Meiosis(Greek meiosis - decrease, decrease) or reduction division. As a result of meiosis, a decrease in the number of chromosomes occurs, i.e. from the diploid set of chromosomes (2p), a haploid (n) is formed.
Meiosis consists of 2 consecutive divisions:
Division I is called reduction or diminutive.
Division II is called equational or equalizing, i.e. goes by the type of mitosis (which means the number of chromosomes in the mother and daughter cells remains the same).
The biological meaning of meiosis is that from one mother cell with a diploid set of chromosomes, four haploid cells are formed, thus the number of chromosomes is halved, and the amount of DNA is four times. As a result of such division, sex cells (gametes) in animals and spores in plants are formed.
The phases are also called as in mitosis, and before the onset of meiosis, the cell also undergoes an interphase.
Prophase I is the longest phase and is conventionally divided into 5 stages:
1) Leptonema (leptotene)- or the stage of fine filaments. There is a spiralization of chromosomes, a chromosome consists of 2 chromatids, thickenings or clumps of chromatin, which are called chromomeres, are visible on even thin filaments of chromatids.
2) Zigonema (zygotene, Greek merging threads) - the stage of paired threads. At this stage, homologous chromosomes (of the same size) converge in pairs, they are attracted and attached to each other along the entire length, i.e. conjugated in the region of chromomeres. It looks like a zipper lock. A pair of homologous chromosomes are called bivalents. The number of bivalents is equal to the haploid set of chromosomes.
3) Pachinema (pachytene, Greek. thick) - the stage of thick threads. Further spiralization of chromosomes is in progress. Then each homologous chromosome is split in the longitudinal direction and it becomes clearly visible that each chromosome consists of two chromatids, such structures are called tetrads, i.e. 4 chromatids. At this time, there is a crossing over, i.e. exchange of homologous regions of chromatids.
4) Diplonema (diplotene)- the stage of double strands. Homologous chromosomes begin to repel, move away from each other, but maintain interconnection with the help of bridges - chiasm, these are the places where crossing over occurs. In each chromatid compound (i.e. chiasma), chromatid regions are exchanged. Chromosomes spiralize and shorten.
5) Diakinesis- the stage of detached double strands. At this stage, the chromosomes are completely condensed and intensely stained. The nuclear membrane and nucleoli are destroyed. The centrioles move to the poles of the cell and form the filaments of the fission spindle. The chromosome set of prophase I is - 2n4c.
Thus, in prophase I:
1. conjugation of homologous chromosomes;
2. the formation of bivalents or tetrads;
3. crossing over.
Depending on the conjugation of chromatids, there can be different types of crossing over: 1 - correct or incorrect; 2 - equal or unequal; 3 - cytological or effective; 4 - single or multiple.
Metaphase I - chromosome spiralization reaches its maximum. Bivalents line up along the cell's equator, forming a metaphase plate. Fission spindle threads are attached to centromeres of homologous chromosomes. Bivalents are connected to different poles of the cell.
The chromosome set of metaphase I is - 2n4c.
Anaphase I - chromosome centromeres do not divide, the phase begins with the division of the chiasm. Whole chromosomes, not chromatids, diverge to the poles of the cell. Only one of a pair of homologous chromosomes gets into daughter cells, i.e. they are randomly redistributed. At each pole, it turns out that the set of chromosomes is 1n2c, and in general the chromosome set of anaphase I is 2n4c.
Telophase I - at the poles of the cell there are whole chromosomes, consisting of 2 chromatids, but their number has become 2 times less. In animals and some plants, chromatids are despiralized. A nuclear membrane is formed around them at each pole.
Then comes cytokinesis
... The chromosome set of cells formed after the first division is - n2c.
There is no S-period between divisions I and II, and DNA replication does not take place. chromosomes are already doubled and consist of sister chromatids, therefore interphase II is called interkinesis - i.e. there is a movement between two divisions.
Prophase II is very short and proceeds without any significant changes; if a nuclear envelope is not formed in telophase I, then fission spindle threads are immediately formed.
Metaphase II - chromosomes line up along the equator. The spindle threads are attached to the centromeres of the chromosomes.
The chromosome set of metaphase II is - n2c.
