What is mitotic division. What have we learned? From this brief consideration, it can be seen that the main feature of mitosis in general is the emergence of fission spindle structures, which are formed in connection with the structures of various structures.
Mitosis is the most common method of dividing eukaryotic cells. During mitosis, the genomes of each of the two resulting cells are identical to each other and coincide with the genome of the original cell.
Mitosis is the last and usually the shortest stage in time. cell cycle. With its ending life cycle cells ends and cycles of two newly formed ones begin.
The diagram illustrates the duration of the stages of the cell cycle. The letter M stands for mitosis. Top speed mitosis is observed in germ cells, the smallest - in tissues with a high degree of differentiation, if their cells divide at all.
Although mitosis is considered independently of the interphase, which consists of periods G 1 , S and G 2 , preparation for it occurs precisely in it. by the most important point is DNA replication occurring in the synthetic (S) period. After replication, each chromosome consists of two identical chromatids. They are close together along their entire length and are connected in the region of the centromere of the chromosome.
In interphase, the chromosomes are located in the nucleus and are a tangle of thin, very long chromatin filaments that are visible only under an electron microscope.
In mitosis, a number of successive phases are distinguished, which can also be called stages or periods. In the classic simplified version of the consideration, four phases are distinguished. This prophase, metaphase, anaphase and telophase. More phases are often distinguished: prometaphase(between prophase and metaphase) preprophase(characteristic of plant cells, precedes prophase).
Another process associated with mitosis is cytokinesis, which occurs mainly during the telophase period. It can be said that cytokinesis is, as it were, integral part telophase, or both processes run in parallel. Cytokinesis is understood as the division of the cytoplasm (but not the nucleus!) of the parent cell. Nuclear fission is called karyokinesis, and it precedes cytokinesis. However, during mitosis, as such, nuclear division does not occur, because first one disintegrates - the parent one, then two new ones are formed - the daughter ones.
There are cases where karyokinesis occurs but cytokinesis does not. In such cases, multinucleated cells are formed.
The duration of mitosis itself and its phases is individual and depends on the cell type. Usually prophase and metaphase are the longest periods.
The average duration of mitosis is about two hours. Animal cells usually divide faster than plant cells.
During the division of eukaryotic cells, a bipolar fission spindle is necessarily formed, consisting of microtubules and proteins associated with them. Thanks to him, equal distribution hereditary material between daughter cells.
Below will be given a description of the processes that occur in the cell in different phases of mitosis. The transition to each next phase is controlled in the cell by special biochemical checkpoints, in which it is “checked” whether everything necessary processes were correctly completed. If there are errors, the division may or may not stop. In the latter case, abnormal cells appear.
Phases of mitosis
In prophase, the following processes occur (mostly in parallel):
Chromosomes condense
Nucleoli disappear
The nuclear envelope is disintegrating
Two poles of the spindle are formed
Mitosis begins with the shortening of chromosomes. The pairs of chromatids that make up them spiralize, as a result of which the chromosomes are greatly shortened and thickened. By the end of prophase, they can be seen under a light microscope.
The nucleoli disappear, because the parts of the chromosomes that form them (nucleolar organizers) are already in a spiralized form, therefore, they are inactive and do not interact with each other. In addition, nucleolar proteins are degraded.
in animal cells and lower plants centrioles of the cell center diverge along the poles of the cell and protrude microtubule organizing centers. Although higher plants there are no centrioles, microtubules are also formed.
Short (astral) microtubules begin to diverge from each center of organization. A structure similar to a star is formed. Plants do not produce it. Their fission poles are wider; microtubules emerge not from a small, but from a relatively wide area.
The breakdown of the nuclear envelope into small vacuoles marks the end of prophase.
On the right in the photomicrograph in green microtubules are highlighted, blue - chromosomes, red - centromeres of chromosomes.
It should also be noted that during the prophase of mitosis, fragmentation of the EPS occurs, it breaks up into small vacuoles; The Golgi apparatus breaks down into individual dictyosomes.
The key processes of prometaphase are mostly sequential:
Chaotic arrangement and movement of chromosomes in the cytoplasm.
Connecting them to microtubules.
Movement of chromosomes in the equatorial plane of the cell.
Chromosomes are in the cytoplasm, they move randomly. Once at the poles, they are more likely to bond to the plus end of the microtubule. Finally, the thread is attached to the kinetochore.
