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At the moment, my thoughts are that the two cell divisions are necessary for recombination to occur, although I am not sure. I cannot really see why technically, the chromosome from each parent cannot just recombine with the other chromsome when each is a single DNA strand and not in the form of two sister chromatids joined at the centromere. Something tells me that there are several reasons why there are two cell divisions. Perhaps the S phase of interphase triiggers some further stages of cell division such as cytokinesis, and the splitting of the cell will not occur unless some DNA replication has taken place? However in meiosis 2 there is no further DNA replication, so this can't quite be right…
For developing a 2N cell, we need a N cell from each parent. In any division(meiosos or meitosis), chromosomes are doubled at first. In firs meiosis a 2N cell in divided into two N cells and as you know these N chromosomes are doubled( 2N chromatids). In second meiosis a N cell is divided into two N cells but this time chromosomes are not doubled(in fact N chromatid cells) 1.Firs division in meiosis is needed because we need N cells to combine, if they are 2N the result of recombination is a 4N cell. 2.Maybe your question is: why a 2N cell is not directly divided into two N cells without doubling? the reason is same in mitosis and meiosos, it is because of a stage of division as you know it is called anaphase in which a kind of protein, connects centromer to centriol, this stage is needed because we need one chromatid of each chromosome in each cells so if this part is not done there is no guarantee to have all N chromosomes in both cells and in fact a kind of distribution is made. ( I hope that I could understand you, well.)
What is the purpose of meiosis I?
The main purpose of meiosis is to create gametes, or sex cells like sperm and eggs. Meiosis creates four unique, haploid gametes with each cycle.
Likewise, what is the definition of meiosis 1? Primary Meanings of meiosis 1. n. (genetics) cell division that produces reproductive cells in sexually reproducing organisms the nucleus divides into four nuclei each containing half the chromosome number (leading to gametes in animals and spores in plants) 2.
Accordingly, what is the purpose of mitosis and meiosis?
The purpose of mitosis is cell regeneration, growth, and asexual reproduction,while the purpose of meiosis is the production of gametes for sexual reproduction. Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new daughter cells.
Meiosis is the process in eukaryotic, sexually-reproducing animals that reduces the number of chromosomes in a cell before reproduction. Many organisms package these cells into gametes, such as egg and sperm. The gametes can then meet, during reproduction, and fuse to create a new zygote.
What Is the Purpose of Meiosis?
Meiosis is the process of cell division that creates offspring in sexually reproducing organisms, explains a University of Illinois at Chicago website. Unlike during mitosis, meiotic cell division starts with double the number of chromosomes in diploid parent cells. Meiosis cuts this number in half forming two haploid daughter cells. When these daughter cells combine and undergo fertilization, a zygote is created and the cell begins to develop.
Meiosis takes diploid parent cells, halves their genetic material, and combines the resulting daughter cells together. The end result is a zygote that consists of genetic material from both parent cells. After the zygote is formed, the organism returns to mitosis, or typical cell reproduction.
The process of meiosis has two main stages: Meiosis I and Meiosis II. Each stage is further broken down into four substages: Prophase, Metaphase, Anaphase and Telophase. Meiosis I involves the creation of the daughter cells from the diploid parent cells. In Prophase I, the chromosomes become visible, move toward the poles of the cell, the membrane disappears and chromosomes begin to swap genetic material. In Metaphase I, the genetic material attaches to centromeres in the cell. These centromeres begin to pull apart in Anaphase I, and they fully divide during Telophase I. Meiosis II involves the combination of the daughter cells. Each substage follows the pattern set in Meiosis I, but results in four haploid daughter cells with a standard amount of chromosomes.
Meiosis can be both sexual and asexual. One advantage of meiotic reproduction is that it can causes variation in the cell that could create new, beneficial adaptations. In this way, meiotic reproduction aids in natural selection.
Meiosis Definition for Kids
There are two ways cells divide and then multiply to form new cells. These processes can be referred to as methods of cell division or duplication. The two methods are known as mitosis and meiosis. Mitosis is asexual division of cells. Meiosis is sexual division of cells.
What is Meiosis?
Meiosis is a biological process in plants and animals, as well as in some microorganisms. The process leads to the division of the nucleus in a cell in organisms that are capable of sexual reproduction. Meiosis is pivotal to reproduction or procreation. There are two larger phases of meiosis that are referred to as Meiosis I and Meiosis II. Unlike mitosis, one cell doesn’t duplicate into two with the same set of chromosomes. Instead, the first cell duplicates, then the chromosomes are halved and two cells contribute half of their chromosomes to form the new cell.
