One of the most important processes in the individual development of a living organism is mitosis. In this article, we will briefly and clearly try to explain what processes occur during cell division, and talk about the biological significance of mitosis.
Definition of the concept
From the textbooks for grade 10 in biology, we know that mitosis is cell division, as a result of which two daughter cells with the same set of chromosomes are formed from one mother cell.
Translated from the ancient Greek language, the term "mitosis" means "thread". It is like a connecting link between old and new cells, in which the genetic code is preserved.
The division process as a whole starts from the nucleus and ends with the cytoplasm. It is referred to as the mitotic cycle, which consists of the stage of mitosis and interphase. As a result of division of a diploid somatic cell, two daughter cells are formed. Thanks to this process, the number of tissue cells increases.
Stages of mitosis
Based on morphological features, the division process is divided into the following stages:
- Prophase ;
At this stage, the nucleus becomes denser, chromatin condenses inside it, which coils into a spiral, chromosomes are viewed under a microscope.
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Under the influence of enzymes, the nuclei and their membranes dissolve, the chromosomes in this period are randomly located in the cytoplasm. Later, the centrioles are divided to the poles, a cell division spindle is formed, the threads of which are attached to the poles and chromosomes.
This stage is characterized by DNA doubling, but the pairs of chromosomes still hold to each other.
Before the prophase stage, the plant cell undergoes a preparatory phase - preprophase. What is the preparation of a cell for mitosis can be understood at this stage. It is characterized by the formation of a pre-phase ring, phragmosomes, and nucleation of microtubules around the nucleus.
- Prometaphase ;
At this stage, the chromosomes begin to move and head towards the nearest pole.
In many teaching aids pre-phase and prometophase are referred to as prophase stage.
- Metaphase ;
On initial stage the chromosomes are in the equatorial part of the spindle, so that the pole pressure acts on them evenly. During this stage, the number of spindle microtubules is constantly growing and renewing.
Chromosomes line up in pairs in a spiral along the spindle equator in strict order. Chromatids gradually detach, but still cling to the spindle threads.
- Anaphase ;
At this stage, the chromatids are lengthened, which gradually diverge towards the poles, as the spindle threads contract. Daughter chromosomes are formed.
This is the shortest phase in time. Sister chromatids suddenly separate and move to different poles.
- Telophase ;
It is the last phase of division, when the chromosomes elongate and a new nuclear envelope is formed near each pole. The threads that made up the spindle are completely destroyed. At this stage, the cytoplasm is divided.
Completion last stage coincides with the division of the mother cell, which is called cytokinesis. It depends on the passage of this process how many cells are formed during division, there can be two or more of them.
Figure: 1. Stages of mitosis
The value of mitosis
The biological significance of the process of cell division is undeniable.
- It is thanks to him that it is possible to maintain a constant set of chromosomes.
- Reproduction of an identical cell is possible only through mitosis. In this way, skin cells, intestinal epithelium, red blood cells, whose life cycle is only 4 months, are replaced.
- Copying, and hence the preservation of genetic information.
- Ensuring the development and growth of cells, due to which a multicellular organism is formed from a unicellular zygote.
- With the help of such division, the regeneration of body parts in some living organisms is possible. For example, the rays of a starfish are restored.
Figure: 2. Regeneration of the starfish
- Ensuring asexual reproduction. For example, hydra budding and vegetative propagation of plants.
Figure: 3. Hydra budding
What have we learned?
Cell division is called mitosis. Thanks to it, the genetic information of the cell is copied and stored. The process takes place in several stages: preparatory phase, prophase, metaphase, anaphase, telophase. As a result, two daughter cells are formed, which are completely similar to the original mother cell. In nature, the importance of mitosis is great, since thanks to it the development and growth of unicellular and multicellular organisms, the regeneration of some parts of the body, and asexual reproduction are possible.
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Assessment of the report
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All cells arise by dividing pre-existing cells. There are several ways of cell division.
Amitosis is a direct cell division in which the interphase state of the nucleus is preserved. The nucleus is divided by constriction into two approximately equal parts without chromosome spiralization. Amitosis occurs in epithelial cells, skeletal muscles, and other cells in certain diseases (for example, in cells of malignant tumors).