Such a kinetochore microtubule begins to grow, which moves the chromosome away from the pole. At some point, another microtubule is attached to the kinetochore of the sister chromatid, growing from the other pole of division. She also begins to push the chromosome, but in the opposite direction. As a result, the chromosome becomes at the equator.
Kinetochores are protein structures at the centromeres of chromosomes. Each sister chromatid has its own kinetochore, which matures in prophase.
In addition to astral and kinetochore microtubules, there are those that go from one pole to another, as if bursting the cell in a direction perpendicular to the equator.
A sign of the beginning of metaphase is the location of chromosomes along the equator, the so-called metaphase, or equatorial, plate. In metaphase, the number of chromosomes, their differences, and the fact that they consist of two sister chromatids connected at the centromere are clearly visible.
Chromosomes are held together by balanced tension forces of microtubules of different poles.
Sister chromatids separate, each moving towards its own pole.
The poles move away from each other.
Anaphase is the shortest phase of mitosis. It begins when the centromeres of chromosomes are divided into two parts. As a result, each chromatid becomes an independent chromosome and is attached to a microtubule of one pole. Threads "pull" chromatids to opposite poles. In fact, microtubules are disassembled (depolymerized), i.e. shortened.
In the anaphase of animal cells, not only daughter chromosomes move, but also the poles themselves. Due to other microtubules, they are pushed apart, astral microtubules are attached to the membranes and also “pull”.
Chromosomes stop moving
Chromosomes decondense
Nucleoli appear
The nuclear envelope is restored
Most of the microtubules disappear
Telophase begins when the chromosomes stop moving, stopping at the poles. They despiralize, become long and filiform.
Microtubules of the fission spindle are destroyed from the poles to the equator, i.e. from their minus ends.
A nuclear envelope is formed around the chromosomes by the fusion of membrane vesicles, into which the maternal nucleus and EPS disintegrated in prophase. Each pole has its own daughter nucleus.
As the chromosomes despiralize, the nucleolar organizers become active and nucleoli appear.
RNA synthesis resumes.
If the centrioles are not yet paired at the poles, then a pair is completed near each of them. Thus, at each pole, its own cell center is recreated, which will go to the daughter cell.
Typically, telophase ends with the division of the cytoplasm, i.e., cytokinesis.
Cytokinesis may begin as early as anaphase. By the beginning of cytokinesis, cell organelles are distributed relatively evenly along the poles.
The division of the cytoplasm of plant and animal cells occurs in different ways.
In animal cells, due to elasticity, the cytoplasmic membrane in the equatorial part of the cell begins to bulge inwards. A furrow is formed, which eventually closes. In other words, the mother cell divides by ligation.
IN plant cells in telophase, the spindle filaments do not disappear at the equator. They move closer to cytoplasmic membrane, their number increases, and they form phragmoplast. It consists of short microtubules, microfilaments, parts of the EPS. Ribosomes, mitochondria, the Golgi complex move here. The Golgi vesicles and their contents at the equator form the median cell plate, cell walls and membrane of daughter cells.
Meaning and functions of mitosis
Thanks to mitosis, genetic stability is ensured: the exact reproduction of genetic material in a number of generations. The nuclei of new cells contain as many chromosomes as the parent cell contained, and these chromosomes are exact copies parental (unless, of course, mutations have occurred). In other words, the daughter cells are genetically identical to the parent.
However, mitosis also performs a number of other important functions:
height multicellular organism,
substitution of cells of various tissues in multicellular organisms,
in some species, regeneration of body parts can occur.
It is a continuous process, each stage of which imperceptibly passes into the next after it. There are four stages of mitosis: prophase, metaphase, anaphase and telophase (Fig. 1). The study of mitosis focuses on the behavior of chromosomes.
Prophase . At the beginning of the first stage of mitosis - prophase - cells retain the same appearance as in interphase, only the nucleus noticeably increases in size, and chromosomes appear in it. In this phase, it is seen that each chromosome consists of two chromatids, spirally twisted relative to each other. Chromatids shorten and thicken as a result of the process of internal spiralization. A weakly colored and less condensed region of the chromosome begins to be revealed - the centromere, which connects two chromatids and is located in a strictly defined place in each chromosome.