The Process of Meiosis
In meiosis, there are a total of four stages. They are called prophase I, metaphase I, anaphase I and telophase I. The nucleus of a cell divides and leads to the production of haploid cells. Haploid cells are unlike the diploid cells as is the case in mitosis where the new cells have equal number of chromosomes as the mother or parent cell. In case of meiosis, there must be two cells in the beginning, both cells would halve the number of chromosomes and the nuclei of the two would get split leading to the formation of the two haploid cells. These haploid cells would then further lead to the formation of the new cell which will have the chromosomes of both the parent cells.
Meiosis Vs Mitosis
In mitosis, a cell lines up its chromosomes in the center, duplicates the nucleus and the chromosomes thus leading to the formation of two cells, both of which will have the same characteristics of the parent cell. In effect, the two new cells are identical to the parent cell. In meiosis, the new cell is not identical to either of the parent cells. The process is a reductionist one as compared to the duplicating method of mitosis.
Humans have sexual and asexual cells. Hence, we need meiosis and mitosis. Excluding single celled organisms that would only multiply by mitosis, all plants and animals rely on meiosis to reproduce. The cellular division inside plants and animals meant for cellular regeneration are done through mitosis while sexual reproduction or procreation is done through meiosis.
What is the purpose of two cell divisions in meiosis? - Biology
3327 days since
3326 days since
70trillion) different possible offspring.
This genetic variation leads on rare occasions to beneficial mutations, which are then (ideally) preserved in the species. Asexual reproduction would not allow that.
Since genetic information is contributed by two parents, each parental cell needs to be haploid (fused diploid gametes would otherwise be tetraploid). When two such haploid cells fuse, the resulting offspring is called a zygote. The haploid cells are produced by males, in which case they are called sperm, and females. Female gametes are called eggs (ova), and they are specialised for the storage of nutrients. They can be much greater in size than sperm, and, unlike sperm, they are usually non-mobile.
The fusion of a male and female gametes is called fertilisation, and the resulting fertilised egg, a zygote. It is diploid, and contains one set of chromosomes from sperm, and one from the egg. The cell division which leads to the production of gametes is called meiosis.
- In leptotene, chromatin condenses into long, thread-like structures, just like at the beinning of mitosis.
- In zygotene, the condensed chromosomes become visible and homologous chromosomes pair together (the process called synapsis) to form bivalents (two homologous chromosomes paired so tightly, that they act as a single unit, also called a tetrad).
- In pachytene, the exchange of DNA segments, or the crossing over, accounts for the genetic recombination.
- In diplotene, bivalents begin to split up, especially at centromeres, but are still condensed where homologous chromosomes exchange DNA (so-called chiasmata).
- Throughout the Prophase I the chromosomes become more and more compact, only to reach their maximally compact state in diakinesis. The centromers sepparate completely, and chromosomes are only held together by the chiasmata. Nuclear envelope disappears, and the spindle forms.
During telophase, chromosomes arrive at the spindle poles, the nuclear envelope reappears, and cytokinesis divides the parent cell into two haploid daughters.
Meiosis II begins after Meiosis I, and its role is similar to that of the mitotic division, to 'pack' the chromatids created during DNA replication to two newly forming cells.
All the stages of Meiosis II resemble those of Mitosis, but with one important difference: in Metaphase II of Meiosis II, only half as many chromosomes align at the metaphase plate.
The final result of the meiotic division is 4 haploid cells (2 from each haploid daughter cell created in meiosis).
What Is the Function of Meiosis?
The function of meiosis is for sexual reproduction as meiosis creates new cells for an organism. Meiosis has two cell divisions known as meiosis I and meiosis II.
Meiosis will create four cells when there was originally only one cell. That means that those four cells will only have half of the amount of DNA that is needed by each cell and means that when a cell goes through the meiosis process, it is not concerned about creating another working cell. Rather it is concerned about reproducing and creating an organism.
Plants, animals and even some fungi undergo meiosis in order to shuffle the cell's genes around. The first step of meiosis I or meiosis II involves pairs of chromosomes lining up at the center of the cell. Then, these cells are pulled to each side at the outermost corner of the cell. During this stage a crossing over happens, which is the exchange of genes in the DNA. These genes are mixed up so that the final result is not a perfect duplicate like mitosis. The cell divides and is left with two new cells with a single pair of chromosomes. The second stage begins immediately and leaves the two cells with four haploid cells. These haploid cells are called gametes. The gametes eventually find other gametes to combine with and form a new organism.
In what cells does meiosis occur?
In respect to this, where does meiosis occur in humans?