Anaphase II - centromeres divide and the spindle filaments separate the chromatids to different poles. Sister chromatids are called daughter chromosomes (or maternal chromatids will be daughter chromosomes).
The chromosomal set of anaphase II is - 2n2c.
Telophase II - chromosomes are despiralized, stretched and then poorly distinguishable. Nuclear membranes and nucleoli are formed. Telophase II ends with cytokinesis.
The chromosome set after telophase II is - nc.
Meiotic division scheme
This article will help you learn about the type of cell division. We will tell you briefly and clearly about meiosis, about the phases that accompany this process, outline their main features, and find out what signs characterize meiosis.
What is meiosis?
Reduction cell division, in other words, meiosis, is a type of nuclear division in which the number of chromosomes is halved.
Translated from the ancient Greek language, meiosis means a decrease.
This process takes place in two stages:
- Reducing ;
At this stage, in the process of meiosis, the number of chromosomes in the cell is halved.
- Equational ;
During the second division, the cells remain haploid.
TOP-4 articleswho read along with this
A feature of this process is that it occurs only in diploid, as well as in even polyploid cells. This is because, as a result of the first division in prophase 1, in odd polyploids there is no way to ensure pairwise fusion of chromosomes.
Phases of meiosis
In biology, division occurs in four phases: prophase, metaphase, anaphase and telophase ... Meiosis is no exception, a feature of this process is that it occurs in two stages, between which there is a short interphase .
First division:
Prophase 1 is a rather complex stage of the entire process as a whole, it consists of five stages, which are included in the following table:
|
Stage |
Sign |
|
Leptotene |
Chromosomes are shortened, DNA condenses and thin strands are formed. |
|
Zygotena |
Homologous chromosomes are paired. |
|
Paquitena |
The longest phase in duration, during which homologous chromosomes are tightly attached to each other. As a result, some areas are exchanged between them. |
|
Diplotena |
Chromosomes are partially decondensed, part of the genome begins to perform its functions. RNA is formed, protein is synthesized, while the chromosomes are still connected to each other. |
|
Diakinesis |
DNA condensation occurs again, the formation processes stop, the nuclear envelope disappears, the centrioles are located in opposite poles, but the chromosomes are interconnected. |
The prophase ends with the formation of a fission spindle, the destruction of nuclear membranes and the nucleolus itself.
Metophase the first division is significant in that the chromosomes line up along the equatorial part of the division spindle.
During anaphase 1 microtubules contract, bivalents are separated and chromosomes diverge to different poles.
Unlike mitosis, at the anaphase stage, whole chromosomes, which consist of two chromatids, extend to the poles.
At the stage telophase chromosomes are despiralized and a new nuclear envelope is formed.
Rice. 1. Scheme of meiosis of the first stage of division
Second division has the following signs:
- For prophase 2 characterized by the condensation of chromosomes and the division of the cell center, the fission products of which diverge to the opposite poles of the nucleus. The nuclear membrane is destroyed, a new fission spindle is formed, which is located perpendicular to the first spindle.
- During metaphases the chromosomes are again located at the spindle equator.
- During anaphase chromosomes divide and chromatids are located at different poles.
- Telophase indicated by despiralization of chromosomes and the appearance of a new nuclear envelope.
Rice. 2. Scheme of meiosis of the second stage of division
As a result, four haploid cells are obtained from one diploid cell by such division. Based on this, we conclude that meiosis is a form of mitosis, as a result of which gametes are formed from the diploid cells of the gonads.
The value of meiosis
During meiosis, at the prophase 1 stage, the process occurs crossing over - recombination of genetic material. In addition, during anaphase, both the first and second division, chromosomes and chromatids diverge to different poles in a random order. This explains the combinative variability of the original cells.
In nature, meiosis is of great importance, namely:
- This is one of the main stages of gametogenesis;
Rice. 3. Scheme of gametogenesis
- Carries out the transfer of the genetic code during reproduction;
- The resulting daughter cells are not similar to the mother cell, and also differ from each other.
Meiosis is very important for the formation of germ cells, since as a result of fertilization of gametes, the nuclei fuse. Otherwise, the number of chromosomes in the zygote would be twice as large. Due to this division, the sex cells are haploid, and during fertilization, the diploidity of the chromosomes is restored.
What have we learned?