During prophase, the nucleoli gradually disintegrate: the nuclear membrane is also destroyed, and the chromosomes are in the cytoplasm. In the late prophase (prometaphase), the mitotic apparatus of the cell is intensively formed. At this time, the centriole divides, and the daughter centrioles diverge to opposite ends of the cell. Thin filaments in the form of rays depart from each centriole; spindle fibers form between the centrioles. There are two types of filaments: pulling filaments of the spindle, attached to the centromeres of chromosomes, and supporting filaments, connecting the poles of the cell.
When the reduction of chromosomes reaches its maximum degree, they turn into short rod-shaped bodies and go to the equatorial plane of the cell.
metaphase . In metaphase, the chromosomes are completely located in the equatorial plane of the cell, forming the so-called metaphase or equatorial plate. The centromere of each chromosome, which holds both chromatids together, is located strictly in the region of the equator of the cell, and the arms of the chromosomes are extended more or less parallel to the spindle threads.
In metaphase, the shape and structure of each chromosome is well revealed, the formation of the mitotic apparatus is completed, and the pulling threads are attached to the centromeres. At the end of metaphase, the simultaneous division of all the chromosomes of a given cell occurs (and the chromatids turn into two completely separate daughter chromosomes).
Anaphase. Immediately after the division of the centromere, the chromatids repel each other and diverge to opposite poles of the cell. All chromatids begin to move towards the poles at the same time. Centromeres play an important role in the oriented movement of chromatids. In anaphase, the chromatids are called sister chromosomes.
The movement of sister chromosomes in anaphase occurs due to the interaction of two processes: contraction of the pulling and lengthening of the supporting threads of the mitotic spindle.
Telophase. At the beginning of telophase, the movement of sister chromosomes ends, and they are concentrated at the poles of the cell in the form of compact formations and clots. Chromosomes despiralize and lose their visible individuality. A nuclear envelope is formed around each daughter nucleus; the nucleoli are restored in the same amount as they were in the mother cell. This completes the division of the nucleus (karyokinesis), cell wall. Simultaneously with the formation of daughter nuclei in telophase, the entire contents of the original mother cell are separated, or cytokinesis.
When a cell divides, a constriction or groove appears on its surface near the equator. It gradually deepens and divides the cytoplasm into
two daughter cells, each with a nucleus.
In the process of mitosis, two daughter cells arise from one mother cell, containing the same set of chromosomes as the original cell.
Figure 1. Scheme of mitosis
The biological significance of mitosis . Main biological significance Mitosis consists in the exact distribution of chromosomes between two daughter cells. A regular and orderly mitotic process ensures the transfer of genetic information to each of the daughter nuclei. As a result, each daughter cell contains genetic information about all the characteristics of the organism.
Meiosis is a special division of the nucleus, which ends with the formation of a tetrad, i.e. four cells with a haploid set of chromosomes. Sex cells divide by meiosis.
Meiosis consists of two cell divisions in which the number of chromosomes is halved so that the gametes receive half as many chromosomes as the rest of the cells in the body. When two gametes unite at fertilization, the normal number of chromosomes is restored. The decrease in the number of chromosomes during meiosis does not occur randomly, but quite naturally: the members of each pair of chromosomes diverge into different daughter cells. As a result, each gamete contains one chromosome from each pair. This is carried out by pairwise connection of similar or homologous chromosomes (they are identical in size and shape and contain similar genes) and the subsequent divergence of the members of the pair, each of which goes to one of the poles. During the convergence of homologous chromosomes, crossing over can occur, i.e. mutual exchange of genes between homologous chromosomes, which increases the level of combinative variability.
In meiosis, a number of processes occur that are important in the inheritance of traits: 1) reduction - a halving of the number of chromosomes in cells; 2) conjugation of homologous chromosomes; 3) crossing over; 4) random segregation of chromosomes into cells.
Meiosis consists of two successive divisions: the first, which results in the formation of a nucleus with a haploid set of chromosomes, is called reduction; the second division is called equational and proceeds according to the type of mitosis. In each of them, prophase, metaphase, anaphase and telophase are distinguished (Fig. 2). The phases of the first division are usually denoted by the number Ι, the second - P. Between Ι and P divisions, the cell is in a state of interkinesis (lat. inter - between + gr. kinesis - movement). In contrast to interphase, DNA is not re(du) replicated in interkinesis and chromosome material is not duplicated.