Originally Answered: Where does meiosis take place in the human body? Meiosis mainly takes place in sperm cell (male) and in egg cell (female). In the male, meiosis takes place after puberty. Diploid cells within the testes undergo meiosis to produce haploid sperm cells with 23 chromosomes.
Secondly, what is the purpose of meiosis? Meiosis, on the other hand, is used for just one purpose in the human body: the production of gametes&mdashsex cells, or sperm and eggs. Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell.
Hereof, in which cells does mitosis occur?
Mitosis is the process in cell division by which the nucleus of the cell divides (in a multiple phase), giving rise to two identical daughter cells. Mitosis happens in all eukaryotic cells (plants, animals, and fungi). It is the process of cell renewal and growth in a plant, animal or fungus.
When and where does meiosis take place?
Meiosis occurs in the sex cell of human body. They are present in the form of sperms cell in males and in female they are present in the form of eggs. Meiosis is the procedure of cells part into four haploid cells, along these lines decreasing the chromosome number considerably in every cell.
Notes on Meiosis: Features, Division and Significance
Two spindle using divisions which reduce the chromosome number from diploid to haploid constitute meiosis. The main function of meiosis is to produce gametes in an organism.
1. Meiosis results in formation of four daughter cells from a single mother cell in each cycle of cell division. In other words, the nuclei divide twice in each cell cycle.
2. Daughter cells are identical to mother cell in shape and size but different in chromosome composition. The daughter cells have haploid chromosome number. The chromosome types also differ in daughter cells due to segregation and recombination.
3. Meiosis occurs in reproductive organs like anthers and ovaries and leads to the production of gametes or spores.
4. The complete process of meiosis consists of two types of division. The first division results in reduction of chromosome number to half and is called reductional division. The second division is like mitotic division.
5. Meiosis results in segregation of chromosomes and genes and their independent assortment. Crossing over and recombination also occur during meiosis.
Notes # Division of Meiosis:
The process of meiosis as indicated earlier, consists of two types of division, viz., first meiotic and second meiotic division. Before initiation of meiosis, there is an interphase which consists of G1, S and G2 phases like mitosis. But here the G2 phase is of very short duration.
The S phase occurs only once in the entire process of meiosis. There is no S phase after first division of meiosis. During S phase 99.7% of the total DNA present in the nucleus is synthesized and remaining 0.3% DNA synthesis takes place during zygotene stage.
A. First Meiotic Division:
The first meiotic division results in reduction of chromosome number in each new cell to just half of the mother cell, therefore, it is referred to as reductional division.
The first meiotic division consists of four different phases, viz:
This phase starts after interphase and is of maximum duration. This consists of five sub stages, viz., leptotene, zygotene, pachytene, diplotene and diakinesis.
Important features of these sub stages are briefly discussed below:
i. Chromosomes look like thin thread under light microscope. They are inter-woven like a loose ball of wool (Fig. 3.3A).
ii. Chromosomes are scattered throughout the nucleus in a random manner.
iii. In some cases, chromomeres are visible on the chromosomes in the form of condensed regions.
iv. RNA and protein syntheses also take place.
i. Homologous chromosomes being to pair.
ii. Chromosomes become shorter and thicker (Fig. 3.3B).
iii. The synthesis of remaining 0.3%. DNA which has not taken place during S phase also occurs during this stage.
iv. Synaptonemal complex also develops during this stage.
i. Chromosomes look like bivalents. Each bivalent has two chromatids. Thus each pair has four chromatids generally known as tetrads.
i. This stage begins after complete terminilization of chiasmata (Fig. 3.3E).
ii. Chromosomes are further condensed.
iii. Bivalents are distributed throughout the cell.
iv. Nucleolus and nuclear membrane disappear towards the end of diakinesis.
2. First Metaphase:
i. The spindle apparatus gradually organises.
ii. Bivalents are arranged on the equatorial plate (Fig. 3.3F).
iii. The centromere of each chromosome divides longitudinally.
3. First Anaphase:
i. From each bivalent, one chromosome moves towards one pole and the other towards other pole. In other words, one homologous chromosome moves towards one pole and another to opposite pole (Fig. 3.3G).
ii. Sister chromatids of each chromosome remain attached at the centromere.
iii. Homologous chromosomes reach the opposite pole at the end of this phase.
4. First Telophase:
i. Chromosomes uncoil and relax and regrouping of chromosomes occurs (Fig 3.3H).
ii. Nucleolus and nuclear membrane reappear.
iii. Two haploid daughter nuclei are formed.
B. Second Meiotic Division:
The first nuclear division (Meiosis I) results in reduction of chromosome number from diploid to haploid. The second nuclear division (Meiosis II) is required to reduce the number of chromatids per chromosome.