Mitosis - indirect division cells, in which there is an accurate distribution of chromosomes containing DNA between daughter cells.
Meiosis - a type of mitosis - a special way of cell division, as a result of which the number of chromosomes is halved, and the cells pass from a diploid state to a haploid one.
Cell (life) cycle -the period of a cell's existence from the moment of its formation as a result of the division of the mother cell to its own division or death.
Mitotic cycle -it is a set of processes occurring in a cell during the preparation of the cell for division and during division. In continuously multiplying cells, the cell cycle coincides with the mitotic cycle.
The mitotic cycle includes:
1.interphase, consisting of presynthetic, synthetic and postsynthetic periods.
2. division itself (mitosis).
Presynthetic(G 1) the period immediately follows the division. During this period, RNA, various proteins, ATP are synthesized, the number of organelles increases. The cell grows and performs its functions. It contains a diploid set of despiralized chromosomes, each chromosome consists of one chromatid. The content of the genetic material will be 2n2с (n is the number of chromosomes in the haploid set, c is the DNA content in the haploid set of chromosomes).
IN syntheticperiod (S) there is a replication (doubling) of DNA molecules under the action of the enzyme DNA polymerase, as well as the synthesis of RNA and proteins. By the end of the period, chromosomes from monochromatid become dichromatid and the content of genetic material will be 2n4c. IN postsyntheticperiod (G 2) the cell stores energy, the synthesis of RNA and proteins continues (fission spindle proteins are synthesized), the content of the genetic material remains the same –2n4c.
Mitotic cycle: A - interphase; B-C - prophase; G-D - metaphase;
E - anaphase; ZH - telophase.
Mitosis -indirect cell division. Somatic cells divide by mitosis, as a result of which daughter cells receive the same set of chromosomes as the mother cell had. In mitosis, several phases are distinguished: prophase, metaphase, anaphase, telophase.
IN prophasespiralization of chromosomes occurs, by the end of prophase they become visible; the nucleolus disappears; the nuclear envelope dissolves, and the chromosomes end up in the cytoplasm; centrioles diverge to the poles of the cell, a division spindle (2n4c) is formed.
IN metaphasechromosomes are maximally spiralized and located in the equatorial plane; each chromosome consists of two chromatids that are connected at the centromere. Fission spindle threads are attached to the centromeres. In this phase, chromosomes are studied and counted (2n4c).
IN anaphaseeach chromosome is divided into two chromatids (daughter chromosomes) in the centromere region. By contracting the filaments of the fission spindle, they stretch the chromatids to the poles of the cell. The genetic material in the cell is 4n4c (2n2c at each pole).
IN telophaseevents are reversed to prophase: chromosomes are despiralized and become invisible under a light microscope; the nuclear envelope and nucleolus are formed; the fission spindle disappears. At the same time, division of the cytoplasm (cytokinesis) takes place: by constriction in animal cells or by constructing a partition from a membrane in plant cells. At the same time, organelles are distributed between cells relatively evenly. The content of genetic material in each formed cell is 2n2c, (before cytokinesis - 4n4c).
Prophase 2n4с. Metaphase 2n4с. Anaphase 4n4s. Telophase 2n2с.
The biological significance of mitosis.
1. As a result of mitosis, daughter cells receive the same set of chromosomes as the mother cell, which ensures the maintenance of a constant number of chromosomes and the preservation
the same set of genetic material in all cell generations.
2. Mitosis provides embryonic development, growth of the organism, the processes of regeneration of tissues and organs.
3. In unicellular organisms, mitosis leads to an increase in the number of individuals.
1. Give definitions of concepts.
Interphase - the phase of preparation for mitotic division, when DNA duplication occurs.
Mitosis - This division, as a result of which there is a strictly identical distribution of exactly copied chromosomes between daughter cells, which ensures the formation of genetically identical cells.
Life cycle
- the period of a cell's life from the moment of its emergence in the process of division to death or the end of subsequent division.