Figure 2. Scheme of meiosis
Reduction division
Prophase Ι
The phase of meiosis during which complex structural transformations of chromosomal material occur. It is longer and consists of a number of successive stages, each of which has its own distinctive properties:
- leptotena - the stage of leptonema (connection of threads). Individual threads - chromosomes - are called monovalents. Chromosomes in meiosis are longer and thinner than chromosomes in the earliest stage of mitosis;
- zygotene - the stage of zygonema (connection of threads). There is a conjugation, or synapsis (connection in pairs), of homologous chromosomes, and this process is carried out not just between homologous chromosomes, but between exactly corresponding individual points of homologues. As a result of conjugation, bivalents are formed (complexes of pairwise homologous chromosomes connected in pairs), the number of which corresponds to the haploid set of chromosomes.
Synapsis is carried out from the ends of chromosomes, therefore, the localization sites of homologous genes in one or another chromosome coincide. Since the chromosomes are doubled, there are four chromatids in the bivalent, each of which eventually turns out to be a chromosome.
- pachytene - the stage of pachinema (thick filaments). The size of the nucleus and nucleolus increase, the bivalents shorten and thicken. The connection of homologues becomes so close that it is already difficult to distinguish between two separate chromosomes. At this stage, crossing over occurs, or chromosomes cross over;
- diplotene - the stage of diplonema (double strands), or the stage of four chromatids. Each of the homologous chromosomes of the bivalent splits into two chromatids, so that the bivalent contains four chromatids. Although the tetrads of chromatids move away from each other in some places, they are in close contact in other places. In this case, the chromatids of different chromosomes form X-shaped figures, called chiasms. The presence of the chiasma holds the monovalents together.
Simultaneously with the continuing shortening and, accordingly, thickening of the chromosomes of the bivalent, their mutual repulsion occurs - divergence. The connection is preserved only in the plane of the intersection - in the chiasms. The exchange of homologous regions of chromatids is completed;
- diakinesis is characterized by the maximum shortening of diploten chromosomes. Bivalents of homologous chromosomes go to the periphery of the nucleus, so they are easy to count. The nuclear envelope is fragmented, the nucleoli disappear. This completes prophase 1.
Metaphase Ι
- begins with the disappearance of the nuclear envelope. The formation of the mitotic spindle is completed, the bivalents are located in the cytoplasm in the equatorial plane. Chromosome centromeres attach to the pulling filaments of the mitotic spindle but do not divide.
Anaphase Ι
- is distinguished by the complete termination of the relationship of homologous chromosomes, their repulsion from one another and the divergence to different poles.
Note that during mitosis, single-chromatid chromosomes diverged to the poles, each of which consists of two chromatids.
Thus, it is anaphase that reduction occurs - the preservation of the number of chromosomes.
Telophase Ι
- it is very short-term and weakly isolated from the previous phase. Telophase 1 produces two daughter nuclei.
Interkinesis
This is a short resting state between 1 and 2 divisions. Chromosomes are weakly despiralized, DNA replication does not occur, since each chromosome already consists of two chromatids. After interkinesis, the second division begins.
The second division occurs in both daughter cells in the same way as in mitosis.
Prophase P
In the nuclei of cells, chromosomes are clearly manifested, each of which consists of two chromatids connected by a centromere. They look like rather thin filaments located along the periphery of the nucleus. At the end of prophase P, the nuclear envelope fragments.
Metaphase P
In each cell, the formation of a division spindle is completed. Chromosomes are located along the equator. Spindle filaments are attached to the centromeres of chromosomes.
Anaphase P
The centromeres divide and the chromatids usually move rapidly to opposite poles of the cell.
Telophase P
Sister chromosomes concentrate at the poles of the cell and despiralize. The nucleus and cell membrane are formed. Meiosis ends with the formation of four cells with a haploid set of chromosomes.
The biological significance of meiosis
Like mitosis, meiosis ensures the precise distribution of genetic material into daughter cells. But, unlike mitosis, meiosis is a means of increasing the level of combinative variability, which is explained by two reasons: 1) there is a free, based on chance, combination of chromosomes in cells; 2) crossing over, leading to the emergence of new combinations of genes within chromosomes.
In each next generation of dividing cells, as a result of the action of these causes, new combinations of genes in gametes are formed, and during the reproduction of animals, new combinations of parental genes in their offspring are formed. This each time opens up new possibilities for the action of selection and the creation of genetically different forms, which allows a group of animals to exist in variable environmental conditions.