Meiosis II differs from Mitosis in the following three main aspects:
1. The interphase prior to meiosis II is very short. It does not have S period because each chromosome already contains two chromatids.
2. The two chromatids in each chromosome are not sisters throughout. In other words, some chromatids have alternate segments of non-sister chromatids due to recombination.
3. The meiosis II (Meiotic mitosis) deals with haploid chromosome number, whereas normal mitosis deals with diploid chromosome number.
Rest of the features of meiosis II is similar to mitosis. It also consists of prophase, metaphase, anaphase and telophase.
The division of cytoplasm takes place either by cell plate method (in plants) or by furrow method (in animals). The cytokinesis may take place after meiosis I and meiosis II separately or sometimes it may take place at the end of meiosis II only. In maize, it occurs after meiosis I and meiosis II. However, in Trillium cytokinesis occurs only at the end of Meiosis II.
2. The chromosome number looks like haploid number.
3. Nucleolus is present and attached to a chromosome.
4. Formation of chiasma and crossing over take place during pachytene stage (Fig. 3.3C).
1. Separation of homologous chromosomes begins. It starts at centromere and moves towards the end (Fig. 3.3D).
2. The separating chromosomes are attached at some points. These points are called chiasmata. These chiasmata are terminalized towards the end of diplotene.
3. Chromosomes are further condensed and become still shorter and thicker.
4. Nucleolus decreases in size.
It is a protein framework which is found between paired chromosomes. It consists of one central and two lateral elements. There are transverse filaments on both the sides of central element. The lateral elements are attached to homologous chromosomes (Fig. 3.4).
Synaptonemal complex is considered to be associated with pairing of homologous chromosomes and recombination. However, its origin and exact role in synapsis is still not properly known which requires further research.
Notes # Genetic Control of Meiosis:
It is believed that meiosis is genetically controlled. Various features of meiosis are controlled by genes.
Some of the features of meiosis which are genetically controlled are described below:
1. Synapsis and Exchange:
Synapsis or pairing between homologous chromosomes may depend on the presence of a specific allele. For example, in maize when this allele is absent, synapsis is prevented between all homologous loci. In the absence of synapsis, no exchange occurs between homologous chromosomes and distribution of chromosomes is also irregular during anaphase I.
In Drosophila male, crossing over does not occur because the homologous chromosomes pair only in the heterochromatic region near centromere. Heterochromatin is considered devoid of active genes, hence exchange is prevented. However, the pairing is normal in females.
2. Centromere Behaviour:
The specific behaviour of centromere during meiosis is genetically controlled. At metaphase II, centromeres of sister chromatids lie very close together. But when one of them faces one pole of spindle, the other one automatically faces the opposite pole.
In Drosophila, in the presence of a particular allele, sister centromeres separate early at metaphase II and orient to the spindle independently. As a result, both sister centromeres sometimes orient to the same pole and hence are not distributed to daughter nuclei properly. However, this allele has no effect on mitosis or meiosis I.
The shape of spindle is governed by a specific allele. The presence of abnormal alleles changes the shape of spindle in maize and Drosophila during meiosis but not mitosis. A normal allele causes the meiotic spindle to have convergent ends.
With such spindle, all the chromosomes come together in group to the pole and are included in a telophase nucleus. In the presence of abnormal allele, divergent spindles are formed. Such spindles lead to the spread of chromosomes at anaphase in such a way that some are left but of telophase nuclei.
4. Spindle Orientation:
The orientation is usually similar in meiosis I and meiosis II. This leads to formation of four nuclei or cells. When this direction is opposite to that of meiosis I, the result is a cluster of four nuclei or cells.
Notes # Significance of Meiosis:
Meiosis plays a very important role in the biological populations in various ways as given below:
1. It helps in maintaining the chromosome number constant in a species. Meiosis results in production of gametes with haploid (half) chromosome number. Union of male and female gametes leads to formation of zygote which receives half chromosome number from male gamete and half from the female gamete and thus the original somatic chromosome number is restored.
2. Meiosis facilitates segregation and independent assortment of chromosomes and genes.
3. The recombination of genes also takes place during meiosis. Recombination of genes results in generation of variability in a biological population which is important from evolution points of view.
4. In sexually reproducing species, meiosis is essential for the continuity of generation. Because meiosis results in the formation of male and female gametes and union of such gametes leads to the development of zygote and thereby new individual.
