haploid cells (1n1c), most often germ cells.
Unlike mitosis, meiosis consists of two successive cell divisions, each of which is preceded by interphase. The first division of meiosis (meiosis I) is called reductionist, since in this case the number of chromosomes is halved, and the second division (meiosis II) - equational, since in its process the number of chromosomes is preserved.
Interphase I proceeds like interphase of mitosis. Meiosis I is divided into four phases: prophase I, metaphase I, anaphase I and telophase I. B prophase I Two important processes occur - conjugation and crossing over. Conjugation- This is the process of fusion of homologous (paired) chromosomes along the entire length. The pairs of chromosomes formed during conjugation are preserved until the end of metaphase I.
Crossing over- mutual exchange of homologous regions of homologous chromosomes. As a result of crossing over, the chromosomes received by the body from both parents acquire new combinations of genes, which causes the appearance of genetically diverse offspring. At the end of prophase I, as in the prophase of mitosis, the nucleolus disappears, the centrioles diverge to the poles of the cell, and the nuclear membrane disintegrates.
IN metaphase I pairs of chromosomes are aligned along the equator of the cell, and spindle microtubules are attached to their centromeres.
IN anaphase I Whole homologous chromosomes, consisting of two chromatids, diverge to the poles.
IN telophase I Nuclear membranes are formed around clusters of chromosomes at the poles of the cell, and nucleoli are formed.
Cytokinesis I ensures separation of the cytoplasms of daughter cells.
The daughter cells (1n2c) formed as a result of meiosis I are genetically heterogeneous, since their chromosomes, randomly dispersed to the cell poles, contain different genes.
Interphase II very short, since DNA doubling does not occur in it, that is, there is no S-period.
Meiosis II also divided into four phases: prophase II, metaphase II, anaphase II and telophase II. IN prophase II the same processes occur as in prophase I, with the exception of conjugation and crossing over.
IN metaphase II chromosomes are located along the equator of the cell.
IN anaphase II chromosomes are split at centromeres and chromatids are stretched towards the poles.
IN telophase II Nuclear membranes and nucleoli are formed around clusters of daughter chromosomes.
After cytokinesis II The genetic formula of all four daughter cells is 1n1c, but they all have a different set of genes, which is the result of crossing over and the random combination of chromosomes of the maternal and paternal organisms in the daughter cells.
Mitosis- the main method of division of eukaryotic cells, in which the doubling first occurs, and then the hereditary material is evenly distributed between the daughter cells.
Mitosis is a continuous process with four phases: prophase, metaphase, anaphase and telophase. Before mitosis, the cell prepares for division, or interphase. The period of cell preparation for mitosis and mitosis itself together constitute mitotic cycle. Below is a brief description of phases of the cycle.
Interphase consists of three periods: presynthetic, or postmitotic, - G 1, synthetic - S, postsynthetic, or premitotic, - G 2.
Presynthetic period (2n 2c, Where n- number of chromosomes, With- number of DNA molecules) - cell growth, activation of biological synthesis processes, preparation for the next period.
Synthetic period (2n 4c) - DNA replication.
Postsynthetic period (2n 4c) - preparation of the cell for mitosis, synthesis and accumulation of proteins and energy for the upcoming division, increase in the number of organelles, doubling of centrioles.
Prophase (2n 4c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle filaments, “disappearance” of nucleoli, condensation of biromatid chromosomes.
Metaphase (2n 4c) - alignment of maximally condensed bichromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of spindle filaments at one end to the centrioles, the other to the centromeres of the chromosomes.
Anaphase (4n 4c) - division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes).
Telophase (2n 2c in each daughter cell) - decondensation of chromosomes, formation of nuclear membranes around each group of chromosomes, disintegration of spindle threads, appearance of a nucleolus, division of the cytoplasm (cytotomy). Cytotomy in animal cells occurs due to the cleavage furrow, in plant cells - due to the cell plate.
1 - prophase; 2 - metaphase; 3 - anaphase; 4 - telophase.
Biological significance of mitosis. The daughter cells formed as a result of this method of division are genetically identical to the mother. Mitosis ensures the constancy of the chromosome set over a number of cell generations. It underlies processes such as growth, regeneration, asexual reproduction, etc.
is a special method of dividing eukaryotic cells, as a result of which the cells transition from a diploid state to a haploid state. Meiosis consists of two successive divisions preceded by a single DNA replication.