Thus, meiosis turns out to be a means of genetic adaptation that increases the reliability of the existence of individuals in generations.
Time from one to the next. It takes place in two successive stages - interphase and division itself. The duration of this process is different and depends on the type of cells.
Interphase is the period between two cell divisions, the time from the last division to cell death or loss of the ability to divide.
During this period, the cell grows and doubles its DNA, as well as mitochondria and plastids. In the interphase, other organic compounds. The synthesis process is most intense in the synthetic period of the interphase. At this time, nuclear chromatids double, energy is accumulated, which will be used during division. The number of cell organelles and centrioles also increases.
Interphase occupies almost 90% of the cell cycle. After it, mitosis takes place, which is the main way of dividing eukaryotic cells (organisms whose cells contain a formed nucleus).
During mitosis, the chromosomes are compacted, and a special apparatus is also formed, which is responsible for uniform distribution hereditary information between cells that are formed as a result of this process.
It goes through several stages. The stages of mitosis are characterized individual features and a certain duration.
Phases of mitosis
During mitotic cell division, the corresponding phases of mitosis pass: prophase, after it comes metaphase, anaphase, the final one is telophase.
The phases of mitosis are characterized by the following features:
What is the biological significance of the process of mitosis?
The phases of mitosis contribute to the accurate transmission of hereditary information to daughter cells, regardless of the number of divisions. At the same time, each of them receives 1 chromatid, which helps to maintain the constancy of the number of chromosomes in all cells that are formed as a result of division. It is mitosis that ensures the transfer of a stable set of genetic material.
1. Define the life and mitotic cycles of a cell.
Life cycle- the time interval from the moment a cell appears as a result of division to its death or until the next division.
Mitotic cycle- a set of consecutive and interrelated processes during the preparation of the cell for division, as well as during mitosis itself.
2. Answer how the concept of "mitosis" differs from the concept of "mitotic cycle".
The mitotic cycle includes mitosis itself and the stages of preparing the cell for division, while mitosis is only cell division.
3. List the periods of the mitotic cycle.
1. period of preparation for DNA synthesis (G1)
2. DNA synthesis period (S)
3. period of preparation for cell division (G2)
4. Expand the biological significance of mitosis.
During mitosis, daughter cells receive a diploid set of chromosomes identical to the mother cell. The constancy of the structure and the correct functioning of organs would be impossible without the preservation of the same set of genetic material in cell generations. Mitosis provides embryonic development, growth, tissue repair after damage, maintaining the structural integrity of tissues with constant loss of cells in the course of their functioning.
5. Indicate the phases of mitosis and make schematic drawings that reflect the events occurring in the cell at a certain phase of mitosis. Fill the table.
Name of the phase of mitosis Schematic drawing 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase In a plant cell
There are two ways of division: 1) the most common, complete division - mitosis (not direct division) and 2) amitosis (direct division). During mitotic division, the cytoplasm is restructured, the nuclear envelope is destroyed, and chromosomes are identified. In the life of a cell, there is a period of mitosis itself and an interval between divisions, which is called interphase. However, the period of interphase (non-dividing cells) in its essence can be different. In some cases, during interphase, the cell functions and simultaneously prepares for the next division. In other cases, cells enter interphase, function, but no longer prepare for division. As part of a complex multicellular organism, there are numerous groups of cells that have lost the ability to divide. These include, for example, nerve cells. Cell preparation for mitosis occurs in interphase. In order to imagine the main features of this process, remember the structure of the cell nucleus.