Meiosis is a process of nuclear division that reduces the number of chromosomes in the resulting cells by half. Thus, meiosis is sometimes called &ldquoreductional division.&rdquo For many organisms the resulting cells become specialized &ldquosex cells&rdquo or gametes . In organisms that reproduce sexually, chromosomes are typically diploid ( 2N ) or occur as double sets ( homologous pairs ) in each nucleus. Each homolog of a pair has the same sites or loci for the same genes. You might recognize that you have one set of chromosomes from your mother and the remaining set from your father. Meiosis reduces the number of chromosomes to a haploid ( 1N ) or single set. This reduction is significant because a cell with a haploid number of chromosomes can fuse with another haploid cell during sexual reproduction and restore the original, diploid number of chromosomes to the new individual. In addition to reducing the number of chromosomes, meiosis shuffles the genetic material so that each resulting cell carries a new and unique set of genes in a process of independent assortment .
As in mitosis, meiosis is preceded by replication of each chromosome to form two chromatids attached at a centromere. However, reduction of the chromosome number and production of new genetic combinations result from two events that don&rsquot occur in mitosis. First, meiosis includes two rounds of chromosome separation. Chromosomes are replicated before the first round, but not before the second round. Thus, the genetic material is replicated once and divided twice. This produces half the original number of chromosomes.
Crossing over between chromatids of homologous chromosomes increases genetic diversity during meiosis I. Synapsis occurs during prophase I as the homologous chromosomes begin to pair up. Credit: Jeremy Seto (CC-BY-NC-SA)
Second, during an early stage of meiosis each chromosome (comprised of two chromatids) pairs along its length with its homolog. This pairing of homologous chromosomes results in a physical touching called synapsis , during which the four chromatids (a tetrad) exchange various segments of genetic material. This exchange of genetic material is called crossing-over and produces new genetic combinations. During crossing-over there is no gain or loss of genetic material. But afterward, each chromatid of the chromosomes contains different segments (alleles) that it exchanged with other chromatid.
Stages and Events of Meiosis
Although meiosis is a continuous process, we can study it more easily by dividing it into stages just as we did for mitosis. Indeed, meiosis and mitosis are similar, and their corresponding stages of prophase, metaphase, anaphase, and telophase have much in common. However, meiosis is longer than mitosis because meiosis involves two nuclear divisions instead of one. These two divisions are called Meiosis I and Meiosis II. The chromosome number is reduced ( reductional division ) during Meiosis I, and chromatids comprising each chromosome are separated in Meiosis II. Each division involves the events of prophase, metaphase, anaphase, and telophase.
Advanced Video Overview of Meiosis
Although the process of meiosis is related to the more general cell division process of mitosis, it differs in two important respects:
usually occurs between identical sister chromatids and does not result in genetic changes
Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs. First, the cell undergoes DNA replication, so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information by homologous recombination often leading to physical connections (crossovers) between the homologs. In the first meiotic division, the homologs are segregated to separate daughter cells by the spindle apparatus. The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and two small polar bodies. Because of recombination, an individual chromatid can consist of a new combination of maternal and paternal genetic information, resulting in offspring that are genetically distinct from either parent. Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection can act.
Meiosis uses many of the same mechanisms as mitosis, the type of cell division used by eukaryotes to divide one cell into two identical daughter cells. In some plants, fungi, and protists meiosis results in the formation of spores: haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers, do not have the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis.
Meiosis does not occur in archaea or bacteria, which generally reproduce asexually via binary fission. However, a "sexual" process known as horizontal gene transfer involves the transfer of DNA from one bacterium or archaeon to another and recombination of these DNA molecules of different parental origin.
Meiosis was discovered and described for the first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig. It was described again in 1883, at the level of chromosomes, by the Belgian zoologist Edouard Van Beneden, in Ascaris roundworm eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann, who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In 1911, the American geneticist Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly Drosophila melanogaster, which helped to establish that genetic traits are transmitted on chromosomes.
The term "meiosis" is derived from the Greek word μείωσις , meaning 'lessening'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905, using the idiosyncratic rendering "maiosis":
We propose to apply the terms Maiosis or Maiotic phase to cover the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming. 
The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions for transliterating Greek. 
Meiosis is divided into meiosis I and meiosis II which are further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle.  Interphase is divided into three phases:
- : In this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1, each of the chromosomes consists of a single linear molecule of DNA. : The genetic material is replicated each of the cell's chromosomes duplicates to become two identical sister chromatids attached at a centromere. This replication does not change the ploidy of the cell since the centromere number remains the same. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I in meiosis. : G2 phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds most closely to the G2 phase of the mitotic cell cycle.