First meiotic division (meiosis 1) is called reduction, since it is during this division that the number of chromosomes is halved: from one diploid cell (2 n 4c) two haploid (1 n 2c).
Interphase 1(at the beginning - 2 n 2c, at the end - 2 n 4c) - synthesis and accumulation of substances and energy necessary for both divisions, increase in cell size and number of organelles, doubling of centrioles, DNA replication, which ends in prophase 1.
Prophase 1 (2n 4c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle filaments, “disappearance” of nucleoli, condensation of biromatid chromosomes, conjugation of homologous chromosomes and crossing over. Conjugation- the process of bringing together and intertwining homologous chromosomes. A pair of conjugating homologous chromosomes is called bivalent. Crossing over is the process of exchange of homologous regions between homologous chromosomes.
Prophase 1 is divided into stages: leptotene(completion of DNA replication), zygotene(conjugation of homologous chromosomes, formation of bivalents), pachytene(crossing over, recombination of genes), diplotene(detection of chiasmata, 1 block of oogenesis in humans), diakinesis(terminalization of chiasmata).
1 - leptotene; 2 - zygotene; 3 - pachytene; 4 - diplotene; 5 - diakinesis; 6 — metaphase 1; 7 - anaphase 1; 8 — telophase 1;
9 — prophase 2; 10 — metaphase 2; 11 - anaphase 2; 12 - telophase 2.
Metaphase 1 (2n 4c) - alignment of bivalents in the equatorial plane of the cell, attachment of spindle filaments at one end to the centrioles, the other to the centromeres of the chromosomes.
Anaphase 1 (2n 4c) - random independent divergence of two-chromatid chromosomes to opposite poles of the cell (from each pair of homologous chromosomes, one chromosome goes to one pole, the other to the other), recombination of chromosomes.
Telophase 1 (1n 2c in each cell) - the formation of nuclear membranes around groups of dichromatid chromosomes, division of the cytoplasm. In many plants, the cell goes from anaphase 1 immediately to prophase 2.
Second meiotic division (meiosis 2) called equational.
Interphase 2, or interkinesis (1n 2c), is a short break between the first and second meiotic divisions during which DNA replication does not occur. Characteristic of animal cells.
Prophase 2 (1n 2c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle filaments.
Metaphase 2 (1n 2c) - alignment of bichromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of spindle filaments at one end to the centrioles, the other to the centromeres of the chromosomes; 2 block of oogenesis in humans.
Anaphase 2 (2n 2With) - division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes), recombination of chromosomes.
Telophase 2 (1n 1c in each cell) - decondensation of chromosomes, formation of nuclear membranes around each group of chromosomes, disintegration of the filaments of the spindle, appearance of the nucleolus, division of the cytoplasm (cytotomy) with the resulting formation of four haploid cells.
Biological significance of meiosis. Meiosis is the central event of gametogenesis in animals and sporogenesis in plants. Being the basis of combinative variability, meiosis provides genetic diversity of gametes.
Amitosis
Amitosis- direct division of the interphase nucleus by constriction without the formation of chromosomes, outside the mitotic cycle. Described for aging, pathologically altered and doomed cells. After amitosis, the cell is not able to return to the normal mitotic cycle.
Cell cycle
Cell cycle- the life of a cell from the moment of its appearance until division or death. Required component cell cycle is the mitotic cycle, which includes the period of preparation for division and mitosis itself. In addition, in the life cycle there are periods of rest, during which the cell performs its inherent functions and chooses its future fate: death or return to the mitotic cycle.
Go to lectures No. 12"Photosynthesis. Chemosynthesis"
Go to lectures No. 14"Reproduction of Organisms"
Textbook for grades 10-11
Section II. Reproduction and development of organisms
Chapter V. Reproduction of organisms
Every second on Earth an astronomical number of living beings die from old age, disease and predators, and only thanks to reproduction, this universal property of organisms, life on Earth does not stop.
It may seem that the processes of reproduction in living beings are very diverse, but all of them can be reduced to two forms: asexual and sexual. Some organisms have different shapes reproduction. For example, many plants can reproduce by cuttings, layering, tubers (asexual propagation) and seeds (sexual propagation).