Meiosis is a type of eukaryotic cell division, in which four haploid cells are formed from one diploid cell, by reducing the number of chromosomes. The whole process takes place in two stages - reduction and equational, each of which consists of four phases - prophase, metaphase, anaphase and telophase. Meiosis is very important for the formation of gametes, for the transfer of genetic information to future generations, and also carries out the recombination of genetic material.
Test by topic
Assessment of the report
Average rating: 4.6. Total ratings received: 772.
Nikolay Mushkambarov, Dr. biol. sciences
Humanity is aging, and everyone wants to live not just for a long time, but also without those diseases that come with age. Over the past half century, many "revolutionary" theories of aging have emerged, almost all of which offer a sure and reliable way to slow down or even stop time. Every year there are new sensations, new discoveries and new statements that are encouraging and promising. Peptide bioregulators, elixir of longevity, life-giving ions, or antioxidant SkQ. Run to the pharmacy, pay and live, according to the attached instructions, up to 100-120 years! To what extent can sensational discoveries be trusted and what is the “truth about aging”?
Professor N. N. Mushkambarov. Photo by Andrey Afanasyev.
August Weismann (1834-1914) was a German zoologist and evolutionist. He created a theory according to which hereditary traits are preserved and transmitted through the ageless germplasm.
Leonard Hayflick is an American microbiologist. In the 1960s, he discovered that under laboratory conditions, human and animal cells can divide only a limited number of times.
Alexey Matveevich Olovnikov is a Russian biochemist. To explain the experiments, Hayflick in 1971 put forward a hypothesis about the shortening of the terminal sections of chromosomes (telomeres) with each cell division.
Science and Life // Illustrations
Elizabeth Blackburn and Carol Grader are American biologists. In 1985, the enzyme telomerase was discovered. The mechanism of action of telomerase is the repetitive coding of new nucleotide sequences at the end portions of telomeres and restoration of their original
Benjamin Gompertz (1779-1865) - British mathematician. He proposed a function that describes the statistics of a person's mortality depending on age. This feature was used to assess risks in life insurance.
The book of M. M. Vilenchik "Biological bases of aging and longevity", published in 1976, was one of the first popular science books on the topic of aging and enjoyed great success.
Meiosis diagram (for example, a pair of homologous chromosomes). In the prophase of the first division of meiosis, the chromosomes are doubled; then homologous chromosomes conjugate with each other and, while maintaining their activity, enter crossing over.
Doctor of Biological Sciences, Professor of the Department of Histology of the Moscow State Medical University named after M.V. I.M.Sechenov Nikolay Mushkambarov.
Nikolai Nikolaevich, you sharply criticize many of the widely known positions of modern gerontology. Please outline the objects of your criticism.
There are more than enough objects! For example, it is now fashionable to refer to Weismann almost as the ultimate truth. This is a famous biologist who, back in the 19th century, postulated that aging did not arise immediately in evolution, but only at some stage of it as an adaptive phenomenon. From this it was concluded that there must be ageless species: first of all, the most primitive organisms. At the same time, they somehow forget that if they do not age, then they must have 100% DNA repair. This is the most primitive! Somehow it does not fit one with the other.
There is a myth associated with the name of another famous biologist - Leonard Hayflick. Since the sixties of the last century, the scientific world has been in the belief that human somatic cells have a limit of 50 divisions, and this limit in biology is called the "Hayflick limit". About twenty years ago, stem cells were isolated that were capable of an allegedly unlimited number of divisions. And this myth (50 for all and infinity for stem cells) persists in the minds to this day. In fact, stem cells, as it turns out, are aging (that is, infinity is canceled), and it is completely incomprehensible where to start counting these 50 divisions. It is so incomprehensible that, most likely, there is no single division limit that is universal for all dividing human cells.
- Well, what about the telomeric theory of aging? Does she also disbelieve you?
This is the most popular myth. According to this theory, the entire aging mechanism is reduced to the fact that dividing cells lack the enzyme telomerase, which lengthens the ends of chromosomes (these ends are called telomeres), and therefore, with each division, telomeres are shortened by 50-100 nucleotide pairs of DNA. The enzyme telomerase does exist, and its discovery was awarded the 2009 Nobel Prize. And the phenomenon of chromosome shortening in dividing cells devoid of telomerase is also beyond doubt (although it is caused by a slightly different reason, which was pointed out by the author of the telomere theory, Aleksey Olovnikov). But reducing aging to this phenomenon is like replacing the most complex score of a symphony with notes of drumming. It is no coincidence that in 2003 A. Olovnikov publicly abandoned his theory, replacing it with the so-called reductive theory (also, by the way, not indisputable). But until now, even in medical universities in the course of biology, telomere theory is presented as the latest achievement of scientific thought. This is, of course, absurd.