Interphase is followed by meiosis I and then meiosis II. Meiosis I separates replicated homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, thus reducing the chromosome number by half. During meiosis II, sister chromatids decouple and the resultant daughter chromosomes are segregated into four daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain only one copy of each chromosome. In some species, cells enter a resting phase known as interkinesis between meiosis I and meiosis II.
Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase II, metaphase II, anaphase II, telophase II).
During meiosis, specific genes are more highly transcribed.   In addition to strong meiotic stage-specific expression of mRNA, there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating the ultimate meiotic stage-specific protein expression of genes during meiosis.  Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis.
Meiosis I Edit
Meiosis I segregates homologous chromosomes, which are joined as tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). Because the ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division. Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c). 
Prophase I Edit
Prophase I is by far the longest phase of meiosis (lasting 13 out of 14 days in mice  ). During prophase I, homologous maternal and paternal chromosomes pair, synapse, and exchange genetic information (by homologous recombination), forming at least one crossover per chromosome.  These crossovers become visible as chiasmata (plural singular chiasma).  This process facilitates stable pairing between homologous chromosomes and hence enables accurate segregation of the chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents (two chromosomes) or tetrads (four chromatids), with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the appearance of chromosomes.
The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads".  : 27 In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within the nucleus.  : 27  : 353 The chromosomes each form a linear array of loops mediated by cohesin, and the lateral elements of the synaptonemal complex assemble forming an "axial element" from which the loops emanate.  Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double strand breaks (around 300 per meiosis in mice).  This process generates single stranded DNA filaments coated by RAD51 and DMC1 which invade the homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologues (to a distance of
Leptotene is followed by the zygotene stage, also known as zygonema, from Greek words meaning "paired threads",  : 27 which in some organisms is also called the bouquet stage because of the way the telomeres cluster at one end of the nucleus.  In this stage the homologous chromosomes become much more closely (
100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and central elements of the synaptonemal complex.  Synapsis is thought to occur in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes.
The pachytene stage ( / ˈ p æ k ɪ t iː n / PAK -i-teen), also known as pachynema, from Greek words meaning "thick threads".  : 27 is the stage at which all autosomal chromosomes have synapsed. In this stage homologous recombination, including chromosomal crossover (crossing over), is completed through the repair of the double strand breaks formed in leptotene.  Most breaks are repaired without forming crossovers resulting in gene conversion.  However, a subset of breaks (at least one per chromosome) form crossovers between non-sister (homologous) chromosomes resulting in the exchange of genetic information.  Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology called the pseudoautosomal region.  The exchange of information between the homologous chromatids results in a recombination of information each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through an ordinary light microscope, and chiasmata are not visible until the next stage.
During the diplotene stage, also known as diplonema, from Greek words meaning "two threads",  : 30 the synaptonemal complex disassembles and homologous chromosomes separate from one another a little. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at the transition to anaphase I to allow homologous chromosomes to move to opposite poles of the cell.
In human fetal oogenesis, all developing oocytes develop to this stage and are arrested in prophase I before birth.  This suspended state is referred to as the dictyotene stage or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later.
Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through".  : 30 This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.
Meiotic spindle formation Edit
Unlike mitotic cells, human and mouse oocytes do not have centrosomes to produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and begin to nucleate microtubules that reach out towards chromosomes, attaching to the chromosomes at the kinetochore. Over time the MTOCs merge until two poles have formed, generating a barrel shaped spindle.  In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes.  Chromosomes then slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores form end-on attachments to microtubules. 
Metaphase I Edit
Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both spindle poles attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. This attachment is referred to as a bipolar attachment. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.  The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension ordinarily requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids (see Chromosome segregation).
Anaphase I Edit
Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center.  Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit"), what prevents the sister chromatids from separating.  This allows the sister chromatids to remain together while homologs are segregated.
Telophase I Edit
The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. However, cytokinesis does not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of meiosis II.  Sister chromatids remain attached during telophase I.
Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.
Meiosis II Edit
Meiosis II is the second meiotic division, and usually involves equational segregation, or separation of sister chromatids. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The end result is production of four haploid cells (n chromosomes, 23 in humans) from the two haploid cells (with n chromosomes, each consisting of two sister chromatids) produced in meiosis I. The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II.
In prophase II, we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic division.
In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. 
This is followed by anaphase II, in which the remaining centromeric cohesin, not protected by Shugoshin anymore, is cleaved, allowing the sister chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. 
The process ends with telophase II, which is similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes re-form and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.
Meiosis is now complete and ends up with four new daughter cells.