During sexual reproduction, each organism develops from one cell, formed from the fusion of two sex cells - male and female.
The basis of reproduction and individual development of an organism is the process of cell division.
§ 20. Cell division. Mitosis
Ability to divide - most important property cells. Without division, it is impossible to imagine an increase in the number of single-celled creatures, the development of complex multicellular organism from one fertilized egg, renewal of cells, tissues and even organs lost during the life of the body.
Cell division occurs in stages. At each stage of division certain processes occur. They lead to the doubling of genetic material (DNA synthesis) and its distribution between daughter cells. The period of cell life from one division to the next is called the cell cycle.
Preparing for division. Eukaryotic organisms, consisting of cells with nuclei, begin preparation for division at a certain stage of the cell cycle, in interphase.
It is during interphase that the process of protein biosynthesis occurs in the cell and chromosomes double. Along the original chromosome available in the cell chemical compounds its exact copy is synthesized, the DNA molecule is doubled. A doubled chromosome consists of two halves - chromatids. Each chromatid contains one DNA molecule.
Interphase in plant and animal cells lasts on average 10-20 hours. Then the process of cell division begins - mitosis.
During mitosis, the cell goes through a series of successive phases, as a result of which each daughter cell receives the same set of chromosomes as was in the mother cell.
Phases of mitosis. There are four phases of mitosis: prophase, metaphase, anaphase and telophase. Figure 29 schematically shows the progress of mitosis. In prophase, centrioles are clearly visible - formations located in the cell center and playing a role in the divergence of the daughter chromosomes of animals. (Recall that only some plants have centrioles in the cell center, which organizes the segregation of chromosomes.) We will consider mitosis using the example of an animal cell, since the presence of a centriole makes the process of chromosome segregation more visual. The centrioles double and move to different poles of the cell. Microtubules extend from the centrioles, forming filaments of the spindle, which regulates the divergence of chromosomes to the poles of the dividing cell.
Rice. 29. Scheme of mitosis
At the end of prophase, the nuclear membrane disintegrates, the nucleolus gradually disappears, the chromosomes spiral and, as a result, shorten and thicken, and they can already be observed under a light microscope. They are even better visible at the next stage of mitosis - metaphase.
In metaphase, chromosomes are located in the equatorial plane of the cell. It is clearly visible that each chromosome, consisting of two chromatids, has a constriction - a centromere. Chromosomes are attached to the spindle filaments by their centromeres. After centromere division, each chromatid becomes an independent daughter chromosome.
Then comes the next stage of mitosis - anaphase, during which the daughter chromosomes (chromatids of one chromosome) diverge to different poles of the cell.
The next stage of cell division is telophase. It begins after the daughter chromosomes, consisting of one chromatid, have reached the poles of the cell. At this stage, the chromosomes despiral again and take on the same appearance as they had before the start of cell division in interphase (long thin threads). A nuclear envelope appears around them, and a nucleolus is formed in the nucleus, in which ribosomes are synthesized. During the process of cytoplasmic division, all organelles (mitochondria, Golgi complex, ribosomes, etc.) are distributed more or less evenly between daughter cells.
Thus, as a result of mitosis, one cell turns into two, each of which has a characteristic number and shape of chromosomes for a given type of organism, and therefore a constant amount of DNA.
The entire process of mitosis takes an average of 1-2 hours. Its duration is somewhat different for different types of cells. It also depends on the conditions external environment(temperature, light conditions and other indicators).
The biological significance of mitosis is that it ensures the constancy of the number of chromosomes in all cells of the body. During the process of mitosis, the DNA of the chromosomes of the mother cell is distributed strictly equally between the two daughter cells arising from it. As a result of mitosis, all daughter cells receive the same genetic information.
- What changes in a cell precede cell division?
- When is the spindle formed? What is his role?
- Describe the phases of mitosis and briefly describe how this process occurs.
- What is chromatid? When does it become a chromosome?
- What is a centromere? What role does it play in mitosis?
- What is biological significance mitosis?
Remember from the course of botany, zoology, anatomy, physiology and human hygiene how reproduction occurs in the organic world.