Another example is from mortality statistics. The main formula for this statistic is the Gompertz equation, proposed in 1825, or, with a correction term, the Gompertz-Makeham equation (1860). These equations have two and three coefficients, respectively, and the values of the coefficients vary greatly for different populations of people. And now, it turns out that the changes in the coefficients of each equation correlate with each other. On the basis of this, global, world-wide regularities are formulated: the so-called Strehler-Mildvan correlation and the compensatory effect of mortality that replaced it in this position - the hypothesis of the Gavrilov spouses.
I made a small model for a conditional population of people and with its help I made sure that all these patterns are most likely an artifact. The fact is that a small error in determining one coefficient creates a sharp deviation from the true value of another coefficient. And this is perceived (in semilogarithmic coordinates) as a biologically significant correlation and serves as a basis for thoughtful conclusions.
- Are you sure you are right when talking about the artifact?
Of course not! It is generally harmful for scientists to be absolutely sure of something, although there are plenty of such examples. But I did my best to check the opposite: that correlations are not an artifact. And I could not be convinced of this opposite. So for now, based on my personal, very modest in scale, analysis, I have more reason to believe that the named correlations are still artificial. They reflect method errors, not biological patterns.
And how do you assess the statements that there are a huge number of ageless organisms in nature and their list is growing from year to year?
Alas, the popular theories about the existence of both ageless cells and ageless organisms lack sufficient basis. Indeed, every year the circle of "ageless" animals is inexorably expanding. At first, they were practically only unicellular, then lower multicellular organisms (hydras, mollusks, sea urchins, etc.) were added to them. And now there are hotheads that "discover" certain ageless species even among fish, reptiles and birds. So it goes - they will soon get to mammals and establish, for example, that elephants also do not age, but simply die because of their excess body weight!
- Are you convinced that there are no ageless animals?
I am not convinced that there are no such animals (although I am inclined to do so), but that there is not a single species of animals for which the absence of aging would be proven absolutely reliably. In relation to human cells (as well as cells and other representatives of the animal world), the degree of confidence is perhaps even higher: stem cells, reproductive cells, and even tumor cells, in principle, age. Stem cells were considered indisputably ageless, and now there are experimental works proving the opposite.
- What is such confidence based on? Did you do the experiments yourself?
Generally speaking, a very long time ago, in 1977-1980, I tried to approach the problem of aging in experiments on mice. But the not very reliable results (although they seem to confirm the initial assumption) convinced that it is better to do not experiment, but analysis. And here is one of the results of this analysis - the concept of "Anerem", or the ameiotic theory of aging. It includes six theses (postulates, if you will), of which one (the very first) is purely my work, and the rest are formulated on the basis of ideas already in the literature. And, of course, it is important that all these theses form a fairly clear picture on the whole.
So, it is the ameiotic concept, if you adhere to it, that excludes the possibility of the existence of both ageless cells in multicellular organisms and ageless organisms (starting with unicellular ones). At the same time, of course, I am aware that all theses of the concept are still hypotheses. But they seem to be much more valid than other views.
So, your concept is like a tester, with which you can evaluate, relatively speaking, the truth of certain assumptions? In that case, tell us more about it.
I will try to make it as accessible as possible. The very name of the concept ("Anerem") is an abbreviation of the words autocatalysis, instability, repair, meiosis. First thesis. Remember, before Engels' definition of life was very well known: “Life is a way of existence of protein bodies”? I revised this definition and gave my own, which formed the first thesis: "Life is a way of autocatalytic multiplication of DNA (less often RNA) in nature." This means that the driving force behind the emergence of life and its subsequent evolution is the indomitable striving of nucleic acids for endless self-reproduction. In essence, any organism is a biomachine improved in evolution, designed to efficiently preserve and multiply the genome contained in it, with the subsequent efficient distribution of its copies in the environment.