The origin and function of meiosis are currently not well understood scientifically, and would provide fundamental insight into the evolution of sexual reproduction in eukaryotes. There is no current consensus among biologists on the questions of how sex in eukaryotes arose in evolution, what basic function sexual reproduction serves, and why it is maintained, given the basic two-fold cost of sex. It is clear that it evolved over 1.2 billion years ago, and that almost all species which are descendants of the original sexually reproducing species are still sexual reproducers, including plants, fungi, and animals.
Meiosis is a key event of the sexual cycle in eukaryotes. It is the stage of the life cycle when a cell gives rise to haploid cells (gametes) each having half as many chromosomes as the parental cell. Two such haploid gametes, ordinarily arising from different individual organisms, fuse by the process of fertilization, thus completing the sexual cycle.
Meiosis is ubiquitous among eukaryotes. It occurs in single-celled organisms such as yeast, as well as in multicellular organisms, such as humans. Eukaryotes arose from prokaryotes more than 2.2 billion years ago  and the earliest eukaryotes were likely single-celled organisms. To understand sex in eukaryotes, it is necessary to understand (1) how meiosis arose in single celled eukaryotes, and (2) the function of meiosis.
The new combinations of DNA created during meiosis are a significant source of genetic variation alongside mutation, resulting in new combinations of alleles, which may be beneficial. Meiosis generates gamete genetic diversity in two ways: (1) Law of Independent Assortment. The independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I and orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I and II, it allows a random and independent distribution of chromosomes to each daughter cell (and ultimately to gametes)  and (2) Crossing Over. The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic information within chromosomes. 
Prophase I arrest Edit
Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis.  In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for decades, four copies of the genome are present in the oocytes. The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to repair damage in the DNA of the germline.  The repair process used appears to involve homologous recombinational repair   Prophase I arrested oocytes have a high capability for efficient repair of DNA damages, particularly exogenously induced double-strand breaks.  DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility. 
In life cycles Edit
Meiosis occurs in eukaryotic life cycles involving sexual reproduction, consisting of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. At certain stages of the life cycle, germ cells produce gametes. Somatic cells make up the body of the organism and are not involved in gamete production.
Cycling meiosis and fertilization events produces a series of transitions back and forth between alternating haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state (diplontic life cycle), during the haploid state (haplontic life cycle), or both (haplodiplontic life cycle, in which there are two distinct organism phases, one during the haploid state and the other during the diploid state). In this sense there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organism phase(s). [ citation needed ]
In the diplontic life cycle (with pre-gametic meiosis), of which humans are a part, the organism is diploid, grown from a diploid cell called the zygote. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes (the spermatozoa for males and ova for females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism.
In the haplontic life cycle (with post-zygotic meiosis), the organism is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Two organisms of opposing sex contribute their haploid gametes to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa utilize the haplontic life cycle. [ citation needed ]
Finally, in the haplodiplontic life cycle (with sporic or intermediate meiosis), the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce spores. The spores proliferate by mitosis, growing into a haploid organism. The haploid organism's gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become a diploid organism again. The haplodiplontic life cycle can be considered a fusion of the diplontic and haplontic life cycles.  [ citation needed ]
In plants and animals Edit
Meiosis occurs in all animals and plants. The end result, the production of gametes with half the number of chromosomes as the parent cell, is the same, but the detailed process is different. In animals, meiosis produces gametes directly. In land plants and some algae, there is an alternation of generations such that meiosis in the diploid sporophyte generation produces haploid spores. These spores multiply by mitosis, developing into the haploid gametophyte generation, which then gives rise to gametes directly (i.e. without further meiosis). In both animals and plants, the final stage is for the gametes to fuse, restoring the original number of chromosomes. 
In mammals Edit
In females, meiosis occurs in cells known as oocytes (singular: oocyte). Each primary oocyte divides twice in meiosis, unequally in each case. The first division produces a daughter cell, and a much smaller polar body which may or may not undergo a second division. In meiosis II, division of the daughter cell produces a second polar body, and a single haploid cell, which enlarges to become an ovum. Therefore, in females each primary oocyte that undergoes meiosis results in one mature ovum and one or two polar bodies.
Note that there are pauses during meiosis in females. Maturing oocytes are arrested in prophase I of meiosis I and lie dormant within a protective shell of somatic cells called the follicle. At the beginning of each menstrual cycle, FSH secretion from the anterior pituitary stimulates a few follicles to mature in a process known as folliculogenesis. During this process, the maturing oocytes resume meiosis and continue until metaphase II of meiosis II, where they are again arrested just before ovulation. If these oocytes are fertilized by sperm, they will resume and complete meiosis. During folliculogenesis in humans, usually one follicle becomes dominant while the others undergo atresia. The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the dictyate stage and lacks the assistance of centrosomes.  