2. How does the growth of unicellular organisms differ from the growth of multicellular organisms?
Growth unicellular organism - this is an increase in the size and complexity of the structure of an individual cell, and the growth of multicellular organisms is also an active division of cells - an increase in their number.
3. Why does interphase necessarily exist in the life cycle of a cell?
In the interphase, preparation for division and DNA doubling takes place. If it did not occur, then with each cell division the number of chromosomes would be halved, and pretty soon there would be no chromosomes left in the cell.
4. Complete the cluster "Phases of mitosis".
5. Using Figure 52 in § 3.4, complete the table.
6. Make up syncwine to the term "mitosis".
Mitosis
Four-phase, uniform
Divides, distributes, crushes
Provides genetic material to daughter cells
Cell division.
7. Establish a correspondence between the phases of the mitotic cycle and the events occurring in them.
Phases
1. Anaphase
2. Metaphase
3. Interphase
4. Telophase
5. Prophase
Developments
A. The cell grows, organelles are formed, DNA doubles.
B. Chromatids diverge and become independent chromosomes.
B. The spiralization of chromosomes begins, the nuclear envelope is destroyed.
D. Chromosomes are located in the equatorial plane of the cell. The spindle threads are attached to the centromeres.
D. The fission spindle disappears, nuclear membranes are formed, chromosomes unwind.
8. Why is the completion of mitosis - the division of the cytoplasm occurs differently in animal and plant cells?
In animal cells there is no cell wall, their cell membrane invades inward, and the cell divides by constriction.
In plant cells, a membrane forms in the equatorial plane inside the cell and, spreading to the periphery, divides the cell in half.
9. Why in the mitotic cycle does interphase take much longer than the division itself?
During the interphase, the cell is intensively preparing for mitosis, synthesis processes, DNA doubling are going on in it, the cell grows, its life cycle goes through, not including the division itself.
10. Choose the correct answer.
Test 1.
As a result of mitosis from one diploid cell are formed:
4) 2 diploid cells.
Test 2.
The division of centromeres and the divergence of chromatids to the poles of the cell occurs in:
3) anaphase;
Test 3.
The life cycle is:
2) cell life from division until the end of the next division or death;
Test 4.
Which term is misspelled?
4) telphase.
11. Explain the origin and total value words (term), based on the meaning of the roots that make it up.
12. Select a term and explain how its modern meaning matches the original meaning of its roots.
The chosen term is interphase.
Conformity. The term corresponds to, and means the period between the phases of mitosis, when preparation for division occurs.
13. Formulate and write down the main ideas § 3.4.
The life cycle is the life of a cell from division until the end of the next division or death. Between divisions, the cell prepares for it during the interphase period. At this time, there is a synthesis of substances, DNA duplication.
The cell divides by mitosis. It consists of 4 stages:
Prophase.
Metaphase.
Anaphase.
Telophase.
The purpose of mitosis: as a result of it, 2 daughter cells with an identical set of genes are formed from 1 mother cell. At the same time, the amount of genetic material and chromosomes remains the same, and the genetic stability of cells is ensured.
The ability of a cell to reproduce is one of the fundamental properties of living things. Cell division is the basis for embryogenesis and regeneration.
Regular changes in the structural and functional characteristics of the cell in time make up the content the life cycle of a cell (cell cycle). The cell cycle is the period of a cell's existence from the moment of its formation by division of the mother cell to its own division or death.
An important component of the cell cycle is mitotic (proliferative) cycle - a complex of interrelated and coordinated in time events occurring in the process of preparing a cell for division and during the division itself. In addition, the life cycle includes cage period multicellular organism specific functions, as well as periods of rest. During periods of rest, the immediate fate of a cell is not determined: it can either begin preparation for mitosis, or proceed to specialization in a certain functional direction.
The duration of the mitotic cycle for most cells is from 10 to 50 hours. Its magnitude varies considerably: for bacteria it is 20-30 minutes, for a shoe 1-2 times a day, for an amoeba about 1.5 days. The duration of the cycle is regulated by changing the duration of all its periods. Multicellular cells also have different ability to divide. In early embryogenesis, they often divide, and in the adult organism they mostly lose this ability, as they become specialized. But even in an organism that has reached full development, many cells must divide to replace worn-out cells that are constantly sloughing off and, finally, new cells are needed to heal wounds.