- It's unusual to feel like a biomachine ...
Nothing, the sensation will pass, but the function, sorry, will remain. The second thesis: “Genome instability is the central element of aging”. This is how aging is understood by the majority of sane scientists in the West, and even here. The fact is that, with all their remarkable abilities, nucleic acids are susceptible to the damaging effects of many factors - free radicals, reactive oxygen species, etc. And although many protective systems (such as the antioxidant system) have been created in evolution, numerous damage constantly occurs in the DNA strands. To detect and correct them, there is another defense system - DNA repair (restoration). The next thesis, the third one, is a filter that filters out everything “ageless”: “The genome repair in mitotic and postmitotic cells is not complete”. That is, any repair system in these cells does not provide 100% correction of all arising DNA defects. And this means the general nature of aging.
- But if everything and everyone is getting old, then how is life on Earth supported at all?
Just about, I became interested in this issue in 1977. And I found, as it seemed to me, my own, albeit lying on the surface, answer. And 25 years later, in 2002, looking through my old books, I realized that this hypothesis was not mine at all, and I had read about it a year before in the book of M.M. your own. These are the quirks of memory. But in the end it is the essence of the matter that matters, not the ambition of the discoverer.
The essence is formulated by the fourth thesis: "Effective repair can be achieved only in meiosis (or in its simplified version - endomixis) - during conjugation (fusion) of chromosomes." What is meiosis, everyone seems to have passed at school, but, unfortunately, sometimes even our medical students do not know this. Let me remind you: meiosis is the last double division in the formation of germ cells - sperm and eggs. By the way, I'll tell you a secret: women do not produce eggs. They have a second meiotic division (at the stage of oocyte II - the development of the female reproductive cell) cannot occur independently - without the help of a sperm. Because the cell has "lost" its centrioles (bodies in the cell, participating in division) somewhere: they were just (during the previous division), but now they have disappeared somewhere. And fertilization of oocyte II is imperative for the sperm to bring in its centrioles and save the day. I regard this as typical "feminine things". So the second division of meiosis eventually occurs, but the resulting cell is no longer an egg, but a zygote.
We got carried away with "female tricks" and did not clarify how complete DNA repair is achieved in meiosis.
The first division of meiosis is preceded by a very long prophase: in male gametogenesis it lasts a whole month, and in female - up to several decades! At this time, homologous chromosomes approach each other and remain in this state almost all the time during prophase.
At the same time, enzymes that cut and cross-link DNA strands are sharply activated. It was believed that this is necessary only for crossing over - the exchange of chromosomes by their regions, which increases the genetic variability of the species. Indeed, the “father's” and “mother's” genes, which are still distributed in each pair of homologous (structurally identical) chromosomes on different chromosomes, turn out to be mixed after crossing over.
But MM Vilenchik, and after him, I, drew attention to the fact that crossing-over enzymes very much resemble DNA repair enzymes, in which, by cutting out damaged areas, DNA strands must also be broken and stitched. That is, at the same time as crossing over, DNA superreparation is probably carried out. It is possible to imagine other mechanisms of major "repair" of genes in the course of meiosis. One way or another, in this case, a radical (more precisely, complete) "rejuvenation" of the cells occurs, which is why the mature germ cells begin to count down the time, as it were, from scratch. If something did not work out, then in the cell self-control sensors for the state of its own DNA are triggered and the process of apoptosis starts - self-
killing cells.
- So, in nature, rejuvenation occurs only in maturing germ cells?
Quite right. But this is quite enough to ensure the immortality of the species - against the background, alas, of the inevitable mortality of all individuals. After all, sex cells - and only they! - that only material substrate of parental organisms, from which a new life is born - the life of offspring.
And the fact that this mechanism concerns only the germ cells is discussed in the two remaining theses of the concept, which dot all the i's. Fifth thesis: "Meiosis improves the state of the genome of only subsequent generations (several generations at once in simple organisms and only one in all others)." The sixth thesis: "Hence follows the inevitability of aging of individuals (individuals) and the relative immortality of the species as a whole."
- And what, meiosis is in all species of animals?