Onion cells in different phases of the cell cycle
Basic structural unit nuclei are chromosomes made up of DNA and protein. In the nuclei of living nondividing cells, as a rule, individual chromosomes are indistinguishable, but most of the chromatin, which is found on stained preparations in the form of thin filaments or grains of various sizes, corresponds to the chromosomes. In some cells, individual chromosomes are also clearly visible in the interphase nucleus, for example, in rapidly dividing cells of a developing fertilized egg and in the nuclei of some protozoa. IN different periods During the life of a cell, chromosomes undergo cyclical changes that can be traced from one division to another. Chromosomes during mitosis are elongated dense bodies, along the length of which two strands can be distinguished - chromatids containing DNA, which are the result of chromosome doubling. Each chromosome has a primary constriction, or centromere. This narrowed part of the chromosome can be located either in the middle or closer to one of the ends, but for each particular chromosome its place is strictly constant. During mitosis, the chromosomes and chromatids are tightly coiled helical filaments (a spiralized or condensed state). In the interphase nucleus, the chromosomes are strongly elongated, i.e., despiralized, due to which they become difficult to distinguish. Consequently, the cycle of chromosome changes consists in spiralization, when they shorten, thicken and become clearly distinguishable, and despiralization, when they are strongly elongated, intertwined, and then it becomes impossible to distinguish each separately. Spiralization and despiralization are associated with the activity of DNA, since it functions only in a despiralized state. The release of information, the formation of RNA on DNA in a spiralized state, that is, during mitosis, stops. The fact that chromosomes are present in the nucleus of a non-dividing cell is also proved by the constancy of the amount of DNA, the number of chromosomes, and the preservation of their individuality from division to division.
Preparing a cell for mitosis. During interphase, a number of processes occur that enable mitosis. Let's name the most important of them: 1) centrioles are doubled, 2) chromosomes are doubled, i.e. the amount of DNA and chromosomal proteins, 3) proteins are synthesized from which the achromatin spindle is built, 4) energy is accumulated in the form of ATP, which is consumed during division, 5) cell growth ends. Of paramount importance in preparing a cell for mitosis is the synthesis of DNA and duplication of chromosomes. The doubling of chromosomes is associated primarily with the synthesis of DNA and the simultaneous synthesis of chromosome proteins. The doubling process lasts 6-10 hours and takes middle part interphases. Chromosome duplication proceeds in such a way that each old single strand of DNA builds a second one for itself. This process is strictly ordered and, starting at several points, spreads along the entire chromosome.
Mitosis
Mitosis is a universal method of cell division in plants and animals, the main essence of which is the exact distribution of duplicated chromosomes between both formed daughter cells. The preparation of a cell for division, as we can see, occupies a significant part of the interphase, and mitosis begins only when the preparation in the nucleus and cytoplasm is completely completed. The whole process is divided into four phases. During the first of them - prophase - centrioles divide and begin to diverge in opposite directions. Around them, achromatin filaments are formed from the cytoplasm, which, together with centrioles, form an achromatin spindle. When the divergence of the centrioles ends, the whole cell is polar, both centrioles are located at opposite poles, and the middle plane can be called the equator. The filaments of the achromatin spindle converge at the centrioles and are widely distributed at the equator, resembling a spindle in shape. Simultaneously with the formation of a spindle in the cytoplasm, the nucleus begins to swell, and a ball of thickened threads - chromosomes - is clearly distinguished in it. During prophase, chromosomes spiralize, shortening and thickening. Prophase ends with the dissolution of the nuclear envelope, and the chromosomes are found to be lying in the cytoplasm. At this time, it can be seen that all chromosomes are already double. Then comes the second phase - metaphase. Chromosomes, randomly arranged at first, begin to move towards the equator. All of them are usually located in the same plane at an equal distance from the centrioles. At this time, part of the spindle threads is attached to the chromosomes, while the other part of them still stretches continuously from one centriole to another - these are the supporting threads. Pulling, or chromosomal, threads are attached to centromeres (primary constrictions of chromosomes), but it must be remembered that both chromosomes and centromeres are already double. Pulling threads from the poles are attached to those chromosomes that are closer to them. There is a short pause. This central part mitosis, after which the third phase begins - anaphase. During anaphase, the pulling filaments of the spindle begin to contract, stretching the chromosomes to different poles. In this case, the chromosomes behave passively, they, bending like a hairpin, move forward by centromeres, for which they are pulled by a spindle thread. At the beginning of anaphase, the viscosity of the cytoplasm decreases, which contributes to the rapid movement of chromosomes. Consequently, the threads of the spindle ensure the exact divergence of chromosomes (doubling even in interphase) to different poles of the cell. Mitosis is completed last stage- telophase. Chromosomes, approaching the poles, are closely intertwined with each other. At the same time, their stretching (despiralization) begins, and it becomes impossible to distinguish between individual chromosomes. Gradually, the nuclear envelope is formed from the cytoplasm, the nucleus swells, the nucleolus appears, and the previous structure of the interphase self is restored.