Therefore, in some populations of cells, division must take place throughout life. With this in mind, all cells can be divided into three categories:
1. In the body of higher vertebrates, not all cells are constantly dividing. There are specialized cells that have lost the ability to divide (neutrophils, basophils, eosinophils, nerve cells). By the time a child is born, nerve cells reach a highly specialized state, losing the ability to divide. In the process of ontogenesis, their number is continuously decreasing. This circumstance also has one good side; if the nerve cells were dividing, then the higher nervous functions (memory, thinking) would be disturbed.
2. Another category of cells is also highly specialized, but due to their constant desquamation, they are replaced by new ones, and this function is performed by cells of the same line, but not yet specialized and have not lost the ability to divide. These cells are called renewing cells. An example is the constantly renewing cells of the intestinal epithelium, hematopoietic cells. Even cells bone tissue are capable of forming from non-specialized ones (this can be observed during reparative regeneration of bone fractures). Populations of non-specialized cells that retain the ability to divide are usually called stem cells.
3. The third category of cells is an exception, when highly specialized cells under certain conditions can enter the mitotic cycle. These are cells that have a long life span and where, after the complete completion of growth, cell division rarely occurs. An example is hepatocytes. But if 2/3 of the liver is removed from an experimental animal, then in less than two weeks it is restored to its previous size. The cells of the glands that produce hormones are the same: in normal conditions only a few of them are able to reproduce, and under altered conditions, most of them can begin to divide.
According to two main events of the mitotic cycle, reproductive and dividing phases corresponding interphase and mitosis classical cytology.
In the initial segment of the interphase (in eukaryotes 8-10 hours) (postmitotic, presynthetic, or G 1 -period)the organization features of the interphase cell are restored, the formation of the nucleolus, which began in the telophase, is completed. A significant (up to 90%) amount of protein enters the nucleus from the cytoplasm. In the cytoplasm, parallel to the reorganization of the ultrastructure, protein synthesis is intensified. This promotes the growth of cell mass. If the daughter cell is to enter the next mitotic cycle, the syntheses take on a directional character: chemical precursors of DNA are formed, enzymes that catalyze the DNA reduplication reaction, and a protein that starts this reaction is synthesized. Thus, the processes of preparing the next period of the interphase - synthetic are carried out. Cells have a diploid set of chromosomes 2n and 2c genetic material DNA (genetic formula of a cell).
IN synthetic or S-period (6-10 h)the amount of hereditary cell material is doubled. With few exceptions reduplication (sometimes DNA doubling is denoted by the term replication, leaving the term reduplication to indicate doubling of chromosomes.) DNA is carried out in a semi-conservative manner. It consists in the divergence of the DNA bis-helix into two strands, followed by the synthesis of a complementary strand near each of them. As a result, two identical bispirals appear. DNA molecules, complementary to the mother, are formed by separate fragments along the length of the chromosome, and at the same time (asynchronously) in different parts of the same chromosome, as well as in different chromosomes. Then the chunks (replication units - replicons) the newly formed DNA is "stitched" into one macromolecule. The human cell contains more than 50,000 replicons. The length of each of them is about 30 microns. Their number changes in ontogeny. The meaning of DNA reduplication by replicons becomes clear from the following comparisons. The DNA synthesis rate is 0.5 μm / min. In this case, the reduplication of a DNA strand of one human chromosome about 7 cm long should take about three months. The sections of chromosomes in which synthesis begins are called points of initiation. Perhaps they are the places of attachment of interphase chromosomes to the inner membrane of the nuclear envelope. It can be thought that the DNA of individual fractions, which will be discussed below, is reduplicated in a strictly defined phase of the S-period. So, most of rRNA genes doubles DNA at the beginning of the period. Reduplication is triggered by a signal entering the nucleus from the cytoplasm, the nature of which is not clear. DNA synthesis in a replicon is preceded by RNA synthesis. In a cell that has passed the S-period of the interphase, the chromosomes contain twice the amount of genetic material. Along with DNA, RNA and protein are intensively formed in the synthetic period, and the amount of histones is strictly doubled.