It should be in all species of animals - according to the Anerem concept, if it turns out to be correct. Indeed, the concept is based on the universality of not only aging, but also meiosis. I have carefully researched this issue from the literature. Of course, in sufficiently developed animals - in fish and "higher" - there is only a sexual mode of reproduction, which also implies the presence of meiosis. In addition, there are huge sectors of both flora and fauna in which mixed breeding types are common. This means that they alternate more or less prolonged acts of asexual reproduction (for example, mitotic divisions, sporulation, budding, fragmentation, etc.) and single acts of sexual or quasi-sexual reproduction. An essential feature of the quasi-sexual process (the so-called endomixis) is that structurally identical chromosomes from the paternal and maternal set (conjugation of homologous chromosomes) are also connected here, although it does not end with their divergence in different cells.
Thus, with mixed reproduction, several generations of organisms live, as it were, gradually aging (similarly to how mitotically dividing cells age in more complex animals), and then the sexual process returns individual organisms to "zero" age and provides
bakes a comfortable life for several more generations. Finally, it is believed that a number of simple animals reproduce only asexually. But with regard to them, I still have some doubt: have these organisms not overlooked something similar to meiosis or endomixis (self-fertilization) in a long series of asexual reproduction?
It turns out that the concept you are developing puts an end to all dreams of extending human life. After all, ordinary (non-sex) cells are doomed to age and age?
No, I am not giving up. Firstly, because for us it is not the very fact of aging that is much more important, but the speed of this process. And you can influence the rate of aging by many means. Some of them are known, some (like Skulachev's ions) are under investigation, some will be discovered later.
Secondly, it is possible that over time it will be possible to initiate some meiotic processes in somatic cells, for example, in stem and non-dividing cells. I mean those processes that restore the state of the genome: this is, apparently, the conjugation of homologous chromosomes, crossing over, or something more subtle and still unknown. I see no reason why this would be impossible in principle. In the lines of germ cells, in general, cells of the same structure as many others enter meiosis. Moreover, even after conjugation of chromosomes in the latter, the activity of the corresponding genes remains. However, for the implementation of this project, it is necessary to first completely determine the genes responsible for various aspects of meiosis, and establish ways of targeted impact on them. This is, of course, a very fantastic project. However, didn't much of what we have today seemed fantastic yesterday ?!
Meiosis - is carried out in the cells of organisms that reproduce sexually.
The biological meaning of a phenomenon is determined by a new set of features in offspring.
In this work, we will consider the essence of this process and for clarity, we will present it in the figure, see the sequence and duration of division of germ cells, and also find out what are the similarities and differences between mitosis and meiosis.
What is meiosis
A process accompanied by the formation of four cells with a single chromosome set from one original.
The genetic information of each newly formed cell corresponds to half of the set of somatic cells.
Phases of meiosis
Meiotic division includes two stages, each consisting of four phases.
First division
Includes prophase I, metaphase I, anaphase I, and telophase I.
Prophase I
At this stage, two cells are formed with half the set of genetic information. The first division prophase includes several stages. It is preceded by premeiotic interphase, during which DNA replication takes place.
Then condensation occurs, the formation of long thin filaments with a protein axis during leptotene. This thread is attached to the membrane of the nucleus by means of end extensions - attachment discs. The halves of the doubled chromosomes (chromatids) are not yet distinguishable. When examined, they look like monolithic structures.
Next comes the zygotene stage. Homologues merge to form bivalents, the number of which corresponds to a single number of chromosomes. The process of conjugation (connection) is carried out between paired, similar in genetic and morphological aspects. Moreover, the interaction begins from the ends, spreading along the bodies of the chromosomes. A complex of homologues linked by a protein component - bivalent or tetrad.
Coiling occurs during the thick filament stage - pachytene. Here, DNA doubling has already been completed, crossing over begins. This is an exchange of sites of homologues. As a result, linked genes are formed with new genetic information. Transcription proceeds in parallel. Dense sections of DNA - chromomeres - are activated, which leads to a change in the structure of chromosomes like "lamp brushes".
Homologous chromosomes condense, shorten, diverge (excluding the junction points - chiasma). This is a stage in the biology of diplotene or dictyotene. Chromosomes at this stage are rich in RNA, which is synthesized at the same sites. In terms of properties, the latter is close to informational.
Finally, bivalents diverge to the periphery of the nucleus. The latter are shortened, lose nucleoli, become compact, not associated with the nuclear envelope. This process is called diakinesis (transition to cell division).