In males, meiosis occurs during spermatogenesis in the seminiferous tubules of the testicles. Meiosis during spermatogenesis is specific to a type of cell called spermatocytes, which will later mature to become spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, proposed to be a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL.   Genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is required postnatally to stimulate spermatogonia differentiation which results several days later in spermatocytes undergoing meiosis, however retinoic acid is not required during the time when meiosis initiates. 
In female mammals, meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo. Some studies suggest that retinoic acid derived from the primitive kidney (mesonephros) stimulates meiosis in embryonic ovarian oogonia and that tissues of the embryonic male testis suppress meiosis by degrading retinoic acid.  However, genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is not required for initiation of either female meiosis which occurs during embryogenesis  or male meiosis which initiates postnatally. 
While the majority of eukaryotes have a two-divisional meiosis (though sometimes achiasmatic), a very rare form, one-divisional meiosis, occurs in some flagellates (parabasalids and oxymonads) from the gut of the wood-feeding cockroach Cryptocercus. 
Recombination among the 23 pairs of human chromosomes is responsible for redistributing not just the actual chromosomes, but also pieces of each of them. There is also an estimated 1.6-fold more recombination in females relative to males. In addition, average, female recombination is higher at the centromeres and male recombination is higher at the telomeres. On average, 1 million bp (1 Mb) correspond to 1 cMorgan (cm = 1% recombination frequency).  The frequency of cross-overs remain uncertain. In yeast, mouse and human, it has been estimated that ≥200 double-strand breaks (DSBs) are formed per meiotic cell. However, only a subset of DSBs (
5–30% depending on the organism), go on to produce crossovers,  which would result in only 1-2 cross-overs per human chromosome.
The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the segregation is not normal, it is called nondisjunction. This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.
Most monosomic and trisomic human embryos are not viable, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome 21. Phenotypes of these aneuploidies range from severe developmental disorders to asymptomatic. Medical conditions include but are not limited to:
- – trisomy of chromosome 21 – trisomy of chromosome 13 – trisomy of chromosome 18 – extra X chromosomes in males – i.e. XXY, XXXY, XXXXY, etc. – lacking of one X chromosome in females – i.e. X0 – an extra X chromosome in females – an extra Y chromosome in males.
The probability of nondisjunction in human oocytes increases with increasing maternal age,  presumably due to loss of cohesin over time. 
In order to understand meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis. 
|End result||Normally four cells, each with half the number of chromosomes as the parent||Two cells, having the same number of chromosomes as the parent|
|Function||Production of gametes (sex cells) in sexually reproducing eukaryotes with diplont life cycle||Cellular reproduction, growth, repair, asexual reproduction|
|Where does it happen?||Almost all eukaryotes (animals, plants, fungi, and protists)   |
In gonads, before gametes (in diplontic life cycles)
After zygotes (in haplontic)
Before spores (in haplodiplontic)
|All proliferating cells in all eukaryotes|
|Steps||Prophase I, Metaphase I, Anaphase I, Telophase I, |
Prophase II, Metaphase II, Anaphase II, Telophase II
|Prophase, Prometaphase, Metaphase, Anaphase, Telophase|
|Genetically same as parent?||No||Yes|
|Crossing over happens?||Yes, normally occurs between each pair of homologous chromosomes||Very rarely|
|Pairing of homologous chromosomes?||Yes||No|
|Cytokinesis||Occurs in Telophase I and Telophase II||Occurs in Telophase|
|Centromeres split||Does not occur in Anaphase I, but occurs in Anaphase II||Occurs in Anaphase|
How a cell proceeds to meiotic division in meiotic cell division is not well known. Maturation promoting factor (MPF) seemingly have role in frog Oocyte meiosis. In the fungus S. pombe. there is a role of MeiRNA binding protein for entry to meiotic cell division. 
It has been suggested that Yeast CEP1 gene product, that binds centromeric region CDE1, may play a role in chromosome pairing during meiosis-I. 
Meiotic recombination is mediated through double stranded break, which is catalyzed by Spo11 protein. Also Mre11, Sae2 and Exo1 play role in breakage and recombination. After the breakage happen, recombination take place which is typically homologous. The recombination may go through either a double Holliday junction (dHJ) pathway or synthesis-dependent strand annealing (SDSA). (The second one gives to noncrossover product). 
Seemingly there are checkpoints for meiotic cell division too. In S. pombe, Rad proteins, S. pombe Mek1 (with FHA kinase domain), Cdc25, Cdc2 and unknown factor is thought to form a checkpoint. 
In vertebrate oogenesis, maintained by cytostatic factor (CSF) has role in switching into meiosis-II.