About 1% of the DNA of an animal cell is found in mitochondria. An insignificant part of mitochondrial DNA is reduplicated in the synthetic, while the main part - in the postsynthetic period of the interphase. At the same time, it is known that the life span of mitochondria of liver cells, for example, is 10 days. Considering that under normal conditions hepatocytes rarely divide, it should be assumed that mitochondrial DNA reduplication can occur regardless of the stages of the mitotic cycle. Each chromosome consists of two sister chromatids ( 2n), contains DNA 4c.
The time interval from the end of the synthetic period to the beginning of mitosis takes postsynthetic (premitotic), or G 2 -neriod interphase ( 2n and 4c) (3-6 hours). It is characterized by intense synthesis of RNA and especially protein. The doubling of the cytoplasm mass in comparison with the beginning of the interphase is completed. This is necessary for the cell to enter mitosis. Some of the proteins formed (tubulins) are later used to build the microtubules of the fission spindle. The synthetic and post-synthetic periods are directly related to mitosis. This allows them to be distinguished in a special period of the interphase - pre-phase.
Exists three ways of cell division: mitosis, amitosis, meiosis.
Question 1. What is the life cycle of a cell?
The life cycle of a cell is the period of its life from the moment it emerges in the process of division to death or the end of subsequent division. The length of the life cycle is highly variable and depends on the type of cells and conditions external environment: temperature, availability of oxygen and nutrients. The life cycle of an amoeba is 36 hours, while in some bacteria it is 20 minutes. For nerve cells or, for example, lens cells, its duration is years and decades.
Question 2. How does DNA duplication occur in the mitotic cycle? What is the meaning of this process?
DNA duplication occurs during interphase. First, two chains of the DNA molecule diverge, and then a new polynucleotide sequence is synthesized on each of them according to the principle of complementarity. This process is controlled by special enzymes with the expenditure of energy ATP. New DNA molecules are absolutely identical copies of the original (mother). There are no changes in genes, which ensures the stability of hereditary information, preventing the disruption of the functioning of daughter cells and the whole organism as a whole. DNA duplication also ensures that the number of chromosomes is consistent from generation to generation.
Question 3. What is the preparation of a cell for mitosis?
Cell preparation for mitosis occurs in interphase. During the interphase, biosynthesis processes are actively going on, the cell grows, forms organelles, accumulates energy, and most importantly, DNA duplication (reduplication) occurs. As a result of reduplication, two identical DNA molecules are formed, connected in the centromere region. Such molecules are called chromatids. Two paired chromatids form a chromosome.
Question 4. Describe successively the phases of mitosis.
Mitosis is conventionally divided into four phases.
Prophase. Chromosomes in the nucleus begin to actively spiralize, acquiring a compact form. As a result, reading information from DNA becomes impossible, and RNA synthesis stops. By the end of prophase, the nuclear envelope disintegrates; centrioles located at the poles of the cell form the filaments of the fission spindle.
Metaphase. At this stage, the maximum spiralization (compaction) of chromosomes is observed. They are located in the equatorial region of the cell. The spindle threads are attached to the centromeres.
Anaphase. In chromosomes, centromeres are opened and, as a consequence, chromatids are separated. Fission spindle filaments pull chromatids (each of which now becomes a separate chromosome) to the poles of the cell.
Telophase. Chromosomes, once at the poles of the cell, unwind; around them at both poles of the cell nuclear membranes are formed. Nuclei are formed containing the same diploid sets of chromosomes. The final division of the cell into two parts takes place.
As a result of mitosis, two daughter cells are formed, identical to the original mother cell.
Question 5. What is biological significance miyuza?
Mitosis is the most important biological process because:
as a result of mitosis, two daughter cells are formed from one mother cell,
identical, which ensures the maintenance of the genetic stability of the cells;
mitosis ensures the growth of organisms;
thanks to mitosis, regeneration and replacement of dying cells is carried out;
mitosis provides vegetative reproduction in plants and asexual reproduction in unicellular eukaryotes.