Metaphase I
Further, the bivalents move to the central axis of the cell. Fission spindles extend from each centromere, each centromere is equidistant from both poles. The threads, small in amplitude, hold them in this position.
Anaphase I
Chromosomes built from two chromatids diverge. There is a recombination with a decrease in genetic diversity (due to the absence in the set of genes located at the loci (sites) of homologues).
Telophase I
The essence of the phase consists in the divergence of chromatids with their centromeres to opposite parts of the cell. In an animal cell, cytoplasmic division occurs, in a plant cell, the formation of a cell wall.
Second division
After the interphase of the first division, the cell is ready for the second stage.
Prophase II
The longer the telophase, the shorter the prophase duration. Chromatids line up along the cell, forming a right angle with their axes relative to the filaments of the first meiotic division. At this stage, they shorten and thicken, the nucleoli undergo decay.
Metaphase II
The centromeres are again located in the equatorial plane.
Anaphase II
Chromatids separate from each other, moving towards the poles. They are now called chromosomes.
Telophase II
Despiralization, stretching of the formed chromosomes, disappearance of the fission spindle, doubling of centrioles. The haploid nucleus is surrounded by a nuclear membrane. Four new cells are being formed.
Comparison table of mitosis and meiosis
Briefly and clearly, the features and differences are presented in the table.
| Specifications | Meiotic division | Mitotic division |
| Number of divisions | carried out in two stages | carried out in one step |
| Metaphase | after doubling, chromosomes are located along the central axis of the cell in pairs | after doubling, chromosomes are located singly along the central axis of the cell |
| Merge | there is | No |
| Crossover | there is | No |
| Interphase | no DNA doubling in interphase II | DNA doubling is characteristic before division |
| Division result | gametes | somatic |
| Localization | in maturing gametes | in somatic cells |
| Play path | sexual | asexual |
The data presented is a diagram of the differences, and the similarities boil down to the same phases, DNA reduplication and spiralization before the start of the cell cycle.
The biological significance of meiosis
What is the role of meiosis:
- Gives new combinations of genes due to crossing over.
- Supports combinative mutability. Meiosis is the source of new traits in the population.
- Keeps a constant number of chromosomes.
Conclusion
Meiosis is a complex biological process during which four cells are formed, with new signs obtained as a result of crossing over.
Meiosis is a method of indirect division of primary germ cells (2p2s), in as a result of which haploid cells (lnlc), most often sex cells, are formed.
Unlike mitosis, meiosis consists of two successive cell divisions, each of which is preceded by interphase (Fig. 2.53). The first division of meiosis (meiosis I) is called reduction, 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 (see Table 2.5).
Interphase I proceeds like the 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 take place - conjugation and crossing over. Conjugation is the process of fusion of homologous (paired) chromosomes along their entire length. The pairs of chromosomes formed during conjugation are retained until the end of metaphase I.
Crossover- mutual exchange of homologous regions of homologous chromosomes (Fig. 2.54). As a result of crossing over, the chromosomes received by the body from both parents acquire new combinations of genes, which leads to 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 envelope disintegrates.
Vmetaphase I pairs of chromosomes line up along the equator of the cell, microtubules of the fission spindle are attached to their centromeres.
V anaphase I whole homologous chromosomes, consisting of two chromatids, diverge to the poles.
V telophase I around the clusters of chromosomes at the poles of the cell, nuclear membranes are formed, nucleoli are formed.
Cytokinesis I provides separation of the cytoplasm of daughter cells.
The daughter cells (1n2c) formed as a result of meiosis I are genetically heterogeneous, since their chromosomes, which randomly diverge to the poles of the cell, contain different genes.
Interphase II very short, since there is no DNA doubling 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. V prophase II the same processes proceed as in prophase I, with the exception of conjugation and crossing over.
V metaphase II chromosomes are located along the equator of the cell.
V anaphase II chromosomes are split in centromeres and chromatids are already stretched to the poles.
V telophase II nuclear membranes and nucleoli are formed around the clusters of daughter chromosomes.
After cytokinesis II the genetic formula of all four daughter cells - 1n1c, however, they all have a different set of genes, which is the result of crossing over and a random combination of the chromosomes of the maternal and paternal organisms in the daughter cells.
